NATIONAL INSTITUTE OF TECHNOLOGY KARNATAKA,SURATHKAL

1. NATIONAL INSTITUTE OF TECHNOLOGY KARNATAKA,SURATHKAL DEPARTMENT OF COMPUTER SCIENCE & ENGGINERING Presentation on ZSIM: FAST AND ACCURATE MICROARCHITECTURAL SIMULATION OF THOUSAND-CORE SYSTEMS Publisher’s: DANIEL SANCHEZ CHRISTOS KOZYRAKIS Presented By: Vaibhav Ashtikar(13IS24F) Govind Dhonddev(13IS06F)

2. ZSIM: FAST AND ACCURATE MICROARCHITECTURAL SIMULATION OF THOUSAND-CORE SYSTEMS Architectural Simulator Piece of software: Modelling computer system/components Input Predicts o/p & performance Evaluating different hardware designs without building costly physical hardware systems. Enabling the opportunities to access non-existing computer components or systems. Obtaining detailed performance metrics: A single execution of simulators can often generate a large set of performance data. Debugging: Debugging on real hardware typically require re-booting and re-running the code to reproduce the problems. In contrast, some simulators have a fully controlled environment and allow software developers to run code backward once an error is detected.

3. ZSIM: FAST AND ACCURATE MICROARCHITECTURAL SIMULATION OF THOUSAND-CORE SYSTEMS IDEAL Architectural Simulator Piece of software: Modelling computer system/components Input Predicts o/p & performance ACCURATE Execute wide range of WORKLOADS Easy to use, easy to modify FAST

4. ZSIM: FAST AND ACCURATE MICROARCHITECTURAL SIMULATION OF THOUSAND-CORE SYSTEMS

5. ZSIM: FAST AND ACCURATE MICROARCHITECTURAL SIMULATION OF THOUSAND-CORE SYSTEMS Problem: Architectural simulation is Time Consuming Current detailed simulators are slow (~200 KIPS) Problem: Time to simulate 1000 cores @ 2GHz for 1second at 200 MIPS: 3 hours 200 KIPS: 4 months • Simulation performance wall • More complex targets (multicore, memory hierarchy, …) •  Hard to parallelize

6. ZSIM: FAST AND ACCURATE MICROARCHITECTURAL SIMULATION OF THOUSAND-CORE SYSTEMS Architectural Simulator Sequential Simulation Parallel simulation More cores to be simulated ; More slower sequential simulation. Scaling poorly due to excessive synchronization. Sacrifice accuracy by allowing event reordering tradeoff

7. ZSIM: FAST AND ACCURATE MICROARCHITECTURAL SIMULATION OF THOUSAND-CORE SYSTEMS Tradeoff between speed and accuracy:

8. ZSIM: FAST AND ACCURATE MICROARCHITECTURAL SIMULATION OF THOUSAND-CORE SYSTEMS ZSIM

9. ZSIM: FAST AND ACCURATE MICROARCHITECTURAL SIMULATION OF THOUSAND-CORE SYSTEMS ZSIM FAST With instruction driven timing models that uses DBT(Dynamic Binary Translation)

10. ZSIM: FAST AND ACCURATE MICROARCHITECTURAL SIMULATION OF THOUSAND-CORE SYSTEMS ZSIM ACCURACY 2 phase parallelization technology that scales parallel simulation on multicore..

11. ZSIM: FAST AND ACCURATE MICROARCHITECTURAL SIMULATION OF THOUSAND-CORE SYSTEMS ZSIM Wide range of workloads To support complex workloads . E.g. multiprogramming client server based applications etc. To bridge user-level/full system gap

12. ZSIM: FAST AND ACCURATE MICROARCHITECTURAL SIMULATION OF THOUSAND-CORE SYSTEMS

13. ZSIM: FAST AND ACCURATE MICROARCHITECTURAL SIMULATION OF THOUSAND-CORE SYSTEMS Dynamic Binary Translation Translated Code • Simulate basic block using host instructions • Binary Code Load t1, simRegs[1] Load t2, 16(t1) Store t2, simRegs[3] Load t1, simRegs[2] Load t2, simRegs[3] Add t3, t1, t2 Store t3, simregs[4] Store 0x48074, simPc J dispatch_loop Load r3, 16(r1) Add r4, r3, r2 Jump 0x48074

14. ZSIM: FAST AND ACCURATE MICROARCHITECTURAL SIMULATION OF THOUSAND-CORE SYSTEMS Dynamic Binary Translation • Modeling thousand-core simulator with parallelization alone is not sufficient. • Instrumentation based approach to eliminate need for functional modeling of X86. Timing Based Model Simple Core Model OOO Core Model

15. ZSIM: FAST AND ACCURATE MICROARCHITECTURAL SIMULATION OF THOUSAND-CORE SYSTEMS Simple Core Model (SCM) • Instrument Load and Store instructions. • SCM counts cycles, instructions, derives memory hierarchy. • Simulated up to 90 MIPS per simulated Core • Pitfalls: • Doesn’t represent ooo model used in desktops, server and processor chips.

