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Academic year: 2021



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Olle Svanfeldt-Winter

Master of Science Thesis Supervisor: Prof. Johan Lilius Advisor: Dr. Sébastien Lafond Embedded Systems Laboratory Department of Information Technologies Åbo Akademi University 2011




This thesis evaluates how the energy efficiency of the ARMv7 architecture based pro-cessors Cortex-A9 MPCpre and Cortex-A8 compare in applications such as a SIP-Proxy and a web server compared to Intel Xeon processors. The focus is on com-paring the energy efficiency between the two architectures rather than just the perfor-mance. As the processors used in servers today have more computational power than the Cortex-A9 MPCore, several of these slower but more energy efficient processors are needed. Depending on the application, benchmarks indicate energy efficiency of 3-11 times greater for the ARM Cortex-A9 in comparison to the Intel Xeon. The top-ics of interconnects between processors and overhead caused by using an increasing number of processors, are left for later research.




Abstract i Contents ii List of Figures iv Glossary vi 1 Introduction 1

1.1 Purpose of this thesis . . . 2

1.2 Cloud Software project . . . 3

1.3 Thesis structure . . . 3

2 Energy efficiency of servers 5 2.1 Throughput and latency . . . 5

2.2 Energy . . . 6

2.3 Large scale energy consumption . . . 7

2.4 Reducing energy consumption . . . 9

2.5 Energy proportional computing . . . 11

2.6 Energy efficient low power processors . . . 13

2.7 Summary . . . 14

3 Evaluated computing platforms 15 3.1 Hardware . . . 16 3.2 Software . . . 23 3.3 Benchmarks . . . 24 3.4 Summary . . . 31 4 Performance comparison 32 4.1 Apache results . . . 32 4.2 Emark results . . . 36 4.3 SIP-Proxy results . . . 41 4.4 Summary . . . 47


5 Conclusions and future work 48

5.1 Conclusions . . . 48

5.2 Future work . . . 51

Bibliography 52 Swedish Summary 55 6 Energieffektivitet hos ARM-arkitektur för applikationer i datormoln 55 6.1 Introduktion . . . 55

6.2 Energiförbrukning . . . 55

6.3 Förbättring av energieffektivitet . . . 56

6.4 Mätningar . . . 58

6.5 Slutsatser . . . 59






2.1 Monthly costs for server, power and infrastructure [1] . . . 9 2.2 CPU contribution to total server power usage for two generations of

Google servers. The rightmost bar shows the newer server when idling [2] . . . 12

3.1 BeagleBoard block diagram [3] . . . 17 3.2 OMAP3530 block diagram [4] . . . 17 3.3 Block diagram of the Versatile Express with the Motherboard Express

µATX, CoreTile Express A9x4 and LogicTile Express [5] . . . 18 3.4 Top level view of the main components of the CoreTile Express A9x4

and with the CA9 NEC chip [5] . . . 20 3.5 Test setup for Apache test . . . 28 3.6 Test setup SIP-Proxy test . . . 30

4.1 Comparison between CoreTile Express, Tegra and an Intel Pentium 4 powered machine running the Apache HTTP server. . . 33 4.2 CPU utilization during test on machine with two Quad Core Intel Xeon

E5430 processors . . . 34 4.3 Number of requests handled for each Joule used by the CPU . . . 36 4.4 Graph showing the CPU utilization for CoreTile Express during

SIP-Proxy benchmark . . . 43 4.5 Power consumption for the CPU in CoreTile Express during SIP-Proxy

test . . . 44 4.6 Graph showing performance of reference machine with two Quad Core

Xeons using an increasing number of schedulers . . . 45 4.7 Number of calls handled for each Joule used by the CPU . . . 46


5.1 Achievable energy dissipation reduction by the usage of more efficient processors . . . 49 5.2 Achievable energy dissipation reduction when moving to more





In symmetric multiprocessing two or more processors are connected to the same main memory.


Dynamic Voltage and Frequency regulation is used to adjust the input voltage and clock frequency according to the momentary need in order to avoid unnec-essary energy consumption

Power gating

Cutting power to parts of a chip when the particular part is not needed.

Data center

Facility that houses computer systems

Server farm

A server farm is a collection of servers. They are used when a single server is not capable of providing the required service

CPU Central processing unit.


Platform for computational and data access services where details of physical location of the hardware is not necessarily of concern for the end user


Granularity describes the extent a system is broken down into smaller parts


Dhrystone MIPS. Obtained when dividing a Dhrystone benchmark score by 1757


1 I


Cloud computing systems often use large server farms in order to provide services. These server farms have high energy consumption. The energy is not only needed to run the servers themselves but also for the systems that keep them cool. Energy con-sumption is seen as both an economical and ecological issue. Regardless of whether one wants to save money or to cause less strain on the environment, the solution is the same; to reduce the energy consumption.

The approach presented in this thesis to reduce the power dissipated by server farms is to replace their processors with ones that are more energy efficient. The architecture of processors used in smartphones and embedded systems have been designed with energy efficiency in mind from the beginning, something that has not been the case with the x86 architecture usually found in servers. This makes the processors used in embedded systems interesting candidates when looking for replacements for regular x86 architecture based server processors.

Although the processors used in mobile devices use less energy for executing a sin-gle instruction, the execution of a sinsin-gle instruction is often not directly comparable to an instruction execution on a regular server processor, due to factors such as differences in the instruction sets. Also the computational power for a single low power processor is generally modest compared to traditional desktop and server processors. Moving to processors with lower individual performance increases the number of processors needed to provide the same service as before. Distributing work on a larger number of processors increases the importance of parallelism on the software side. Applica-tions that use a lot of I/O operaApplica-tions will get less of a performance drop than those that are more computationally intensive, when switching to processors with lower in-dividual performance, as the speed of I/O is dependent on other components than just the CPU. Applications designed to be run on server farms are already designed to be distributable in order to use the added resources from a server farm, compared to that of a single server.


Applications such as gateways in telecommunication have a large number of re-quests to serve but the rere-quests are light and generally independent of each other. Also tasks performed by web servers such as serving static webpages are suitable candidates to be run in clouds of low power processors, as the services provided are not CPU in-tensive. The suitability to provide services using low power energy efficient processors will be evaluated using benchmarks.

General performance benchmarks for the Erlang virtual machine (VM) will be used in addition to Apache benchmarking and a SIP-Proxy running on top of Erlang. Gen-eral performance benchmarks will be used to evaluate the performance of the Erlang run time system (erts) for a variety of tasks such as message passing and pure number crunching thereby comparing the performance between the different architectures. The obtained results are used to evaluate why some tasks run better on some hardware than others and explain the performance differences for realistic applications.

The extent of this thesis is limited to how well a single Cortex-A9 MPCore and a Cortex-A8 perform in comparison to processors such as the Intel Xeon. The topics of interconnects between processors and overhead caused by using an increasing number of processors are left for later research.


