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simple-MKL-DGEMM-test

This project contains a simple benchmark of the single-node DGEMM kernel from Intel's MKL library.

The Makefile is configured to produce four different executables from the single source file. The executables differ only in the method used to allocate the three arrays used in the DGEMM call.

  • dgemm_test_default.exe uses malloc() to allocate the arrays.
  • dgemm_test_MMAP_1GB.exeuses mmap() to allocate the arrays, using the default page size.
  • dgemm_test_MMAP_2MB.exeuses mmap() to allocate the arrays, using 2MiB hugepages.
  • dgemm_test_MMAP.exeuses mmap() to allocate the arrays, using 1GiB hugepages.

The first two should use Transparent Huge Pages if the system is configured to use them by default. The MMAP_2MB version requires that the hugepages be pre-allocated. This is typically done with a command of the form echo 4096 >/proc/sys/vm/nr_hugepages The MMAP_1GB version requires that 1 GiB pages be pre-allocated. This typically requires a reboot, with the kernel command line containing the requested hugepage size and number of hugepages using options like hugepagesz=1G hugepages=32

The script run_ensemble.sh provides a simple way to run any of the binaries repeatedly, as part of a search for performance variability. The output files are a bit verbose, but the most important values can be extracted with a simple command like grep ^MIN log.default.* for the output using the default version of the code. This gives the minimum, average, and maximum GFLOPS for the run (excluding the first iteration).
Interesting things to look for include

  • Runs with low averages, but good maximum GFLOPS.
    • These have probably run into interference with other processes.
  • Runs with low averages, but with small spread across min, avg, max GFLOPS.
    • These may indicate a systematic performance degradation related to the set of physical addresses used in the run.

As an example, the command grep ^MIN ExampleResults/log.default.* shows the results from 20 runs. Sorting by the average value (column 6) shows log.default.003 with low min, avg, and max values (suggesting a systematic slowdown), while log.default.017 has a min GFLOPS value that is much lower than the (low) avg and max values. This suggests both a systematic performance issue AND an interference problem in the slowest iteration (iteration number 3).