message: bench: add usage description and documentation
  This adds some usage description with tips to `bench_bitcoin -h`.
diff: #include <bench/bench.h>
  +#include <clientversion.h>
  #include <crypto/sha256.h>
  #include <util/strencodings.h>
  #include <util/system.h>
  @@ -50,7 +51,51 @@ int main(int argc, char** argv)
  }
  if (HelpRequested(argsman)) {
  - std::cout << argsman.GetHelpMessage();
  + std::cout << "Usage: bench_bitcoin [options]\n"
  + "\n"
  + << argsman.GetHelpMessage()
  + << "Description:\n"
  + "\n"
  + " bench_bitcoin executes microbenchmarks. The quality of the benchmark results\n"
  + " highly depend on the stability of the machine. It can sometimes be difficult\n"
  + " to get stable, repeatable results, so here are a few tips:\n"
  + "\n"
  + " * Use pyperf [1] to disable frequency scaling, turbo boost etc. For best\n"
  + " results, use CPU pinning and CPU isolation (see [2]).\n"
  + "\n"
  + " * Each call of run() should do exactly the same work. E.g. inserting into\n"
  + " a std::vector doesn't do that as it will reallocate on certain calls. Make\n"
  + " sure each run has exactly the same preconditions.\n"
  + "\n"
  + " * If results are still not reliable, increase runtime with e.g.\n"
  + " -min_time=5000 to let a benchmark run for at least 5 seconds.\n"
  + "\n"
  + " * bench_bitcoin uses nanobench [3] for which there is extensive\n"
  + " documentation available online.\n"
  + "\n"
  + "Environment Variables:\n"
  + "\n"
  + " To attach a profiler you can run a benchmark in endless mode. This can be\n"
  + " done with the environment variable NANOBENCH_ENDLESS. E.g. like so:\n"
  + "\n"
  + " NANOBENCH_ENDLESS=MuHash ./bench_bitcoin -filter=MuHash\n"
  + "\n"
  + " In rare cases it can be useful to suppress stability warnings. This can be\n"
  + " done with the environment variable NANOBENCH_SUPPRESS_WARNINGS, e.g:\n"
  + "\n"
  + " NANOBENCH_SUPPRESS_WARNINGS=1 ./bench_bitcoin\n"
  + "\n"
  + "Notes:\n"
  + "\n"
  + " 1. pyperf\n"
  + " https://github.com/psf/pyperf\n"
  + "\n"
  + " 2. CPU pinning & isolation\n"
  + " https://pyperf.readthedocs.io/en/latest/system.html\n"
  + "\n"
  + " 3. nanobench\n"
  + " https://github.com/martinus/nanobench\n"
  + "\n";
  return EXIT_SUCCESS;
  }
  