16. ZSIM: FAST AND ACCURATE MICROARCHITECTURAL SIMULATION OF THOUSAND-CORE SYSTEMS OOO Core Model • Models • Branch prediction • Instruction length • Pre-decoder instruction decoding • Issue stalls • Register renaming • Conventional Simulators with OOO model execute around 100 KIPS. • ZSIM accelerates OOO core by pushing most of the work at instrumentation phase

17. ZSIM: FAST AND ACCURATE MICROARCHITECTURAL SIMULATION OF THOUSAND-CORE SYSTEMS OOO core modeling • Basic BlockInstrumented basic block +Basic Block Descriptor • Instruction Driven Approach: Simulate all stages at once for each instruction / μ-operation. • Schedule of given μ operation must not depend on future μ operation. • Execution time of every μ operation must be known in advance. BasicBlock(DecodedBBL) Load(addr = -0x38(%rbp)) mov -0x38(%rbp),%rcx lea -0x2040(%rbp),%rdx add %rax,%rdx mov %rdx,-0x2068(%rbp) Store(addr = -0x2068(%rbp)) cmp $0x1fff,%rax jne10840530a mov (%rbp),%rcx add %rax,%rbx mov %rdx,(%rbp) ja 40530a Ins →μop decoding μop dependencies, functional units, latency Front-end delays

18. ZSIM: FAST AND ACCURATE MICROARCHITECTURAL SIMULATION OF THOUSAND-CORE SYSTEMS Parallelism and Interference • Path-altering Interference • Two accesses if simulated in out of order changes their paths through memory hierarchy. • Root cause: • Two accesses address to same line (except both reads) • Second access if executed out of order, causes first access as miss.

19. ZSIM: FAST AND ACCURATE MICROARCHITECTURAL SIMULATION OF THOUSAND-CORE SYSTEMS Parallelism and Interference • Path-Preserving Interference • Two accesses if simulated out of order changes their timing but path to memory hierarchy remains unaffected.. • Ex. 2 accesses to different Cache sets in same bank. • In small intervals(1-10K cycles) path altering interference is very rare(<1 in 10K accesses)

20. ZSIM: FAST AND ACCURATE MICROARCHITECTURAL SIMULATION OF THOUSAND-CORE SYSTEMS Bound-weave Algorithm • Need accuracy on path-altering interference Bound-weave Algorithm 1. Bound Phase 2. Weave Phase In this interval, each core is simulated for specific small interval Zero load latency during the interval. In this interval, each core is simulated for specific small interval Parallel simulation of core for prior knowledge of events to scale efficiently.

21. ZSIM: FAST AND ACCURATE MICROARCHITECTURAL SIMULATION OF THOUSAND-CORE SYSTEMS Bound Phase • Limit the skew between simulated Cores • Thread execution in parallel and sync for interval barrier. • Moderate parallelism • Allow as many threads as host hardware threads run concurrently. • Ex. 1024-core simulation on host with 32 hardware threads, barrier only wakes up 32 threads at each time interval. • Avoiding systematic bias • At end of time interval, barrier shuffles thread wake up order to avoid consistently prioritizing a few threads.

22. ZSIM: FAST AND ACCURATE MICROARCHITECTURAL SIMULATION OF THOUSAND-CORE SYSTEMS Bound-Weave Example • 2-core host simulating 4-core system • 1000-cycles intervals • Dividing components among 2 domains

23. ZSIM: FAST AND ACCURATE MICROARCHITECTURAL SIMULATION OF THOUSAND-CORE SYSTEMS Complex Workloads • Multi process simulation • Scheduler – simple round robin scheduler • Avoiding Simulator-OS Deadlock • Time Virtualization – virtualize rdtsc counter. • System Virtualization – pregenerated virtual instruction. • Fast forwarding – DBT to perform pre-processing fast close to native speed. • Challenge: Accounting for OS execution time.

24. ZSIM: FAST AND ACCURATE MICROARCHITECTURAL SIMULATION OF THOUSAND-CORE SYSTEMS Accuracy • 18 out of 29 benchmarks, zsim is within 10% of real system (average performance error around 9.7 ) • Absolute performance error Simulated Real

25. ZSIM: FAST AND ACCURATE MICROARCHITECTURAL SIMULATION OF THOUSAND-CORE SYSTEMS Accuracy- Cache Level • MPKI Error • Error rate increases along with memory hierarchy. • Currently TLB misses not modelled.

26. ZSIM: FAST AND ACCURATE MICROARCHITECTURAL SIMULATION OF THOUSAND-CORE SYSTEMS Thousand Core performance • Single system in sequence simulation: • 1.32 trillion instructions takes 1.8 hours(IPC1-NC) to 8.9 hours(OOO-C).

27. ZSIM: FAST AND ACCURATE MICROARCHITECTURAL SIMULATION OF THOUSAND-CORE SYSTEMS Speed Up • Performance simulation of single threaded ZSIM on SPEC2006 using 4 models: IPC1 or OOO cores with and without contention

28. ZSIM: FAST AND ACCURATE MICROARCHITECTURAL SIMULATION OF THOUSAND-CORE SYSTEMS Speed Up • Average ZSIM speedup on workloads as we increase host threads from 1 to 32(16 cores with 2 hardware threads/cores)

29. ZSIM: FAST AND ACCURATE MICROARCHITECTURAL SIMULATION OF THOUSAND-CORE SYSTEMS Comparison With other simulators • Parallel simulators reports 1-10 MIPS. • Many constraints – host, workloads, memory intensive application, leads to potential difference. • ZSIM is 2-3 orders of magnitude faster than other simulators.

30. ZSIM: FAST AND ACCURATE MICROARCHITECTURAL SIMULATION OF THOUSAND-CORE SYSTEMS Conclusion • New techniques to achieve speed and accuracy. • DBT based Timing Model • Bound-Weave Parallelization • Lightweight virtualization of user process • Leads to speedup of 1-1500 MIPS on thousand core simulation.

31. ZSIM: FAST AND ACCURATE MICROARCHITECTURAL SIMULATION OF THOUSAND-CORE SYSTEMS References • [1] “ZSIM: Fast and Accurate Micro architectural Simulation of Thousand-Core Systems ” Daniel Sanchez, Christos Kozyrakis, ISCA 2013