harfbuzz/perf

Building and Running

Benchmarks are implemented using Google Benchmark.

To build the benchmarks in this directory you need to set the benchmark option while configuring the build with meson:

meson build -Dbenchmark=enabled --buildtype=debugoptimized

The default build type is debugoptimized, which is good enough for benchmarking, but you can also get the fastest mode with release build type:

meson build -Dbenchmark=enabled --buildtype=release

You should, of course, enable features you want to benchmark, like -Dfreetype, -Dfontations, -Dcoretext, etc.

Then build a specific benchmark binaries with ninja, eg.:

ninja -Cbuild perf/benchmark-set

or just build the whole project:

ninja -Cbuild

Finally, to run one of the benchmarks:

./build/perf/benchmark-set

It's possible to filter the benchmarks being run and customize the output via flags to the benchmark binary. See the Google Benchmark User Guide for more details.

The most useful benchmark is benchmark-font. You can provide custom fonts to it too. For example, to run only the "paint" benchmarks, against a given font, you can do:

./build/perf/benchmark-font NotoColorEmoji-Regular.ttf --benchmark_filter="paint"

Some useful options are: --benchmark_repetitions=5 to run the benchmark 5 times, --benchmark_min_time=.1s to run the benchmark for at least .1 seconds (defaults to .5s), and --benchmark_filter=... to filter the benchmarks by regular expression.

To compare before/after benchmarks, you need to save the benchmark results in files for both runs. Use --benchmark_out=results.json to output the results in JSON format. Then you can use:

./subprojects/benchmark-1.8.4/tools/compare.py benchmarks before.json after.json

Substitute your version of benchmark instead of 1.8.4.

Profiling

If you like to disable optimizations and enable frame pointers for better profiling output, you can do so with the following meson command:

CXXFLAGS="-fno-omit-frame-pointer" meson --reconfigure build -Dbenchmark=enabled --buildtype=debug
ninja -Cbuild

However, this will slow down the benchmarks significantly and might give you inaccurate information as to where to optimize. It's better to profile the debugoptimized build (the default).

Then run the benchmark with perf:

perf record -g build/perf/benchmark-subset --benchmark_filter="BM_subset_codepoints/subset_notocjk/100000" --benchmark_repetitions=5

You probably want to filter to a specific benchmark of interest and set the number of repititions high enough to get a good sampling of profile data.

Finally view the profile with:

perf report

Another useful perf tool is the perf stat command, which can give you a quick overview of the performance of a benchmark, as well as stalled cycles, cache misses, and mispredicted branches.