Purpose of this thesis

The purpose of this thesis is to evaluate the energy efficiency of ARM Cortex-A9 MP-Core based processors compared to x86 based processors for telecom systems and other Cloud like services. In addition to the energy efficiency of the Cortex-A9 MP-Core processors single core Cortex-A8 processor will also be evaluated. To make an energy efficiency comparison possible the performance of the processors will first be evaluated. The main interest is the comparison between the energy efficiency for the two architectures rather than on the energy efficiency between certain processor mod-els.

To achieve this, several processors based on the same underlying architecture will be evaluated. The ability to provide simple web services will also be evaluated on the same ARM based test machines. For testing how well the testmachines perform in providing simple web services Apache 2.2 will be used. The focus is on the ability to provide static web pages. For the telecommunication part the focus will be on a SIP-Proxy running on top of the Erlang VM. Micro benchmarks that stress different


aspects of the Erlang VM are used to evaluate how efficient the ARM based processors can handle different tasks. The important metric is how much performance is achieved compared to the amount of energy used, rather than pure performance of the proces-sors. The potential for energy saving achieved from using ARMv7 based processors in servers compared to servers based on x86 processors will be evaluated. To make the comparison realistic only the efficiency of the processors will be considered. The potential for energy efficiency improvement for the rest of the components in the test machines are not considered. The impact to total data center infrastructure and power cost will be analyzed using a cost model for a hypothetical data center.


Cloud Software project

The Cloud Software Program (2010-2013) is a SHOK-program financed through TEKES and coordinated by Tivit Oy. Its aim is to improve the competitive position of the Finnish software intensive industry in the global market [6]. The content of this thesis is part of the project.

The research focus for the project in the Embedded Systems Laboratory at The Department of Information Technologies at Åbo Akademi is to evaluate the potential gain for energy efficiency by using low power nodes to provide services. In addition to energy efficiency the total cost of ownership for the cloud server infrastructure is in a central role.


Thesis structure

Chapter 2 begins with the introduction of concepts such as energy and energy effi-ciency. Followed by why energy efficiency is an issue for cloud service providers and how much and where energy is consumed in data centers. Methods to reduce energy consumption as well as the concept of energy proportional computing are presented in chapter 2. The chapter ends with a discussion on the motivations and theories on why the usage of energy efficient low power processors is a viable option. Chapter 3 presents the hardware and software used in the evaluations as well as the benchmarks. Chapter 4 presents the results from the benchmarks presented in Chapter 3. Compar-isons between the results for the different test machines in the benchmarks are also presented in this chapter. Chapter 5 shows conclusions together with suggestions for


2 E


In this chapter concepts such as energy, energy efficiency and energy proportionality will be presented. Metrics necessary to evaluate energy efficiency will also be briefly discussed, continued by the subject of why energy consumption is both an economical and practical issue. An example of how much energy is being used by server farms and the cost associated with it is shown. Both direct energy cost and costs related to energy related infrastructure. The chapter ends by presenting methods used to decrease energy consumption and by discussing why using energy efficient low power nodes would be an option.


Throughput and latency

It is important to notice that latency and throughput do not always correlate. Even if two systems have the same throughput the time to serve a single request is not nec-essarily the same. A system that uses one node to provide a certain service has to process the individual requests faster than one consisting of several units. If the num-ber of nodes used to provide a certain service is doubled, the requirements are naturally cut by half for each unit. The formula below shows the relation between throughput, latency and the number of nodes.

T hroughput=Availablenodes∗(1/Latency)

The unit of throughput can vary greatly. If the performance of a web server is eval-uated, the throughput can be defined as the number of requests served each second. The latency is the time taken to serve a request and the available nodes simply indi-cates how many nodes that are available to provide the service. The throughput can be kept on the same level even if the latency increases, provided that the number of nodes is increased to compensate. For example if one server with the ability to serve


a hundred requests per second were be replaced by servers with the ability to serve 10 requests each, ten of the less powerful servers would be needed to achieve the same performance. It is implied that the minimum latency for the service cannot be lower than the minimum latency for a node. How long latency is acceptable depends on the service being provided: a phone call is likely to have stricter requirements for the response time than a service for downloading files.



Energy is generally described as the ability to perform work. It can be of many forms such as kinetic, thermal or electric. In this thesis the focus is on electric and thermal energy as computers use electric energy and transform it to thermal energy. Power is the rate at which energy is being transformed into another form. In the case of computers the conversion is from electrical energy to thermal energy. The unit for power is Watt (W). Electric energy is measured in Joule (J) and is defined as power multiplied with time.

Energy=AvgP ower∗T ime

Energy efficiency is the amount of work done compared to the amount of energy used. The exact way to measure and compare energy efficiency varies depending on the particular application. When considering the energy efficiency for services provided by a cloud or a server it could be for example how many Joules is needed for a transaction, or retrieving a file from a web server. Depending on the application the metrics can greatly vary.

As it is stated by the first law of thermodynamics, energy is never created or de-stroyed, only converted to other forms. All the energy that a computer, or part of a computer consumes will be converted into heat. Depending on the amount of energy the component consumes, the greater the problem with heat dissipation becomes. The ability to dissipate heat is dependent on many factors, such as surface area and ma-terial. Heat sinks are often used to increase the ability of the component in question to dissipate heat. Regardless of thermal dissipation capabilities and the amount of thermal energy dissipated, the heat has to be transferred somewhere in order to avoid overheating issues.



Large scale energy consumption

Small computer systems such as personal computers can generally be cooled by a few fans as the space they are kept in is relatively large in comparison to the amount of heat that is generated. When having a large number of servers in the same place the amount of heat builds adds up. Also the modest energy consumption of a regular home computer is often not a great economical issue as the power needed is on the same scale of magnitude as a few incandescent light bulbs. The more densely hardware is stacked in order to fit as much equipment as possible in the smallest possible space; the more heat is also produced in the same space.

The biggest consumer of energy in a server is the CPU with approximately 45 percent of the total consumption [2]. The energy consumption ratios between different components vary depending on the configuration of the server. In servers where several disk drives are used for data storage the energy consumption of the disk drives also becomes significant [7]. According to Schäppi et al. the total energy consumption of data centers has been increasing for years [8]. In 2006 the energy consumption of servers in Western Europe (EU 15 and Switzerland) was 14,7 TWh [8]; this does not include any energy consumed by the infrastructure such as cooling, lighting and UPS. Schnäppi et al. also states that the complete energy consumption for the data centers in the same region is 36,9 TWh. It is not uncommon for data center service providers to boast of high energy efficiency, both for their servers and for the data centers as a whole. Companies do not, however, generally present exact data on energy consumption and the technical specification of their centers for the public, making accurate estimates difficult. Several different metrics for energy efficiency on a data center scale are used. Power Usage Effectiveness (PUE) and DCiE are metrics defined by Green Grid in a white paper called “The green grid power efficiency metrics: PUE & DCiE“ [9]. The definitions on PUE and DCiE are shown below.

