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Optimizing Process and Thread Placement on Zeus for Enhanced Performance

This presentation by Raghu Reddy at the NCEP MTT Meeting focuses on the effective management of process and thread placement on the Zeus architecture. Using a GFS T574 test case with 241 MPI ranks, it explores the implications of idle cores, memory bandwidth, and NUMA architecture. Through specific examples, it demonstrates how strategic placement can significantly enhance application performance, particularly for memory bandwidth-limited applications. Key takeaways include recommendations for configuration settings to optimize resource usage, memory access, and overall execution efficiency.

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Optimizing Process and Thread Placement on Zeus for Enhanced Performance

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  1. Controlling Process/ThreadPlacement on Zeus Raghu Reddy (Raghu.Reddy@noaa.gov) NCEP MTT Meeting

  2. Placement example • GFS, T574 case, with 241 MPI ranks • Nodes=121, PPN=2, NUMTHD=5 • 2 cores left idle • Artificial example to illustrate the problem  NCEP MTT Meeting

  3. Default or “basic” omplace qsub -l nodes=121:ppn=2 jobfile mpiexec_mpt -np 241 omplace –nt 5 ./a.out NCEP MTT Meeting

  4. One Zeus node Node Socket C C C C C C C C C C C C • Two Sockets • Each with it’s own memory (12 GB each, 24 GB Total) • QPI makes it possible for 1 sock to access the other’s memory • Non Uniform Memory Access (NUMA) • Different BW • Difference Latency • Memory BW is shared by 6 cores NCEP MTT Meeting

  5. Omplace with bsand st set NCEP MTT Meeting

  6. Controlling placement • bs = (ppn*numthds)/2 • mpiexec_mpt-np 241 omplace–c 0-:bs=$bs+st=6 –nt5 ./a.out -c CPU list BS Block Size ST Stride NCEP MTT Meeting

  7. Impact of Proper Placement? • Depends on the application • If not memory BW limited, minimal impact • DGEMM • Memory BW limited applications will see good benefit • STREAM • Most applications fall in-between NCEP MTT Meeting

  8. Impact of Placement: Kernels • HPCC Benchmark • MPI benchmark to characterize performance • “Single” (we will ignore this for today) • “Star” – Independent copy per core NCEP MTT Meeting

  9. Controlling placement: GFS • bs = (ppn*numthds)/2 • mpiexec_mpt-np 241 omplace–c 0-:bs=$bs+st=6 –nt5 ./a.out • fe2% dd 20:13:36 20:17:39 nt-5-ppn-2-noomplace • 243 nt-5-ppn-2-noomplace • fe2% dd 16:26:01 16:28:47 nt-5-ppn-2 • 166 nt-5-ppn-2 • fe2% dd 16:30:32 16:33:00 nt-5-ppn-2-bs-5 • 148 nt-5-ppn-2-bs-5 • fe2% NCEP MTT Meeting

  10. A More Practical example • Your MPI (non-threaded application) needs more memory than what is available per core • So you have to use ppn=6 instead of ppn=12 • If you run it without omplace, all ranks would be put on 1 socket • 6 ranks on one socket, 0 ranks on the second! • Use bs = 3 (ppn*numthd/2) • This will put 3 ranks on each socket • Improves memory bandwidth NCEP MTT Meeting

  11. Test run of GFS (no OpenMP) • GFS, T574 case, with 241 MPI ranks • Nodes=41, PPN=6, NUMTHD=1 noomp runs ------------ fe2% dd 13:10:50 13:16:35 nt-1-ppn-6 345 nt-1-ppn-6 fe2% dd 13:26:55 13:32:13 nt-1-ppn-6-nt-2 318 nt-1-ppn-6-nt-2 fe2% dd 13:57:15 14:02:27 nt-1-ppn-6-bs-3 312 nt-1-ppn-6-bs-3 fe2% NCEP MTT Meeting

  12. GFS:Nodes=41, PPN=6, THDS=2 fe2% dd 14:51:07 14:54:52 nt-2-ppn-6 (no TAU) 225 nt-2-ppn-6 fe2% dd 14:27:57 14:32:17 tau-3files-nt-2-ppn-6 260 tau-3files-nt-2-ppn-6 fe2% NCEP MTT Meeting

  13. Summary • Under certain circumstances using proper placement can be beneficial • In general, if you’re using all the available cores this may not be important. • This may be significant if you are leaving some cores idle where it may be beneficial. • Especially so, if you idle cores and use “remote” memory NCEP MTT Meeting

  14. Questions? • Thanks! NCEP MTT Meeting

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