CPU-GPU Queueing System

In this subsection, we test the performance of the sampling based ENMPC approach on the CPU- GPU queueing system proposed in [59], which is the motivating application for the results in this chapter; see Figure5.3for the model of the system. The objective is to drive the injection rate of the GPU queue (i.e., λGP U) to track a target number of Frames Per Second (FPS) of the display, and the controllable variables are the operating frequencies of the CPU and GPU (i.e., fCP U and fGP U) which determine the injection rates of the corresponding queues (i.e., λCP U and λGP U).

The dynamics of the queueing system is given as follows:

qCP U[k + 1] = qCP U[k] + λCP U[k]Ts− λGP U[k]Ts qGP U[k + 1] = qGP U[k] + λGP U[k]Ts− µGP UTs,

where qCP U (resp. qGP U) and λCP U (resp. λGP U) are the state and injection rate of the CPU queue (resp. GPU queue), respectively, Ts is the sampling time, and µGP U is a constant target FPS.

Figure 5.3: CPU-GPU queueing system.

The injection rates λCP U and λGP U are determined by fCP U and fGP U as follows: λCP U[k] = Φ  T_{CP U}ref [k], fCP U[k]  λGP U[k] = Φ  T_{GP U}ref [k], fGP U[k]  ,

where the parameter T_{CP U}ref [k] (resp. T_{GP U}ref [k]) is the average processing time for the tokens in the CPU queue (resp. GPU queue) at time-step k when the CPU (resp. GPU) is operating at some specified reference frequency. Note that T_{CP U}ref and T_{GP U}ref only depend on the characteristics of the tokens to be processed (i.e., the input stream to the system). For a given sequence of tokens (i.e., given T_{CP U}ref or T_{GP U}ref ), the function Φ represents the mapping from the frequency adopted (i.e., fCP U or fGP U) to the corresponding injection rate; we omit the specific form of the function Φ and refer to [59] for more details.

In order to construct the ENMPC controller, we regard the variables qCP U, qGP U, TCP Uref and T_{GP U}ref as parameters of the optimization problem. We use N = 5000 sampling points over the sampling region which is qCP U, qGP U ∈ [0, 5] and TCP Uref , T

ref

GP U ∈ [10, 20].1 We choose the basis functions to be the Legendre polynomials up to degree 2 with M = 15 coefficients and the prediction horizon is chosen to be 2. Furthermore, the sampling time Ts = 50 (unit: millisecond) and the target FPS µGP U = 60.

In Figure5.4, we illustrate the performance of the ENMPC controller using data collected from a mobile platform. From Figure5.4d, we can see that the controller is able to drive the

1_{Note that the unit of T}ref CP U and T

ref

injection rate of the GPU queue close to the desired target (i.e., µGP U). Moreover, from Fig- ure5.4aand Figure5.4b, we can see that the queue occupancies are kept in a desired range (i.e., qCP U, qGP U ∈ [0, 5]). 0 0.5 1 1.5 0 1 2 3 4 t [in sec] qC P U

(a) CPU queue state qCP U.

0 0.5 1 1.5 0 1 2 3 t [in sec] qG P U

(b) GPU queue state qGP U.

0 0.5 1 1.5 0 20 40 60 80 t_{[in sec]} λ CPU

(c) CPU queue injection rate λCP U.

0 0.5 1 1.5 0 20 40 60 80 t_{[in sec]} λ GPU Actual Target

(d) GPU queue injection rate λGP U.

Figure 5.4: Performance of the ENMPC controller on the CPU-GPU queueing system. The red dashed line in Figure5.4dis the target FPS to track.

5.8

Summary

In this chapter, we studied the output tracking problem for nonlinear constrained systems and proposed a sampling based ENMPC approach to address the problem. The basic idea is to sample the augmented state and reference signal space using a low-discrepancy sequence and approximate the optimal control surface based on the information at the sampling points. We also extended the tube-based robust control in [88,89] to robust ENMPC by using the sampling based approach. As we showed, the proposed approaches achieve asymptotic tracking and guarantee stability and feasibility of the system, given that certain mild conditions are satisfied. Moreover, the sampling based (robust) ENMPC is easy to implement and is suitable for applications with limited online computation and storage resources, such as MPSoCs.

Chapter 6

Conclusions and Future Research

6.1

Conclusions

In this thesis, we studied a set of estimation and control problems, driven by applications in Multi-Processor Systems on Chips (MPSoCs). These applications stimulated new formulations for extensively studied problems (e.g., the state estimation problems in Chapter2and Chapter3), motivated new objectives for existing problems in the literature (e.g., the design-time sensor se- lection problem in Chapter4), and brought new challenges which required us to invent applicable techniques (e.g., the resource constrained output tracking problem in Chapter5).

In Chapter2and Chapter3, we studied the state estimation problem for linear dynamical sys- tems with unknown inputs when the system is not strongly detectable. In other words, we studied the case where it is impossible to exactly reconstruct the system states. Under this situation, in Chapter2, we considered the problem of constructing an unknown input norm-observer, which can be regarded as a relaxed estimation objective for cases where perfect estimation cannot be achieved, and proposed the notion of BIBOBS stability to solve the unknown input norm esti- mation problem. We showed that under certain conditions, the inputs and initial condition can be chosen so that the states corresponding to the eigenvalues with magnitude 1 are persistently excited or triggered while the states of the other systems are maintained in a bounded orbit; thus, care must be taken to avoid such situations.

In Chapter3, we explored the influence of the other assumption on the system: the property of positivity. We showed that the additional information on positivity is not helpful in relaxing the conditions under which perfect estimation is achievable. We also considered the case where the positivity of the observers is needed and provided a construction method for positive observers.

In Chapter4, we studied the priori and posteriori KFSS problems for linear dynamical sys- tems. We showed that these problems are both NP-hard (even under the additional assumption that the system is stable). We also provided upper bounds for the performance of the worst-case selection of sensors and highlighted the factors that dominate the worst-case performance. Then we studied a priori covariance based and a posteriori covariance based greedy algorithms for sensor selection. We showed that these algorithms are optimal for two classes of systems. For general systems, we provided a negative result showing that the corresponding cost functions are neither supermodular nor submodular; however, simulations indicate that these algorithms perform well in practice. For the Lyapunov equation based greedy algorithm (which attempts to minimize an upper bound on the original objective functions), we showed that this algorithm achieves optimal performance with respect to its cost function. Although this algorithm performs less well than the original algorithm in terms of minimizing the steady state Kalman filtering er- ror covariance, the run-time scales better with the system size.

Finally, in Chapter5, we proposed an efficient sampling based explicit nonlinear MPC (EN- MPC) for the nonlinear output tracking problem by augmenting the state space with the reference space. We provided feasibility and asymptotic tracking guarantees for the nominal controlled system. We designed an ancillary ENMPC to eliminate the influence of additive disturbances and provided ultimate bounds for the robust ENMPC controlled system states. The proposed ap- proaches are efficient for online implementation and can be easily modified to balance the trade off between performance and online computational complexity.

To summarize, the theory and techniques developed in this thesis provide efficient algorithms for estimation, configuration and control of MPSoCs and yield insights into the underlying struc- ture of the corresponding problems.