1. KAIST Computer Architecture Lab. The Effect of Multi-core on HPC Applications in Virtualized Systems Jaeung Han¹, Jeongseob Ahn¹, Changdae Kim¹, Youngjin Kwon¹, Young-ri Choi², and Jaehyuk Huh¹ ¹ KAIST (Korea Advanced Institute of Science and Technology) ² KISTI (Korea Institute of Science and Technology Information)

2. Outline Virtualization for HPC Virtualization on Multi-core Virtualization for HPC on Multi-core Methodology PARSEC – shared memory model NPB – MPI model Conclusion 2

3. Outline Virtualization for HPC Virtualization on Multi-core Virtualization for HPC on Multi-core Methodology PARSEC – shared memory model NPB – MPI model Conclusion 3

4. Benefits of Virtualization 4 Hardware Virtual Machine Monitor VM Improve system utilization by consolidation

5. Benefits of Virtualization 5 Hardware Virtual Machine Monitor VM Windows VM Linux VM Solaris Improve system utilization by consolidation Support for multiple types of OSes on a system

6. Benefits of Virtualization 6 Hardware Virtual Machine Monitor VM Windows VM Linux VM Solaris Improve system utilization by consolidation Support for multiple types of OSes on a system Fault isolation

7. Benefits of Virtualization 7 Hardware Virtual Machine Monitor VM Windows VM Linux VM Solaris Hardware Virtual Machine Monitor Improve system utilization by consolidation Support for multiple types of OSes on a system Fault isolation Flexible resource management

8. Benefits of Virtualization 8 Improve system utilization by consolidation Support for multiple types of OSes on a system Fault isolation Flexible resource management Hardware Virtual Machine Monitor VM Windows VM Linux VM Solaris Hardware Virtual Machine Monitor

9. Benefits of Virtualization 9 Improve system utilization by consolidation Support for multiple types of OSes on a system Fault isolation Flexible resource management Cloud computing VM Windows VM Linux VM Solaris Cloud Hardware Virtual Machine Monitor

10. Virtualization for HPC Benefits of virtualization – Improve system utilization by consolidation – Support for multiple types of OSes on a system – Fault isolation – Flexible resource management – Cloud computing HPC is performance-sensitive Virtualization can help HPC workloads 10  resource-sensitive

11. Outline Virtualization for HPC Virtualization on Multi-core Virtualization for HPC on Multi-core Methodology PARSEC – shared memory model NPB – MPI model Conclusion 11

12. Virtualization on Multi-core 12 core More VMs on a physical machine More complex memory hierarchy (NUCA, NUMA) VM core VM core VM core VM core VM core VM Shared cache Memory core VM core VM

13. Challenges VM management cost Semantic gaps – vCPU scheduling, NUMA 13 Virtual Machine Monitor VM Scheduling, Memory, Communication, I/O multiplexing… MemMem MemMem MemMem MemMem co re Virtual Machine Monitor co re OS Memory $ $ $ $

14. Outline Virtualization for HPC Virtualization on Multi-core Virtualization for HPC on Multi-core Methodology PARSEC – shared memory model NPB – MPI model Conclusion 14

15. Virtualization for HPC on Multi-core Virtualization may help HPC Virtualization on multi-core may have some overheads For servers, improving system utilization is a key factor For HPC, performance is a key factor. 15 How much overheads are there? Where do they come from?

16. Outline Virtualization for HPC Virtualization on Multi-core Virtualization for HPC on Multi-core Methodology PARSEC – shared memory model NPB – MPI model Conclusion 16

17. Machines Single Socket System – 12-cores AMD processor – Uniform memory access latency – Two 6MB L3 caches shared by 6 cores Dual Socket System – 2x 4-core Intel processor – Non-uniform memory access latency – Two 8MB L3 caches shared by 4 cores 17 P L2 P L3 P L2 P P P P P L3 P L2 P P P Single socket: 12-core CPU Memory P L2 P P P L3 P L2 P P P L3 Dual socket: 2x 4-core CPUs

18. Workloads PARSEC – Shared memory model – Input: native – On one machine Single and Dual socket – Fix: One VM – Vary: 1, 4, 8 vCPUs NAS Parallel Benchmark – MPI model – Input: class C – On two machines (dual socket) 1Gb Ethernet switch – Fix: 16 vCPUs – Vary: 2 ~ 16 VMs 18 MemMem MemMem MemMem MemMem core Virtual Machine Monitor core OS Memory $ $ $ $ Virtual Machine Monitor VM Hardware Virtual Machine Monitor VM Hardware Semantic gapsVM management cost

