Update benchmarking docs. (#375)

Author: g-easy

opencensus/doc/benchmarks.md

@@ -16,20 +16,44 @@ library. This accepts several helpful flags, including
    benchmark_repetitions, report only summary statistics and not single-run
    timings.
 
+Don't forget to use `-c opt` to get an optimized build.
+
 ## Profiling
 
 Benchmarks can be profiled using the
-[gperftools](https://github.com/gperftools/gperftools) library. On
-Debian/Ubuntu, install the `google-perftools` (profile analysis tools) and
-`libgoogle-perftools-dev` (profiling library) packages. When running the
-benchmark, set `LD_PRELOAD=/usr/lib/libprofiler.so` to enable the profiler and
-`CPUPROFILE=PATH` to save a profile, and analyze the profile with
-`google-pprof` (which needs a path to the binary for symbolization). For
-example,
+[gperftools](https://github.com/gperftools/gperftools) library. To install on
+Debian / Ubuntu:
+
+```shell
+sudo apt install google-perftools libgoogle-perftools-dev
+```
+
+When running the benchmark, set:
+ - `LD_PRELOAD=/usr/lib/x86_64-linux-gnu/libprofiler.so.0` to enable the
+   profiler.
+ - `CPUPROFILE=path_to_output_file`
+ - Optional: `CPUPROFILE_FREQUENCY=4000` to increase resolution.
+ - Optional: for visibility into inlined functions, build with `-g1` or above.
+
+Analyze the profile with `google-pprof`, which needs a path to the binary for
+symbolization.
+
+Example:
+
 ```shell
-bazel build -c opt opencensus/stats:stats_manager_benchmark
-LD_PRELOAD=/usr/lib/libprofiler.so CPUPROFILE=/tmp/prof \
-    ./bazel-bin/opencensus/stats/stats_manager_benchmark
+bazel build -c opt --copt=-g1 opencensus/stats:stats_manager_benchmark
+LD_PRELOAD=/usr/lib/x86_64-linux-gnu/libprofiler.so.0 \
+  CPUPROFILE=/tmp/prof \
+  CPUPROFILE_FREQUENCY=4000 \
+  ./bazel-bin/opencensus/stats/stats_manager_benchmark
 google-pprof --web bazel-bin/opencensus/stats/stats_manager_benchmark /tmp/prof
 ```
-pprof supports many analysis types--see its documentation for options.
+
+`google-pprof` supports many analysis types - see its
+[documentation](https://gperftools.github.io/gperftools/cpuprofile.html) for
+options.
+
+Note that `CPUPROFILE_FREQUENCY` defaults to 100 samples per second.
+The maximum value `libprofiler` will accept is 4000.
+Most Linux kernels are built with `CONFIG_HZ=250`, so a single-threaded
+benchmark will produce at most 250 samples per second.