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User-Requested Performance Counters
When running benchmarks, the user may choose to request collection of performance counters. This may be useful in investigation scenarios - narrowing down the cause of a regression; or verifying that the underlying cause of a performance improvement matches expectations.
This feature is available if:
- The benchmark is run on an architecture featuring a Performance Monitoring Unit (PMU),
- The benchmark is compiled with support for collecting counters. Currently, this requires libpfm be available at build time, and
- Currently, there is a limitation that the benchmark be run on one thread.
The feature does not require modifying benchmark code. Counter collection is handled at the boundaries where timer collection is also handled.
To opt-in:
- Install
libpfm4-dev, e.g.apt-get install libpfm4-dev. - Enable the cmake flag BENCHMARK_ENABLE_LIBPFM.
To use, pass a comma-separated list of counter names through the
--benchmark_perf_counters flag. The names are decoded through libpfm - meaning,
they are platform specific, but some (e.g. CYCLES or INSTRUCTIONS) are
mapped by libpfm to platform-specifics - see libpfm
documentation for more details.
The counter values are reported back through the User Counters mechanism, meaning, they are available in all the formats (e.g. JSON) supported by User Counters.