1. Static Bus Schedule aware Scratchpad Allocation in Multiprocessors Sudipta Chattopadhyay Abhik Roychoudhury National University of Singapore

2. Scratchpad Memory (Basics) Scratchpad Memory  A fast and software controlled on-chip memory  Each memory access is predictable Problems  Cumbersome and error prone if managed by user  Need extensive compiler support for automatic management

3. Scratchpad Allocation Worst case optimization vs Average case optimization  This work is on worst case Actual BCET Actual WCET Execution Time Observed WCET Estimated BCET Actual Observed Over-estimation WCET = Worst-case Execution Time BCET = Best-case Execution Time Observed BCET Estimated WCET Actual WCET

4. Scratchpad Allocation SPM Task SPM-0SPM-n........ Task graph Previous work in our group (Suhendra et. al. RTSS’05) Software cache locking (Puaut ECRTS’07) Previous work in our group (Suhendra et. al. TOPLAS’10) SPM-0SPM-n..…… Task graph This paper with next level of memory accessed by shared bus

5. An MPSoC Architecture PE-0PE-1PE-N SPM-0SPM-1SPM-N Shared off-chip data bus Off-chip memory External Memory Interface MPSOC …… Fast on-chip communication media

6. SPM architecture Bypassing memory hierarchy  Each memory access is predictable – crucial for time predictable embedded systems Non-bypassing memory hierarchy  Acts like a fully associative cache  Spilling and reloading of memory blocks lead to unpredictable execution time

7. Allocation strategy  Consider data memory allocation  Variable locations (private SPM, remote SPM or external memory) are computed at compile time  If two variables share the same space, they are guaranteed to have disjoint lifetime  No reloading cost required

8. Motivation Why shared bus makes it different ? Bus slots for other cores (150 cycles) m m’m’ Total delay for ref(m) = (150 + LAT) * freq(m) Total delay for ref(m’) = LAT * freq(m’) 150 * freq(m) > LAT * (freq(m’) – freq(m)) (m likely to reduce WCET more than m’) this core slot this core slot Because shared bus delay is variable freq(m’) > freq(m)

9. Motivation Why shared SPM space makes it different ? SPM - 0SPM - 1 Not so critical task An SPM allocator unaware of shared scratchpad space cannot allocate memory blocks accessed in critical tasks to SPM-1 Critical task Allocator’s View

10. Motivation Why shared SPM space makes it different ? SPM - 0SPM - 1 Critical task Not so critical task Allocator’s View Exploiting shared scratchpad space, more performance can be obtained as the critical tasks can also allocate in SPM-1

11. Allocation framework Bus-delay aware WCET analysis Task WCET Total delay (bus delay + memory latency) to access variables along WCEP WCRT analysis Variable lifetime and critical path information Bus aware SPM allocator Enough space? SPM allocation decision Yes Optimized WCRT No Application task graph

12. Allocation framework Bus-delay aware WCET analysis Task WCET Total delay (bus delay + memory latency) to access variables along WCEP WCRT analysis Variable lifetime and critical path information Bus aware SPM allocator Enough space? SPM allocation decision Yes Optimized WCRT No Application task graph

13. Bus delay aware WCET analysis Shared bus introduces variable latency for each memory access. Our previous work approximates the total delay incurred by a static memory reference. This delay is used as a metric by the greedy SPM allocator.

14. Allocation framework Bus-delay aware WCET analysis Task WCET Total delay (bus delay + memory latency) to access variables along WCEP WCRT analysis Variable lifetime and critical path information Bus aware SPM allocator Enough space? SPM allocation decision Yes Optimized WCRT No Application task graph

15. WCRT analysis t3 t2 t4 t1 (1) (2) (1) Assigned core Task graph Task lifetime : [eStart, lFinish] eStart(t1) = 0 eStart(t4) >= eFinish(t2) eFinish(t4) >= eFinish(t3) eFinish = eStart + BCET lStart(t4) >= lFinish (t2) lStart(t4) >= lFinish (t3) t3 can be preempted by t2 lFinish (t3) = lStart(t3) + WCET(t3) + WCET(t2) + 2 * BUS_SLOT_LENGTH Computed WCRT = lFinish(t4) Earliest time computation Latest time computation All tasks have the same period – the period of the entire task graph