P U E=T otalF acilityP ower/IT EquipmentP ower[9]

DCiE = 1/P U E =IT EquipmentP ower/T otalF acilityP ower∗100%[9]

IT Equipment power includes the servers but also network equipment and equip-ment used to monitor and control the data center. Total facility power includes in


addition to the IT equipment cooling, UPS, lighting and distribution losses external to the IT equipment [9].

In an ideal data center the PUE would be 1 and would mean that all power used by the center is used to power the IT equipment. According to "The green grid power efficiency metrics: PUE & DCiE“ preliminary data shows that many data centers have a PUI of 3.0 or greater [9].

Companies that provide cloud services need large amounts of computer resources. When using cloud computing the user does not need to worry about the resources locally, and many new data centers are being built to provide the required resources. Several companies including Google and Microsoft are building data centers [10] with increasing numbers of servers. Many of the centers are so large that instead of using a server rack as the basic unit shipping containers are used [10] [11]. For example Google uses shipping containers to house servers in their data centers. One container is reported to house 1160 servers, and the power consumption of just one container is reported to be up to 250 KW [11]. Using the reported values one server would use approximately 216 W.

In 2008 Microsoft announced that they were building a data center containing 300 000 servers [12]. If the power consumption of the servers in Microsoft’s new server farm is the same as that reported by Google the power consumption of the servers in the farm is approximately 65 MW. The fact that the servers are packed tightly also means that the challenges for the cooling is increasing. The problem with large heat dissipation is being addressed in different ways, for example Intel provides energy efficient versions of some of its Xeon processors intended especially for high density blade servers [13]. The more energy efficient versions are generally more expensive. For example the Intel Xeon L5434 costs 562 e[14] and the E5430 costs 455e[15]. Having to pay less for keeping the servers running and still providing the same services makes new business opportunities possible and increases the profit for current business areas.

In order to evaluate the potential savings caused by a reduction of the energy con-sumption the total cost structure for a server farm must be analyzed. Hamilton [1] presents a cost analysis for a hypothetical data center. To enable the comparison be-tween cost elements such as infrastructure, hardware and power, amortization times are defined for the investments. The infrastructure in Hamilton’s hypothetical data center is designed to have a 15-year amortization time for infrastructure and a 3-year


amorti-zation time for the servers. A five percent annual cost for the capital used to build the data center is assumed. The cost of power is set at $0.07/KWh for this example. The costs of the data center can be seen in the pie chart shown in Figure 2.1. The chart shows that the direct cost of power is 19 percent of the total cost. Hamilton contin-ues pointing out that for the hypothetical data center 82 percent of the infrastructure costs consist of power and cooling infrastructure, and that thereby the maximum power consumption of the servers is reflected in the infrastructure costs. In Hamilton´s hy-pothetical data center the combined cost of power and cooling infrastructure, and the actual power is 42 percent. Hamilton writes that the power consumption contribution is 23 percent of the total cost. The numbers in the graph do not support that statement. The contribution of power and cooling infrastructure to the total cost on the other hand is 23 percent and the cost of power is 19 percent, according to the graph. These are the values that will be used in chapter five.

Figure 2.1: Monthly costs for server, power and infrastructure [1]


Reducing energy consumption

In order to improve energy efficiency for a computer, the causes for the energy con-sumption must be known. Knowing the contributions of the main components in a modern computer helps to focus only on the critical components. Modern computer systems are built using CMOS circuits. The causes for energy consumption in a CMOS circuit are divided into Static power consumption and Active power consumption. The static power consumption is caused by unintended leakage currents within the circuits.


The static power consumption can be reduced by bringing down the number of active transistors and turning parts of the chip off when not needed. Another factor that af-fects the static power consumption is the supply voltage. Active power consumption is caused by switching the states of the transistors and is thereby dependent of the usage of the circuit.

The time taken to charge and discharge a capacitor is dependent on the voltage used. A higher voltage allows a shorter switching time and thereby a higher clock frequency. The lowest voltage possible should be used for the planned clock frequency in order to be energy efficient. Voltage and Frequency Scaling (DVFS) is a method to reduce the energy consumption of a processor at times when it is not required to run at full capacity. DVFS works by varying both the voltage and clock frequency of the processor, depending on the performance required at a specific time [16].

The number of transistors in a CPU has increased approximately as predicted by Moore’s law for the last forty years, which means doubling roughly every two years [17]. The number of transistors is reflected in chip performance. David A. Patterson points out that the bandwidth (performance) improvement of CPUs has been faster than for other components [18]. The annual improvements can be seen in Table 2.1. The performance increases shown in the table are without units as it only shows the annual improvement for the type of component in comparison to similar components from previous years. While the difference in improvement per year is not huge, the difference has been building up for many years. Patterson points out that bandwidth between components such as the CPU and memory can always be improved by adding more communications paths between them, but that it is costly and causes an increase in energy consumption and the size of the circuits. In addition to the un proportional increase in performance for the components in Table 2.1, Patterson raises concerns that latency has improved less than bandwidth. Patterson continues to point out that marketing has been one reason for this inbalance, that an increase in bandwidth is easier to sell than a decrease in latency. Finally Patterson reminds us that certain methods created to improve bandwidth, such as buffering, has a negative effect on latency [18].

The time it takes for a computer to execute a process is not only dependent on the speed of its CPU. Other components in a computer such as the random-access memory (RAM) are not as fast as the CPU. The speed difference causes the CPU to waste many clock cycles waiting for memory transactions. If data has to be fetched from a hard


CPU DRAM LAN HDD 1.50 1.27 139 1.28

Table 2.1: Relative annual bandwidth improvement of different computer components during the last 20-25 years [18]

drive disk (HDD) the waiting period is further increased.

If there is more than one task running on the same system, and the tasks are running independently from each other, the system might well be able to execute other tasks while one is waiting for I/O. This works well if most tasks do not require I/O operations and access to memory. If the purpose of a system is to mainly run tasks that are I/O intensive and require lots of memory accesses, much time is potentially wasted for the CPU.

As Hamilton [1] points out there are at least two ways of dealing with the perfor-mance inbalance problem. One is to simply invest in better bandwidth and commu-nication paths between the memory and CPU. Another way is to avoid the problem by using lower-powered and cheaper CPUs that does not need as fast memory [1]. Hamilton also points out that because server hardware is built with higher quality re-quirements, and in lower volumes than client hardware it is more expensive. Hamilton continues that ”When we replace servers well before they fail, we are effectively pay-ing for quality that we’re not uspay-ing“[1]. The energy efficiency is in general better for newer hardware adding to the pressure to upgrade to newer servers.


