Evaluate Profile-Guided Optimization (PGO) and LLVM BOLT

[zamazan4ik]

Hi!

Recently I checked Profile-Guided Optimization (PGO) improvements on multiple projects. The results are here. E.g. PGO helps with optimizing Envoyproxy. PGO results for other proxies like HAProxy, Nginx, httpd you can be found in the repo above. According to the multiple tests, PGO can help with improving performance in many other cases. That's why I think trying to optimize g3 with PGO can be a good idea.

I can suggest the following action points:

• Perform PGO benchmarks on g3. And if it shows improvements - add a note about possible improvements in g3 performance with PGO.
• Providing an easier way (e.g. a build option) to build scripts with PGO can be helpful for the end-users and maintainers since they will be able to optimize g3 according to their own workloads.
• Optimize pre-built binaries

Maybe testing Post-Link Optimization techniques (like LLVM BOLT) would be interesting too (Clang and Rustc already use BOLT as an addition to PGO) but I recommend starting from the usual PGO.

For the Rust projects, I recommend starting experimenting with PGO with cargo-pgo.

Here are some examples of how PGO optimization is integrated in other projects:

• Rustc: a CI script for the multi-stage build
• GCC:
  • Official docs, section "Building with profile feedback" (even AutoFDO build is supported)
  • A part in a "wonderful" configure script
• Clang: Docs
• Python:
  • CPython: README
  • Pyston: README
• Go: Bash script
• V8: Bazel flag
• ChakraCore: Scripts
• Chromium: Script
• Firefox: Docs
  • Thunderbird has PGO support too
• PHP - Makefile command and old Centminmod scripts
• MySQL: CMake script
• YugabyteDB: GitHub commit
• FoundationDB: Script
• Zstd: Makefile
• Foot: Scripts
• Windows Terminal: GitHub PR
• Pydantic-core: GitHub PR
• file.d: GitHub PR
• OceanBase: CMake flag

[zh-jq-b]

Thank you for bring this up.

PGO will be on our roadmap next year (2024).

There are some steps to do:

1. Add testcases for profile purpose, may be based on the ones in scripts/coverage dir.
2. Enable PGO for rust code, and do a benchmark test.
3. Enable PGO when compile the vendored c source code such openssl or c-ares, then do a benchmark test.
4. Add guides for users.