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36 lines
1.5 KiB
Markdown
36 lines
1.5 KiB
Markdown
<a name="perf-counters" />
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# User-Requested Performance Counters
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When running benchmarks, the user may choose to request collection of
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performance counters. This may be useful in investigation scenarios - narrowing
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down the cause of a regression; or verifying that the underlying cause of a
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performance improvement matches expectations.
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This feature is available if:
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* The benchmark is run on an architecture featuring a Performance Monitoring
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Unit (PMU),
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* The benchmark is compiled with support for collecting counters. Currently,
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this requires [libpfm](http://perfmon2.sourceforge.net/), which is built as a
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dependency via Bazel.
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The feature does not require modifying benchmark code. Counter collection is
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handled at the boundaries where timer collection is also handled.
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To opt-in:
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* If using a Bazel build, add `--define pfm=1` to your build flags
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* If using CMake:
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* Install `libpfm4-dev`, e.g. `apt-get install libpfm4-dev`.
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* Enable the CMake flag `BENCHMARK_ENABLE_LIBPFM` in `CMakeLists.txt`.
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To use, pass a comma-separated list of counter names through the
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`--benchmark_perf_counters` flag. The names are decoded through libpfm - meaning,
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they are platform specific, but some (e.g. `CYCLES` or `INSTRUCTIONS`) are
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mapped by libpfm to platform-specifics - see libpfm
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[documentation](http://perfmon2.sourceforge.net/docs.html) for more details.
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The counter values are reported back through the [User Counters](../README.md#custom-counters)
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mechanism, meaning, they are available in all the formats (e.g. JSON) supported
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by User Counters.
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