16. Allocation framework Bus-delay aware WCET analysis Task WCET Total delay (bus delay + memory latency) to access variables along WCEP WCRT analysis Variable lifetime and critical path information Bus aware SPM allocator Enough space? SPM allocation decision Yes Optimized WCRT No Application task graph

17. Bus aware SPM allocator Using WCRT analysis we also obtain the lifetime information of each variable Interference graph  Each node is a variable accessed in some task  An edge exists between two nodes if their lifetimes interfere Nodes have weights  Higher the total access delay (including bus delay), higher the weight  Higher the weight if accessed in critical path

18. Bus aware SPM allocator M1 = 10 C1 = 30 C2 = 55 C3 = 10 M2 = 10 N = 10 N = 5 C4 = 10 M1 = 10 Task T1 in PE-0 C5 = 40 C6 = 55 N = 10 M3 = 10 Task T2 in PE-1 Assume M1, M2 and M3 are only memory accesses Critical Task = T1 M2 suffers more delay to access than M1. Reduce WCRT by reducing the WCRT of critical task T1

19. Bus aware SPM allocator M1 = 10 C1 = 30 C2 = 55 C3 = 10 M2 = 10 N = 10 N = 5 C4 = 10 M1 = 10 Task T1 in PE-0 C5 = 40 C6 = 55 N = 10 M3 = 10 Task T2 in PE-1 SPM-0 SPM-1 M1 M2 M3 [0,690] [455, 480] [375,650] M2 t = 530 t = 480 T1 T2 WCRT = 530 cycles (empty) Allocation (iteration 1) Critical Task = T1 Reduce WCRT by reducing the WCRT of critical task T1

20. Bus aware SPM allocator M1 = 10 C1 = 30 C2 = 55 C3 = 10 M2 = 10 N = 10 N = 5 C4 = 10 M1 = 10 Task T1 in PE-0 C5 = 40 C6 = 55 N = 10 M3 = 10 Task T2 in PE-1 Critical Task = T2 SPM-0 SPM-1 M1 M2 M3 [0,530] [455, 480] [375,510] M2 t = 464 T1T2 WCRT = 480 cycles M1 t = 480 Allocation (iteration 2)

21. Bus aware SPM allocator M1 = 10 C1 = 30 C2 = 55 C3 = 10 M2 = 10 N = 10 N = 5 C4 = 10 M1 = 10 Task T1 in PE-0 C5 = 40 C6 = 55 N = 10 M3 = 10 Task T2 in PE-1 M2 and M3 have disjoint lifetimes, allocate same space SPM-0 SPM-1 M1 M2 M3 [0,464] [455, 480] [405,450] (M2, M3) t = 464 t = 463 T1T2 WCRT = 464 cycles M1 Allocation (iteration 3)

22. Experimental evaluation Two real world applications  An unmanned aerial vehicle (UAV) controller (papabench)  A fragment of an in-orbit spacecraft software (Debie) Compare WCRT improvement with different  SPM size (default: 5% of total data size)  Bus slot length (default: 50 cycles to each core)  Remote SPM latency (default: 4 cycles) Compare improvement with bus unaware SPM allocation

23. WCRT Impr. w.r.t. SPM size MIS = SPM allocation using our framework NOBUS = Bus unaware SPM allocator Improvement over bus unaware allocator = 50%

24. WCRT impr. w.r.t. bus slot length Average improvement = 52%

25. Worst case vs average case SIM(MIS) - Average case improvement MIS - Worst case improvement

26. Allocation framework Bus-delay aware WCET analysis Task WCET Total delay (bus delay + memory latency) to access variables along WCEP WCRT analysis Variable lifetime and critical path information Bus aware SPM allocator Enough space? SPM allocation decision Yes Optimized WCRT No Application task graph

27. Allocation framework Bus-delay aware WCET analysis Task WCET Total delay (bus delay + memory latency) to access variables along WCEP WCRT analysis Variable lifetime and critical path information Bus aware SPM allocator Enough space? SPM allocation decision Yes Optimized WCRT No Application task graph Analysis of different bus arbitration policies

28. Summary We have proposed an SPM allocation framework for MPSoCs Our goal is to reduce the worst case response time (WCRT) of an application We consider variable bus delays in SPM allocation Currently, we have the model for TDMA bus only, but the SPM allocation framework can be used for different types of bus arbitration policies