Energy proportional computing

According to Barroso and Holzle [2] a server is generally operating at 10 to 50 percent of its maximum capacity but is rarely completely idle. Having data on several servers improves the availability of the data; a side effect is, however, that more servers must be online. In a case where servers would be completely idle for significant times, powering down a part of the server farm would allow significant power savings. In practice some sort of load and task migration/management system would be needed to distribute tasks in a favorable manner between the available servers, in order to allow powering down a larger number of servers. Barroso and Holzle continue to state that even when a server is close to idle it still consumes about half of its peak


power consumption. In a completely energy proportional server no energy would be used while the server is idle. Complete energy proportionality is not feasible with the manufacturing techniques and materials of today’s processors, due to leakage currents. Regardless of the average power consumption during standard operation, a data center must still have the infrastructure to support the maximum power that the servers can use, or are allowed to use. Reducing the peak power consumption also reduces the demand on the power and cooling infrastructure, the part of the infrastructure that is responsible for 82 percent of the total infrastructure costs [1]. The energy consumption of a server is not necessarily the same as the combined peak power of the components the server is built from [7]. The maximum peak power consumption measured for a server constructed for the example was less than 60 percent of the combined peak power consumption for its components. Fan et al. [7] continue to state that the power consumption is also application specific. From the tests performed in preparation for this thesis, it is clear that even if the system reported full CPU utilization the actual power consumption of the CPU can vary. Furthermore Fan et al. [7] state that in case of an actual data center the consumption is 72 percent of the actual peak power consumption.

Figure 2.2: CPU contribution to total server power usage for two generations of Google servers. The rightmost bar shows the newer server when idling [2]

Figure 2.2 from [2] shows the percentage of energy consumption that the CPU con-tributes to the total energy consumption of the server. The data are from two servers


used by Google in 2005 and 2007. The graphs show that the contribution to the to-tal consumption is approximately 45 percent during its peak power consumption and approximately 27 percent when idle for the newer server. The power saving mech-anisms on the server is unknown but from the data provided in the graph the power saving works better on the processor itself than on the server as a whole because the contribution made by it is smaller when the server is idling. Barroso and Holzle also point out that they have experienced that the dynamic power ranges for DRAM are below 50 percent: 25 percent for disc drives and 15 for networking switches [2]. The observations are in line with the results in Figure 2.2.

The authors in [7] claim that peak power consumption is the most important factor for guiding server deployment in data centers but that the power bill is defined by the average consumption. A lower peak power consumption for the servers allows for a larger number of servers within the same energy budget, leading to a higher utilization level of the cooling and power infrastructure and thereby a more effective use of the available resources and budget. The requirements for both cooling and power, including UPS are reduced with lower peak power consumption.


Energy efficient low power processors

Servers have generally been constructed for high performance, using high performance processors rather than energy efficient ones. Processors that originate from embedded systems are in contrast mainly built for energy efficiency. This is due to both thermal constraints and power constraints from battery powered devices. This kind of proces-sors hardly ever needs active cooling, regardless of them having small physical size.

By replacing an energy hungry high performance processor with a set of energy efficient processors originating from battery powered embedded systems, the energy consumption can be reduced. General purpose processors for embedded systems are produced in large numbers. To get the same amount of work done, a larger number of the slower processors is needed. When increasing the number of processors the gran-ularity of the power consumption also increases. In order to improve energy efficiency with the changing of processors, the processors used must be at least as energy efficient as the one that should be replaced. Processors used in battery powered devices where computational power is required, are ideal for the evaluating the energy saving poten-tial. ARM Cortex-A8 and the Cortex-A9 MPCore are tested for this purpose. When


using DVFS to reduce the energy consumption of the processor the server continues in an operational state. A much greater energy reduction can, however, be achieved by entering a sleep state, where a processor is turned off and thereby not able to do any calculations until it is waken up. The time to enter and return from a sleep state is generally longer than when changing between power states using DVFS. In a server with multiple processors it could be possible to put the ones that are not needed at the moment in a sleep state, in order to reduce power consumption. In this case it is, how-ever, important to be able to predict how long time switching between different power states takes, and know if the service deadlines allow for such a latency.



In a perfectly power proportional server the instantaneous energy consumption is pro-portional to the required service level. An idling server would not use any energy and a server functioning at half capacity would use half of the server’s peak power consumption. Techniques such as DVFS and power gating are used to increase en-ergy proportionality. In a modern computer based on CMOS circuits complete enen-ergy proportionality is not achievable due to leakage currents. While the average power consumption determines the amount of actual used energy, the peak power consump-tion is what defines the required capacity of the cooling and power infrastructure. The cost of power and cooling infrastructure combined with direct energy consumption in a data center is 42 percent, and approximately 45 percent of a server’s peak power consumption is caused by the server’s processor or processors.

Processors intended for usage in embedded devices are designed for energy effi-ciency, in contrast to server processors that are designed for performance. The poten-tial benefit from replacing server grade processors based on the x86 -architecture, used in modern servers with more energy efficient ARM Cortex-A9 MPCore processors is evaluated in this thesis.


3 E


This chapter describes the hardware and software that has been used for the bench-marking. The evaluated hardware uses the ARMv7 -architecture based Cortex-A8 and Cortex-A9 MPCore processors. The platforms that will be used for testing are the Bea-gleBoard, Versatile Express with a CoreTile Express A9 MPCore daughter board and a Tegra 250 development board. The benchmarks presented in this chapter are used to evaluate the performance of the following applications: Apache 2 HTTP server, an Erlang based SIP-Proxy used for session management and some benchmarks testing various aspects of the Erlang virtual machine itself. What SIP and a SIP-Proxy is will be covered as well as Erlang.

In order to evaluate how well a cluster of the ARM Cortex-A8 and Cortex-A9 MPCore processors are suited to replace processors used in todays servers the perfor-mance of single processors must first be evaluated. It is possible to determine how the energy efficiency compares between the different architectures by running com-parison benchmarks with machines that are built using processors based on the x86 architecture. The performance of the processor architecture will be better shown by using two different Cortex-A9 MPCore processors compared to using only one. As the two Cortex-A9 MPCore machines have different clock frequencies as well as differ-ent number of cores, the scaling properties for the two performance increasing options can be evaluated. How the energy efficiency is affected by these factors must also be evaluated. In practice evaluate, if increasing a processors clock frequency or adding additional cores is more beneficial, when looking at the performance per watt. If the results for a single ARM processor show worse energy efficiency compared to modern server processors a cluster of the low power processors will then also have a worse energy efficiency.






The BeagleBoard [3] is a low cost system based on the ARM Cortex-A8 processor with low power requirements. The version of the BeagleBoard that was used for the measurements is the C3. It is equipped with a TI-OMAP3530 chip with an ARM Cortex-A8 processor running at 600MHz. The main storage device is a Micro SD-card and there is 256MB DDR RAM available. A block diagram of the BeagleBoard is shown in figure 3.1 and a block diagram of the OMAP3530 chip in figure 3.2. The BeagleBoard that was used for the benchmarking had Ångström Linux installed with kernel version 2.6.32.

The first tests were run on a BeagleBoard B5. The B5 was later replaced by a C3 as it has double the amount of RAM compared to the B5, allowing for a wider range of tests to be run. Neither model has an Ethernet port built in, a USB to Ethernet adapter was therefore added to get Ethernet connectivity. An improvement from the B5 model to the C3 model is a USB A-port in addition to the OTG mini USB port on the B5 board. Ethernet connectivity has not been built in before the new xM model and also on the xM it is a USB based Ethernet solution [19]. A USB to Ethernet adapter was found to be the best way to get network connectivity to the BeagleBoard. Due to the USB to Ethernet adapter the maximum bandwidth is limited by the speed of USB 2.0 to 480 Mbps. This was not a serious limitation, due to the limited performance of the BeagleBoard in the benchmarks.