19. Outline Virtualization for HPC Virtualization on Multi-core Virtualization for HPC on Multi-core Methodology PARSEC – shared memory model NPB – MPI model Conclusion 19

20. PARSEC – Single Socket Single socket No NUMA effect Very low virtualization overheads 20 2~4 % Execution times normalized to native runs

21. PARSEC – Single Socket Single socket + pin vCPU to each pCPU Reduce semantic gaps by prevent vCPU migration vCPU migration has negligible effect 21 Execution times normalized to native runs Similar to unpinned

22. PARSEC – Dual Socket Dual socket, unpinned vCPUs NUMA effect  semantic gap Significant increase of overheads 22 16~37 % Execution times normalized to native runs

23. PARSEC – Dual Socket Dual socket, pinned vCPUs May reduce NUMA effect also Reduced overheads with 1 and 4 vCPUs 23 Execution times normalized to native runs

24. XEN and NUMA machine Memory allocation policy – Allocate up to 4GB chunk on one socket Scheduling policy – Pinning to allocated socket – Nothing more Pinning 1 ~ 4 vCPUs on the socket mem. allocated is possible Impossible with 8 vCPUs 24 MemMem MemMem co re $ $ $ $ MemMem MemMem VM 0 VM 0 VM 1 VM 1 VM 2 VM 2 VM 3 VM 3

25. Mitigating NUMA Effects Range pinning – Pin vCPUs of a VM on a socket – Work only if # of vCPUs < # of cores on a socket – Range-pinned (best): memory of VM in the same socket – Range-pinned (worst): memory of VM in the other socket NUMA-first scheduler – If there is an idle core in the socket memory allocated, pick it – If not, anyway, pick a core in the machine – All vCPUs are not active all the time (sync. or I/O) 25

26. Range Pinning For 4 vCPUs case Range-pinned(best) ≈ Pinned 26 Execution times normalized to native runs

27. NUMA-first Scheduler For 8 vCPUs case Significant improvement by NUMA-first scheduler 27 Execution times normalized to native runs

28. Outline Virtualization for HPC Virtualization on Multi-core Virtualization for HPC on Multi-core Methodology PARSEC – shared memory model NPB – MPI model Conclusion 28

29. VM Granularity for MPI model Fine-grained VMs – Few processes in a VM – Small VM: vCPUs, memory – Fault isolation among processes in different VMs – Many VMs on a machine – MPI communications mostly through the VMM Coarse-grained VMs – Many processes in a VM – Large VM: vCPUs, memory – Single failure point for processes in a VM – Few VMs on a machine – MPI communications mostly within a VM 29 VMM Hardware VMM Hardware VMM Hardware VMM Hardware

30. NPB - VM Granularity Work to do are same for all granularity 2 VMs: each VM has 8 vCPUs, 8 MPI processes 16 VMs: each VM has 1 vCPU, 1 MPI processes 30 Execution times normalized to native runs 11~54 %

31. NPB - VM Granularity Fine-grained VMs  significant overheads (avg. 54%) – MPI communications mostly through VMM Worst in CG with high communication ratio – Small memory per VM – VM management costs of VMM Coarse-grained VMs  much less overheads (avg. 11%) – Still dual socket, but less overheads than shared memory model  the bottle neck is moved to communication – MPI communication largely within VM – Large memory per VM 31

32. Outline Virtualization for HPC Virtualization on Multi-core Virtualization for HPC on Multi-core Methodology PARSEC – shared memory model NPB – MPI model Conclusion 32

33. Conclusion Questions on virtualization for HPC on multi-core system – How much overheads are there? – Where do they come from? For shared memory model – Without NUMA  little overheads – With NUMA  large overheads from semantic gaps For MPI model – Less NUMA effect  communication is important – Fine-grained VMs have large overheads Communication mostly through VMM Small memory / VM management cost Future Works – NUMA-aware VMM scheduler – Optimize communication among VMs in a machine 33

34. 34 Thank you!

35. 35 Backup slides

36. PARSEC CPU Usage Environments: native linux, turn on only 8 cores (use 8 threads mode) Get CPU usage every seconds, then average them For all workloads, less than 800% (fully parallel)  NUMA-first can work 36