Flexible Network Load Scheduling Analysis

In order to determine baseline performance, we initially assume there are no optical buffers and no packet retransmission in the network, meaning that no contention management techniques are ap- plied to resolve output contention. The network performance, measured in terms of the packet loss rate and the average input buffering delay, is evaluated under worst-case uniform traffic pattern and hotspot non-uniform traffic pattern for different inter-rack traffic loads a. There are two major ob- jectives in this section. One is to examine the benefits of the proposed flexible network scheduling technique in the developed optical network, by comparing the performance of the flexible schedul- ing with that of fixed resource scheduling. Note that in the fixed scheduling, the resource scheduling result is obtained based on the uniform traffic matrix and will not change with the traffic demands in the network. For that reason, under uniform traffic pattern, the fixed and flexible routing algorithms

(a) Blocking probability (b) Input buffering latency

Figure 5.7: Performance evaluation of the flexible resource scheduling under uniform traffic pattern.

(a) Blocking probability (b) Input buffering latency

Figure 5.8: Performance evaluation of the flexible resource scheduling under hot-spot traffic pattern.

achieve the same network performance. Thus, the performance comparison between the fixed and flexible routing algorithms is only performed for the non-uniform traffic pattern. The other aim is to decide the proper values for the number of OPSs in the optical network, P , and the number of transmission channels per input/output port of the OPS, K, by illustrating how packet loss rate and input buffering delay scale with P × K under a wide range of inter-rack traffic loads a.

Figures 5.7 and 5.8 plot the performance measurements of the proposed optical network with flexi- ble global resource scheduling as a function of the inter-rack traffic load a under, respectively, all-to- all communication pattern and non-uniform hotspot traffic pattern. For the purpose of comparison, Figure 5.9 depicts the system performance of the optical network with fixed resource scheduling under hot-spot traffic pattern. From Figures 5.7 and 5.8, it can be seen that in all scenarios, with the increase of the inter-rack traffic load a, the network performance degrades significantly. This can

be explained by the fact that as the amount of inter-rack communication increases, the volume of datacentre traffic that needs to traverse through the optical network grows progressively, and as a consequence, more traffic competes for the network resources, resulting in more packet contentions and larger buffering delay. It is further shown that the observations in both figures exhibit the same trend, that is, with the increase of P × K, the packet loss rate and the input buffering delay decrease substantially. This is expected as increasing the effective network switching capacity always yields an improvement in network performance. Notably, in Figure 5.7(a), for a set of implementations where the product of P and K are the same, the packet loss rates have very similar values, due to the uniform distribution feature of the all-to-all communication pattern. In contrast, Figure 5.8 shows that under non-uniform traffic, when P × K is the same, the higher is the value of P , the higher is the packet loss rate, but the lower is the input buffering delay. This is because the proposed access scheduling alleviates the input congestion by spreading the traffic among P input fibres of P OPSs, but it potentially imbalances the traffic loads among output fibres and consequently increases the output congestion. Thus there exists a trade-off between the packet loss rate and the input buffering delay. Furthermore, the latency results show that, for a certain value of P , as K increases, the delay first reduces drastically. This is due to the fact that a larger number of transmission channels give less input congestion, and thus the waiting time in the queue is reduced. But beyond a certain value of K, the input buffering delay is sufficiently low and the latency improvement is less significant. Based on these measurements, it can be concluded that a compromise needs to be made between the packet loss rate and the latency performance when dimensioning the optical network.

As previously stated, in datacentre applications, only a small fraction of the rack traffic, less than 25%, is destined towards remote racks, thereby the network configuration with P ×K=12 is chosen, leading to a ratio of 4 : 3 between the downlink capacity and the uplink capacity in the cluster switch. Consider the above mentioned trade-off between packet loss and input queueing delay, P is dimensioned to be 3 and K to be 4. The experiments in the following sections will be performed in this network implementation. Given the configuration settings, the relationship between the offered network load ρ and the inter-rack traffic load a can be quantified. As explained in Section 5.5.1.1, in the uniform traffic pattern, ρ is calculated as ρ=_{P K}aM=4₃a≈1.33a, whereas in the non-uniform traffic pattern, ρ=βaM_{P K}=0.835∗16₁₂ a≈1.11a. Apparently, the network is under-dimensioned, that is to say, if a=1, the offered traffic load to the optical network is significantly higher than 1.

More importantly, the results in Figures 5.8 and 5.9 indicate that the proposed adaptive global scheduling algorithm greatly enhances optical network utilisation, packet loss performance and

(a) Blocking probability (b) Input buffering latency

Figure 5.9: Performance evaluation for the fixed scheduling under hot-spot traffic pattern.

communication latency, compared to commonly used fixed scheduling. The figures show that using the flexible scheduling method, the packet loss rate varies from 0.006 to 0.2, whereas for the fixed scheduling, the loss rate ranges within the interval [0.02, 0.2], thus the traffic is less likely to over- flow from the optical network in flexible scheduling. Note also that the overall network throughput is M N a ( = 2048a), so even a small reduction in loss rate results in a considerable decrease in the amount of blocked traffic. The reason behind the loss improvement is that the access scheduling phase balances the traffic demands among P OPSs and then the OPS resource allocation procedure further balances the traffic loads among different outgoing links at each output port, which sig- nificantly alleviates the congestion conditions in the network and enhances resource utilisation, as discussed in Section 5.3. Further, it is observed that using the flexible scheduling method, the input buffering delay is greatly reduced from >1µs to less than 300ns, as the adaptive access scheduling enables load-balancing and thus alleviates the input congestion, which in turn reduces the required queueing time in the buffer queue. The performance improvements highlight the fact that the pro- posed flexible scheduling algorithm significantly outperforms the fixed scheduling scheme, because it allocates the network resources to the transmission requests based on the traffic demands. Figures 5.7 and 5.8 indicate that a significant fraction of the total traffic overflows from the net- work, even when the switching capacity is over-provisioned. This is the main inherent drawback of optical packet switching. In order to mitigate the contention problem in the proposed optical packet-switched datacentre network, different congestion control techniques are investigated, in- cluding optical buffering, packet retransmission and hybrid FDL/retransmission scheme. In what follows, extensive experiments are carried out to study the performance of the proposed contention control strategies in the presented flexible optical packet-switched network.