Although the BeagleBoard did give some indications on how the test programs performed on an ARM based system, the tests were not conclusive. This, because of the many differences compared to “normal” computer systems not just caused by the processor architecture, but also from the small amount of RAM and the speed of the RAM. The slow speed of the Micro SD-card used for main storage was also slowing down the entire system. In the test with Erlang, a non SMP version of the Erlang run time system (erts) was used, as the Cortex-A8 only has one core.


Figure 3.1: BeagleBoard block diagram [3]

Figure 3.2: OMAP3530 block diagram [4]


Versatile Express

The Versatile Express [5] development platform that was used consisted of the Versa-tile Express Motherboard (V2M-P1) with a CoreTile Express A9 MPCore [5] (V2P-CA9) daughter board. In addition to the Quad Core Cortex-A9 MPCore the daughter board has 1GB of DDR2 memory with a 266MHz clock frequency [5]. A block


dia-gram of the Versatile Express can be seen in figure 3.3. The diadia-gram shows a second daughter board, a LogicTile Express, in addition to the CoreTile Express, the particular machine used for the benchmarking did not have a LogicTile Express installed.

Figure 3.3: Block diagram of the Versatile Express with the Motherboard Express

µATX, CoreTile Express A9x4 and LogicTile Express [5]

The ARM processor on the daughter board is a CA9 NEC [5] chip clocked at 400MHZ with limited power management functions. Power gating and DVFS are not


supported on the chip [5], which needs to be noted when considering the power con-sumption of the system. A top level view of the chip can be seen in figure 3.4. As powering on and off cores is the main power reduction technique available on this particular chip, the power consumption is not precisely matched to the required perfor-mance. In a system where power gating and DVFS are available, the possibilities for power proportional computing are better.

The Versatile Express does, however, allow monitoring of both operating voltage and power consumption. To use this functionality a kernel module was created and loaded to the kernel on the V2P-CA9 to enable usage of the necessary registers for collecting voltage, current and power consumption data. The registers used are VD10 _ S2 and VD10 _ S3. VD10 _ S3 is the power measurement device for the Cortex-A9 system supply, cores, MPEs SCU and PL310 logic [5]. VD10 _ S3 is the most interesting power measurement supply for this comparison. VD10 _ S2 is the current measuring device for the PL310, L2 cache and SRAM cell supply. A program that read the values for the voltage, current and power for both supplies once every second and stored them for further use, was created. The use of the program allowed continuous monitoring during benchmarking. Data logging at shorter intervals was also tested, but was discontinued in order to reduce the interference caused by the data collecting, and because the added value brought by it was negligible.

Furthermore, several possibilities exist for changing settings on the chip, such as the speed of the memory and the clock frequency for the cores. Suitable frequency combinations for the different clocks must be carefully calculated in order for the sys-tem to work properly. These changes must be done while the syssys-tem is off line as the settings are stored on a memory card on the Versatile Express Motherboard, and read from there on startup. A Debian installation was provided with the Versatile Express. The installation was provided with a 2.6.28 Linux kernel. Official support for the Ver-satile Express in the Linux kernel was not added before version 2.6.33. To determine the reasons for unexpected performance differences between the test platforms, mainly between the CoreTile Express, and the Tegra 250, impacts of different parts of the test platforms were examined. Kernel version 2.6.33 was used to evaluate if unexpected performance differences were caused by the kernel version previously used. The main reasons for choosing version 2.6.33 for the test, are that it supports the Versatile Ex-press and is the closest possible version to the 2.6.32 used on the Tegra. Having the Tegra 250, and the CoreTile Express using similar software is useful, in order to find


differences caused by the hardware. The operating system was installed on a USB flash drive, as the native memory card on the Versatile Express was significantly slower than the USB flash drive.

Figure 3.4: Top level view of the main components of the CoreTile Express A9x4 and with the CA9 NEC chip [5]




The Tegra [20] is a Tegra 200 series developer kit with a Tegra 250 system intended to support software development. The Tegra 250 chip includes a dual core Cortex-A9 MPCore chip running at 1GHz. The board also contains 1GB of DDR2-667 RAM and is equipped with a SMSC LAN9514 USB hub with integrated 10/100 Ethernet. The used Tegra 250 board had an additional PCI express Gigabit Ethernet card isntalled in order to avoid networking bottlenecks. Compared to the older chip on the Versatile Express, the newer Cortex-A9 has both more advanced power management features and a different networking implementation. By evaluating the performance of both the Tegra and the CoreTile Express, the aim is to identify how varying number of cores and difference in clock frequencies is reflected in the performance for running different applications. Ubuntu 10.04 was installed on the board with a Linux kernel version 2.6.32, which was provided by Nvidia. As the only compatible kernel version available for the Tegra was the 2.6.32, and the Versatile Express was not supported before 2.6.33, the two Cortex-A9 systems could not use the same kernel version. For the benchmarks the Tegra 250 board was controlled through its serial port and the two Ethernet ports.

As information of the power consumption of either the parts of, or the entire Tegra 250 chip was not available, the values used are estimates derived from the information released by ARM [21]. The Tegra 250 chip also includes several other specialized processors in addition to the Cortex-A9 MPCore. This makes the process of measur-ing the energy consumption of the Cortex-A9 even more challengmeasur-ing. There is little information available for the exact configuration and manufacturing process for the Tegra 250. According to ARM, a Dual Core Cortex-A9 built using the TSMC (Taiwan Semiconductor Manufacturing Company) 40G process, which is a 40 nm manufactur-ing process, in a speed optimized implementation uses 1.9 W at 2 GHz, resultmanufactur-ing in 10000 DMIPS. A power optimized implementation uses 0.5W at 800 MHz providing 4000 DMIPS [21]. In this thesis the power consumption is estimated to be 1 W for the Cortex-A9 in the Tegra 250.


Reference and client machines

In addition to the test machines with ARMv7-A processors other test machines with x86 processors are needed to make a comparison between the energy efficiency of the


processor architectures. A variety of machines were used during the benchmarking, both to make comparisons possible but also to enable the benchmarking in both the SIP-Proxy and the Apache HTTP server tests. The results that are presented for refer-ence values originate mainly from three different machines. The first has a Dual Core Intel E6600 processor, the second has two Intel Quad Core E5430 processors and the third has two Quad Core Intel L5430 processors.

According to the data sheet for the 5400 series [13], there are three different sub series within the 5400 series, targeting different markets, the X5400, E5400 and L5400 sub-series. The X5400 series is described as a performance version and the E5400 as a mainstream performance version. The L5400 is described as a lower voltage and lower power version intended specifically for dual processor server blades. The listed thermal dissipation power (TDP) for X5400, E5400 and L5400 series is 130 W, 80 W and 50 W.



