1 files changed, 35 insertions, 0 deletions
diff --git a/docs/perf_counters.md b/docs/perf_counters.md
new file mode 100644
index 0000000..f342092
--- /dev/null
+++ b/docs/perf_counters.md
@@ -0,0 +1,35 @@
+<a name="perf-counters" />
+
+# 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.