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Typical performance bottlenecks and how they can be found

Typical performance bottlenecks and how they can be found. Bert Wesarg ZIH , Technische Universität Dresden. Outline. Case I: Finding load imbalances in OpenMP codes Case II: Finding communication and computation imbalances in MPI codes. Case I: Sparse Matrix Vector Multiplication.

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Typical performance bottlenecks and how they can be found

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  1. Typical performance bottlenecks and how they can be found Bert Wesarg ZIH, TechnischeUniversitätDresden

  2. Outline • Case I: • Finding load imbalances in OpenMP codes • Case II: • Finding communication and computation imbalances in MPI codes

  3. Case I: Sparse Matrix Vector Multiplication • Matrix has significant more zero elements => sparse matrix • Only non-zero elements of are saved efficiently in memory • Algorithm: foreach row r in A y[r.x] = 0 foreach non-zero element e in row y[r.x] += e.value * x[e.y]

  4. Case I: Sparse Matrix Vector Multiplication • Naïve OpenMP Algorithm: • Distributes the rows of A evenly across the threads in the parallel region • The distribution of the non-zero elements may influence the load balance in the parallel application #pragma omp parallel for foreach row r in A y[r.x] = 0 foreach non-zero element e in row y[r.x] += e.value * x[e.y]

  5. Case I: Load imbalances in OpenMP codes • Measuring the static OpenMP application % cd ~/Bottlenecks/smxv % make PREP=scorep scorepgcc -fopenmp -DLITTLE_ENDIAN \ -DFUNCTION_INC='"y_Ax-omp.inc.c"' -DFUNCTION=y_Ax_omp \ -o smxv-ompsmxv.c -lm scorepgcc -fopenmp -DLITTLE_ENDIAN \ -DFUNCTION_INC='"y_Ax-omp-dynamic.inc.c"‘ \ -DFUNCTION=y_Ax_omp_dynamic -o smxv-omp-dynamic smxv.c -lm % OMP_NUM_THREADS=8 scan –t ./smxv-ompyax_large.bin

  6. Case I: Load imbalances in OpenMP codes: Profile • Two metrics which indicate load imbalances: • Time spent in OpenMP barriers • Computational imbalance • Open prepared measurement on the LiveDVD with Cube % cube ~/Bottlenecks/smxv/scorep_smxv-omp_large/trace.cubex [CUBE GUI showing trace analysis report]

  7. Case I: Time spent in OpenMP barriers These threads spent up to 20% of there running time in the barrier

  8. Case I: Computational imbalance Master thread does 66% of the work

  9. Case I: Sparse Matrix Vector Multiplication • Improved OpenMP Algorithm • Distributes the rows of A dynamically across the threads in the parallel region #pragma omp parallel for schedule(dynamic,1000) foreach row r in A y[r.x] = 0 foreach non-zero element e in row y[r.x] += e.value * x[e.y]

  10. Case I: Profile Analysis • Two metrics which indicate load imbalances: • Time spent in OpenMP barriers • Computational imbalance • Open prepared measurement on the LiveDVD with Cube: % cube ~/Bottlenecks/smxv/scorep_smxv-omp-dynamic_large/trace.cubex [CUBE GUI showing trace analysis report]

  11. Case I: Time spent in OpenMP barriers All threads spent similar time in the barrier

  12. Case I: Computational imbalance Threads do nearly equal work

  13. Case I: Trace Comparison • Open prepared measurement on the LiveDVD with Vampir: % vampir ~/Bottlenecks/smxv/scorep_smxv-omp_large/traces.otf2 [VampirGUI showing trace]

  14. Case I: Time spent in OpenMP barriers Improved runtime Less time in OpenMP barrier

  15. Case I: Computational imbalance Great imbalance for time spent in computational code Great imbalance for time spent in computational code

  16. Outline • Case I: • Finding load imbalances in OpenMP codes • Case II: • Finding communication and computation imbalances in MPI codes

  17. Case II: Heat Conduction Simulation • Calculating the heat conduction at each timestep • Discretized formula for space and time

  18. Case II: Heat Conduction Simulation • Application uses MPI for boundary exchange and OpenMP for computation • Simulation grid is distributed across MPI ranks

  19. Case II: Heat Conduction Simulation • Ranks need to exchange boundaries before next iteration step

  20. Case II: Profile Analysis • MPI Algorithm • Building and measuring the heat conduction application: • Open prepared measurement on the LiveDVD with Cube foreach step in [1:nsteps] exchangeBoundaries computeHeatConduction % cd ~/Bottlenecks/heat % make PREP=‘scorep --user’ [... make output ...] % scan –t mpirun –np 16 ./heat-MPI 3072 32 % cube ~/Bottlenecks/heat/scorep_heat-MPI_16/trace.cubex [CUBE GUI showing trace analysis report]

  21. Case II: Hide MPI communication with computation • Step 1: Compute heat in the area which is communicated to your neighbors

  22. Case II: Hide MPI communication with computation • Step 2: Start communicating boundaries with your neighbors

  23. Case II: Hide MPI communication with computation • Step 3: Compute heat in the interior area

  24. Case II: Profile Analysis • Improved MPI Algorithm • Measuring the improved heat conduction application: • Open prepared measurement on the LiveDVD with Cube foreach step in [1:nsteps] computeHeatConductionInBoundaries startBoundaryExchange computeHeatConductionInInterior waitForCompletionOfBoundaryExchange % scan –t mpirun –np 16 ./heat-MPI-overlap 3072 32 % cube ~/Bottlenecks/heat/scorep_heat-MPI-overlap_16/trace.cubex [CUBE GUI showing trace analysis report]

  25. Case II: Trace Comparison • Open prepared measurement on the LiveDVD with Vampir: % vampir ~/Bottlenecks/heat/scorep_heat-MPI_16/traces.otf2 [VampirGUI showing trace]

  26. Acknowledgments • Thanks to Dirk Schmidl, RWTH Aachen, for providing the sparse matrix vector multiplication code

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