The benchmarks that required interaction between several machines were connected in a number of different ways, depending on the test in question, and partly by the available resources. Due to the design of the test machines, Gigabit Ethernet, was not always available. The BeagleBoards had the option of Ethernet over USB, or a USB to Ethernet adapter. In order to make the tests more comparable, the USB to Ethernet adapter option was used. Both the Versatile Express, and the Tegra 250 had 10/100 Mbps Ethernet capabilities. The Tegra had in addition to the built in fast Ethernet a Gigabit Ethernet card.

At first the machines that were to be benchmarked were connected directly to the benchmarking machine without any switches. To increase the number of clients a fast Ethernet switch was used in order to add up to six benchmarking machines. In order to enable benchmarks by using a larger number of more powerful machines, a Gigabit Ethernet LAN was used. Through this network ten client machines were controlled by an eleventh machine. These machines were used to create the necessary traffic for the benchmark. To avoid and detect problems caused by other users of the same network, the tests were performed in the evenings outside office hours. Tests were also re done later to confirm the results. As the network bandwidth was limited the file requested in the test was small, in order to keep the bandwidth requirements as low as possible. The theoretical maximum bandwidth for the LAN is a gigabit, or 131 072 KBps. The


bandwidth of the network was not a problem for the ARM test machines. However, for the machine used for the reference results, it was a potential bottleneck considering the ability of the reference machines to serve tens of thousands of request per second.





Erlang [22] is a functional programming language and a Virtual Machine. The Erlang syntax resembles that of prolog, not surprisingly, as it started out as a modified version of prolog. The first version of Erlang was created at the Ericsson Computer Science Laboratory by Joe Armstrong, Robert Virding and Mike Williams. The development of Erlanf began in the eighties and Erlang is still used by Ericsson in telecommunication applications [22]. It is designed to be highly concurrent and designed for fault tolerant soft real-time systems [23]. The aim was to create a language that would be suitable for creating telecommunication systems, consisting of millions of lines of code. These systems are not only large, they are also meant to constantly be running. To be able to run them continuously for as long times as possible, software upgrades must be possible without stopping the system [24].

Erlang/OTP is often implied when discussing Erlang. OTP is short of Open Tele-com Platform. It contains tools, libraries and procedures for building Erlang appli-cations. It provides readymade components, such as a complete web server and FTP server. It is also useful when creating telecommunication applications. Both the Erlang VM and OTP are open source licensed.

The Erlang run time system (erts) implements its own lightweight processes and garbage collection mechanism. Erlang is run as a single process in the host operating system, and schedules the Erlang processes within it. SMP Erlang enables the use of more than one CPU core on the host machine by using multiple schedulers. All sched-ulers are run as separate processes in order to enable their simultaneous execution. In general equally many schedulers are run as there are available CPU cores. From the users perspective there is no difference if the cores are on the same CPU, or on differ-ent CPUs in the same SMP machine. The number of schedulers that are used can vary, but by default it is the same as the number of available CPU cores. There is no shared memory between Erlang processes, which means that all communication is done using


message passing, enabling the construction of distributed systems [23].



SIP is short for Session Initialization Protocol, a standard defined by IETF [25]. IETF or Internet Engineering Task Force is an organization that develops and promotes In-ternet standards. IETF does not have any formal membership or membership require-ments [25]. SIP is an application-layer protocol for controlling sessions with one or more participants. It is used for creating, modifying and terminating sessions. The ses-sions can be multimedia, including video or voice calls, and the session modification possibilities include the ability to add or remove media and participants, and change addresses. The protocol itself can be run on top of several different transport protocols, such as Transmission Control Protocol (TCP) or User Datagram Protocol (UDP). SIP includes features such as the possibility for a user to move around in a network, while maintaining a single visible identifier. It is also possible to be connected to the net-work from several different places, by for example using several different phones that are associated with the same identifier. The SIP protocol does not provide services on its own, it does, however, provide primitives that can be used to implement a variety of services. There is a great variety of extensions for the SIP protocol to make it usable for many use cases and environments.

SIP enables the creation of an infrastructure, consisting of proxy servers that users can use to access a service. A SIP-proxy is a server that helps route requests to the current location of the user, and makes requests on behalf of the client, the proxy also authenticates and authorizes users for the provided services. The protocol allows for registration of the users locations to be used by the proxy servers.



To evaluate the performance of our hardware, several benchmarks were used. First, the performance of the Erlang rts is benchmarked to see how well it performs on the hard-ware. This shows how well an application running on top of Erlang could be expected to run. By benchmarking the SIP-Proxy, and Apache 2 server, the performance for actual services is evaluated for all the hardware. More precise information about the benchmark setups are presented in the following sections, and the results are presented and analyzed in the following chapter.



Apache 2

Apache 2.2 HTTP server was used to determine how well the Cortex-A9 MPCore machines can perform with traditional server tasks. Apache 2.2 was chosen as it is both freely available, open source and has been one of the most popular servers for a long time. The Apache HTTP server is available for many platforms, such as Linux, Mac OS/X and is used with a variety of architectures. As these benchmarks are targeting the x86 and ARMv7-A architectures, the ability for the Apache HTTP server to run on both is crucial. These tests are focusing on use cases with small static files. The initial tests were run using Apache Bench (AB), a tool for quick performance testing. The use of AB was later discontinued in favor of autobench, in order to produce more reliable results.

Autobench [26] was used to measure how well Apache performs on the different machines and with different test parameters. Autobench is a tool that helps automate the use of httperf. Httperf is a program for benchmarking the performance of a HTTP server. It creates connections to a server, in order to fetch a file. During one connec-tion, one or several requests for the file is made, depending on the test parameters. By changing the rate the connections are created and the number of requests for each con-nection, the load on the server varies. By running httperf several times with different rates of connections the servers response to different load can be evaluated. By running the test from several machines simultaneously, the load generating capabilities of the test setup can be taken beyond that of a single machine, but the instances must be con-trolled separately. The results from such tests must also generally be later combined manually. To decrease the error caused by differences in the starting time for the test from different test machines, the testing time should be relatively long [27].

