Redesigned "caar loop pre-boundary exchange", tuned for Frontier, also faster on Perlmutter GPU #6972
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Trey, for clarity, would you mind explaining how you take a line from a timer file and derive the entry in the table? As an example, from this line we see that the maximum is 43.766 sec and the average is 1.360819e+04/320 = 42.52559375. The quantity being measured is "time spent in DIRK over the course of the run", where each rank makes 4.608000e+06/320 = 14400.0 calls to DIRK over the course of the run. |
The times listed above are from the fourth column of numbers in the timer output, Frontier GPU original: Frontier GPU new: Perlmutter GPU original: Perlmutter GPU new: Perlmutter CPU Gnu original: Perlmutter CPU Gnu new: Perlmutter CPU Intel original: Perlmutter CPU Intel new: |
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Thanks, Trey. That clears things up. Note that in your table you wrote 1.244864e+04 as 1245 rather than 12449, and similarly for the other CPU numbers. |
Oops! Fixed. |
This pull request attempts to provide Frontier optimizations even better than those used in the 2023 Gordon Bell Climate runs, but with a software architecture that meets the requirements of https://acme-climate.atlassian.net/wiki/x/oICd6, and with additional changes to reduce slowdown on Perlmutter CPUs.
It replaces pull request #6522.
Summary of changes:
structs inSphereOperators.hppto use for the new Caar pre-boundary exchange.SphereOperators.hppthat allow code that uses implicit parallelism and vector registers on GPUs to add explicit loops and temporary arrays on CPUs.#if (WARP_SIZE == 1)preprocessor directives inSphereOperators.hppto try to minimize CPU-specific code.zbelowinSphereOperators.hppto supportScalartypes withVECTOR_SIZE> 1 on CPUs.CaarFunctorImpl.cppthat implements the newcaar_computefunction and template functions with Kokkos loops. The single source code supports both GPUs and CPUs by relying on functions and macros defined inSphereOperators.hpp.CaarFunctorImpl.hppwith new functions, slight changes to temporary buffers, and#ifto turn on/off the newcaar_compute. If we adopt the new implementation permanently, significant code can be eliminated from this file.viewAsRealfunctions inViewUtils.hpp.LaunchBounds<512,1>calls, which I think are incorrect for AMD GPUs, where the Kokkos teams sometimes use 1024 threads.frontier-bfb.cmake,frontier-bfb-serial.cmake, andpm-cpu-bfb.cmakefiles for bit-for-bit unit testing of Caar.I confirmed that the modified code passes the
caar_utunit test, and I ran a single-node NE30 test from Noel Keen on Frontier, Perlmutter GPU, and Perlmutter CPU. Here is a comparison of total "caar compute" times, summed over all MPI tasks on the node (8 on Frontier GPU, 4 on Perlmutter GPU, 128 on Perlmutter CPU).#if 0#if 1The good news is that the new code is faster on both Frontier and Perlmutter GPUs. The bad news is that it slows down Perlmutter CPUs. In particular, it appears to inhibit whatever optimization the Intel compiler is able to do over the Gnu compiler.