# 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](http://perfmon2.sourceforge.net/), which is built as a dependency via Bazel. The feature does not require modifying benchmark code. Counter collection is handled at the boundaries where timer collection is also handled. To opt-in: * If using a Bazel build, add `--define pfm=1` to your build flags * If using CMake: * Install `libpfm4-dev`, e.g. `apt-get install libpfm4-dev`. * Enable the CMake flag `BENCHMARK_ENABLE_LIBPFM` in `CMakeLists.txt`. 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](http://perfmon2.sourceforge.net/docs.html) for more details. The counter values are reported back through the [User Counters](../README.md#custom-counters) mechanism, meaning, they are available in all the formats (e.g. JSON) supported by User Counters.