One of the most useful features of autobench is that it runs multiple tests while steadily increasing the load on the server according to the users instructions. In order to run autobench, a number of parameters must be set. The parameters can either be set in a configuration file, or given as command line arguments when starting the test. An example for benchmarking a single server giving the arguments from the command line, looks like the following.

autobench -single_host -host1 www.google.com -uri1 /10B -quiet -low_rate 100 -high_rate 1000 -rate_step 20 -num_call 10 -num_conn 10000 -timeout 5 -file benchmarkresult.tsv


ad-dress to the server, in this case www.google.com. Uri1 is the file that is requested in the test, a file called 10B, is used in this example. The quiet option must be used if the results are expected to be sent to STDOUT, as it restricts the amount of data that httperf produces. Too much output from httperf causes autobench to cre-ate badly formatted report tables. The benchmarking stats from the point indiccre-ated by

low_rate, and continues to the upper limit given byhigh_rateusing the step size from rate_step. In this case the test starts from 100 connections and is run until 1000 connections per seconds is achieved, always adding 20 connections for each new run. Num_callregulates how many requests for the particular file should be made for each connection, 10 in this case. If the option to keep alive connections is disabled on the server, the test will report a number of unsuccessful attempts at retrieving the file. The number of attempted requests is calculated by multiplying the num_call

with the connection rate. Num_conn specifies the number of connections to create for each step of requests. As the time taken to attempt a number of connections is dependent on the rate the connections are created, a more convenient alternative is re-placingnum_connwithconst_test_time. Const_test_timespecifies how long the test should continue, automatically calculating the value for the number of connections for all request rates during the testing. Timeout sets the value in sec-onds to how long a request is allowed to last before it is considered a failure. A longer

timeout value causes a greater fd (file descriptor) usage than a shorter, increasing the risk for stability issues for the system under test. Thefileoption simply specifies the name of the file where the results are stored.

In theory, almost any benchmarking program could have been run on several ma-chines simultaneously, and then manually combine the results, autobench helps auto-mate this process for httperf. Two programs are provided together with autobench: autobench_admin and autobenchd. Their purpose is to make the usage of several ma-chines for testing convenient. The idea with these programs is that autobenchd is run on all client machines and one of them runs autobench_admin. Autobench_admin distributes the required requests between all client machines. After the instances of autobenchd have completed their part of the test, the results are collected and com-bined by autobench_admin. With the basic configuration the requests are distributed equally among all autobenchd instances. This has two implications. The first is that the requested number of connections must be evenly dividable between all instances. The second is that in order to get clean results, all client machines should only be requested


to create a number of requests that the used machines are able to produce. The results from a test where the client machine has not been able to produce enough request due to limitations of its own, look similar to results where the server that is being tested is not able to reply to the requests.

In the results file, autobench stores the number of requests that were supposed to be requested, followed by the number that actually was attempted and the number of connections created. Statistics on what the minimum, mean and maximum number of requests during the test is also provided. The amount of bandwidth used, and the number of errors is also listed. As the tests are run several times, trends are easily detectable. Especially short and sudden interference is generally obvious, interference that remains constant is more difficult to detect. By running the tests several times, and with an increasing load, more information is generated on how the server responds to a varying number of connections and requests. Testing several times with similar test parameters also gives an overview and helps verify if something has interfered with the test.

As illustrated in Figure 3.5, the test was set up so that the machines that were to be benchmarked, were connected through Ethernet to the client machines that created the requests. In order to make sure that the server was the bottleneck in the final tests, i.e. that the data produced were meaningful; the test was run several times with slight variations in the test parameters. To guarantee that the client machines were able to create enough traffic, the same tests were run using an increasing number of client machines. If the results remained close to the same, even if the number of client machines was increased, the load created by the client machines was sufficient. The number of served requests was set as the metric for performance. The only requirement was that the requests being counted would be served within five seconds, the rest of the responses were discarded. Any other quality of service aspects such as the number of unanswered request were ignored, as the focus was on maximum performance rather than quality of service.

The original plan was to leave all installations of the Apache 2 http server in their default configurations to get a generic comparison. It soon became clear that the de-fault configurations for the Apache server installations were not identical. Although the same version of Apache (Apache 2.2) was provided with all Linux distributions that were used, the configurations varied. It was expected that there would be differ-ences such as where configuration files were located on the different distributions, but


not that the default configurations would differ. The main difference was that on the Apache installation on the Fedora machine, the keep alive option was disabled, this was changed to be enabled on all machines.

By running the same static page fetching test, but using files of different sizes and monitoring the bandwidth usage, it was decided if a file could be used safely without getting problems with the available network bandwidth. Small file size was required as the Versatile Express was equipped with only a 10/100 Mbps Ethernet card rather than Gigabit Ethernet. The performance of the reference machine was expected to serve a large number of requests resulting in a high bandwidth usage even with small files. In tests where small files are used, the overhead caused by the underlying protocols becomes significant.

Figure 3.5: Test setup for Apache test


Basic Erlang performance benchmarks

To evaluate the performance of the Erlang Virtual Machine (VM) running on the test machines some general performance benchmarking was done. A set of micro bench-marks running on the Erlang VM was used. Some of the benchbench-marks were able to make use of more than one core on the host machine, while others did not gain any notable benefit compared to running on just one core. The micro benchmarks are de-signed to stress different parts of the VM. While these tests do not emulate a realistic service producing scenario, they do give information of the general performance levels


of different parts of the systems. This information can then be used to compare the performance of the VMs running on different platforms, and provide a way to estimate how well different applications could be expected to run. The results are included as Appendix A. Due to reasons such as insufficient memory in the test hardware, not all benchmarks were run on all available test platforms. If a benchmark was not able to run on all machines, the results from the benchmark was also not analyzed for the rest of the machines either.



An Erlang based SIP-Proxy (Session Initiation Protocol Proxy) was tested to find out how well an ARM Cortex-A9 would perform in telecom applications. The perfor-mance of the SIP-Proxy was measured in the number of calls per second it could han-dle. The metric for energy efficiency for the proxy was decided to be the number of calls the proxy could handle for each Joule used. As the proxy is running on top of Erlang, the results from this particular proxy reflect the result from the Erlang micro benchmark. The value this benchmark brings comes from giving a performance and energy efficiency evaluation for a realistic service, rather than just parts of the system as the micro benchmarks did.

To measure the performance of the proxy, two other machines were used as shown in Figure 3.6. One machine was used to create the messages that should be passed and the other was used as the receiver. Both of these machines were running SIPp, an open source test and traffic generation tool made available by HP. The version used was 3.1 and was compiled from the source. The bandwidth required for the proxy running on a machine capable of only a limited number of calls each second is not big, bandwidth, was thereby not expected to be an issue here. The proxy had a fd leak and in order to avoid issues caused by this, the maximum number of fd:s both system wide, and for each user was increased. During the time the proxy was running there was constantly a small increase in the memory used by the underlying erts. The total memory usage of the system was, however, all in all modest. To get more reliable results and avoid as much interference from the unwanted accumulative use of system resources caused by the fd leak, the proxy was restarted between every test.

In order to evaluate the performance of the proxy a definition for when the proxy passed a test was needed. In the reference results provided by Ericsson the proxy had been expected to run for a few minutes. The reference results used in this benchmark


are from a machine with two Quad Core Intel Xeon L5430 processors and 8 GB of RAM. As the testing focused on processors with lower performance more strict re-quirements were set up. This was done to enable a more accurate comparison between the different ARM Cortex-A9 MPCore processors that were to be tested.

Figure 3.6: Test setup SIP-Proxy test

The test machines were required to be able to pass all requested messages for a two minute period. The two minute requirement was as a result of two different factors. The first was the requirement used by Ericsson that the new requirement would need to be in line with. The second was caused by the way the proxy used system resources. When running the proxy, the CPU utilization level of the system increased rapidly for a while. After the rapid increase a very slow increase was observable for the entire time the proxy was running. The rapid increase was observed to halt well before the two minute mark for all the tested ARM Cortex-A9 MPCore systems. There was not much need for discussion for an acceptable error rate, as a low sustainable error rate was not encountered during the testing. If the load was not significantly decreased quickly after a failed message caused by a high load, the fail rate would increase rapidly.

The erts on both the Versatile Express and the Tegra 250 was recompiled to support profiling using Gprof. Some optimizations had to be disabled from the make files for the erts in order for Gprof to work. Gprof shows function calls executed by a specific program, Oprofile was also used as it has the ability to perform system wide profiling. To enable profiling using Oprofile the kernels on both machines were recompiled. As the profiling has a negative effect on system performance the profiling enabled versions


of both the erts and the kernels were not used when obtaining results for maximum performance in any benchmarks. The versions that supported profiling were only used to find reasons for unexpected anomalies and performance differences.



To evaluate the energy efficiency of the ARM Cortex-A8 and the Cortex-A9 MPCore, compared to processors built on the x86-architecture for server tasks, a set of test hard-ware has been used. To evaluate the performance of the Cortex-A8 a BeagleBoard is used. Evaluating the Cortex-A9 is done using a Tegra 200 development kit with a Tegra 250 chip, and a Versatile Express with a CoreTile Express. An Apache 2.2 HTTP server is used to evaluate the energy efficiency of the Cortex-A9 MPCore processor, compared to an Intel Xeon processor when serving static files to clients. The perfor-mance for running the Erlang VM on the test machines is evaluated directly using a set of micro benchmarks, as well as a Erlang based SIP-Proxy.


4 P


This chapter explains the individual executions of the benchmarks. After each bench-mark execution the corresponding results are presented. The results from the execu-tions are followed by performance comparisons. After the pure performance compar-isons the energy efficiencies will be compared. All energy efficiency comparcompar-isons in this chapter focus on the energy consumption off the processors themselves, rather than on the total consumption of the computers being benchmarked.


Apache results

After reaching its peak performance in this particular test the performance of the Ver-satile Express dropped quickly, and in the end made the server unresponsive. This can be seen in Figure 4.1. This happened sometimes before reaching full CPU utilization. A reason for this could be the network implementation on the Versatile Express. If enough interrupts are generated in order to handle TCP packets it eventually leads to a situation where an increasing part of the runtime is used up by interrupts and thus leaving a decreasing amount of resources available to actually provide the intended service. This has not been proven to be the case here, but is a viable possibility. To deal with the unresponsiveness the test machine was restarted between tests.

When benchmarking the machine with the two Intel Quad Core Xeon E5430 pro-cessors using the same test parameters as for the rest of the machines, the test proved to not be CPU intensive enough as full CPU utilization was not achieved. In order to find the bottleneck, the number of clients was increased. The test was run using both ten and five client machines. As the results were the same in both tests the performance of the client machines was not a bottleneck. The bandwidth was tested by redoing the test using a larger file than the original. The result from the test with the larger file was close to that of the original test, with the biggest difference being a higher bandwidth


Figure 4.1: Comparison between CoreTile Express, Tegra and an Intel Pentium 4 pow-ered machine running the Apache HTTP server.

usage. The system reported no shortage of available memory in any of the Apache HTTP server benchmarks.

The machine with the two Quad Core Xeons was able to serve 36000 requests per second when a hundred requests were made for each connection. For ten requests for each connection the result was only 6200 requests/s. The data transfer during the test with 36000 requests per second was reported by Autobench to be 11600 KBps. Compared to the theoretical maximum bandwidth for a gigabit network (131072 KBps) the used bandwidth was less than 10 percent. Although the theoretical bandwidth is generally not achieved in a real life network, more than 10 percent is achievable. In addition, higher data transfer rates from the same server, using the same network was achieved using a larger file. As the network seemed an unlikely bottleneck, other parts of the test setup was inspected. The machine running the Apache server reported 60


percent CPU utilization for the test with 36000 requests and 10 percent for the test with 6200 requests. If the CPU utilization level and web server performance would continue having the same relation to each other, the performance in both cases is around 60000 requests per second with full CPU utilization. Figure 4.2 show the CPU utilization at a few points during the Apache test for the dual E5430 machine. Assuming the performance for one E5430 running at 100 percent is the same as the performance of two running at 50 percent, the performance for one E5430 is 33000 requests per second.

Figure 4.2: CPU utilization during test on machine with two Quad Core Intel Xeon E5430 processors

The results from the Apache test shown in Table 4.1 are from fetching a static file of size 10 Bytes. 10 Calls per connection and 100 calls per connection were requested in the tests. The better results from the two benchmarks were used. The performance for the Tegra 250 was more or less the same when making ten or a hundred requests for each connection. For the comparison machine with the two Xeon processors the difference was approximately a multiple of ten. The better results were used for the


Machine Request / second Requests / Joule Quad Core Intel Xeon E5430 (2.66 GHz, 80 W) 33000 413

Pentium 4 (2.8GHz) 7100 80

Dual Core Cortex-A9 MPCore (1 GHz) 4600 4600 Quad Core Cortex-A9 MPCore (400 MHz) 3400 2833

Cortex-A8 (600 MHz) 760 760

Table 4.1: Ability of Apache 2.2 to serve a 10 byte static files using different hardware

comparison. The test results can be seen in Table 4.1. As it can be seen in the table the Tegra 250 managed to serve 4600 requests per second and the Versatile Express 3400 requests per second. The performance difference of the Versatile Express compared to the Tegra 250 is likely caused by both the slower clock frequency of the CPU and the network implementation. The difference in combined clock frequencies between the two processors on its own is slightly less than the performance difference. The combined number of clock ticks for the Versatile Express is (400 * 4) 1600 and 2000 (1000 * 2) for the Tegra 250. Comparing these, the Versatile Express has 80 percent of the clock ticks of the Tegra 250. The performance of the Versatile Express is in comparison slightly less, 74 percent of that of the Tegra 250.

The results from the Apache tests were mainly compared against a machine with two Intel Quad Core Xeon processors running at 2.66 GHz. To provide a more com-prehensive comparison, and more reference points a machine with a Pentium 4 (2.8 GHz) was also benchmarked. While the machine that has the more traditional server processors outperforms the tested Cortex-A9 processors, the Cortex-A9 processors do well taking their energy consumption into account. The Intel Xeon processor (E5430) that was used in the reference machine has a reported maximum thermal design power (TDP) of 80 W, while the Quad Core Cortex-A9 according to performed tests has a maximum measured power consumption of 1.2 W.

The rightmost column in table 4.1 shows the number of answered calls produced per Joule used. A clear improvement in energy efficiency is visible, starting from the Pentium 4 to the Dual Core ARM Cortex-A9 MPCore. Figure 4.3 shows the energy efficiency comparison as a bar diagram.



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