Evaluate using Profile-Guided Optimization (PGO) and Post-Link Optimization (PLO)

[zamazan4ik]

Hi!

Recently I checked Profile-Guided Optimization (PGO) improvements on multiple projects. The results are available here. According to the tests, PGO can help with achieving better performance in many cases similar to lol-html. I think trying to optimize lol-html with PGO can be a good idea.

I already did some benchmarks and want to share my results.

Test environment

• Fedora 39
• Linux kernel 6.5.12
• AMD Ryzen 9 5900x
• 48 Gib RAM
• SSD Samsung 980 Pro 2 Tib
• Compiler - Rustc 1.74
• lol-html version: the latest for now from the master branch on commit 44a7659d1ce018c27ae1f7e7913884bdedf72d71
• Disabled Turbo boost

Benchmark

For benchmark purposes, I use cargo bench benchmark. For PGO optimization I use cargo-pgo tool. The same benchmark suite was used for the PGO training phase built with cargo pgo bench. PGO optimized results I got with cargo pgo optimize bench.

Results

I got the following results:

• PGO optimized compared to Release: https://gist.github.com/zamazan4ik/e838bdc873d026bab9fbe21bfed99a8e
• (just for reference) PGO instrumentation compared to Release: https://gist.github.com/zamazan4ik/c94afd268a1ad39d8f8bbe3f17cf1109

As I interpret the results, PGO measurably improves lol-html performance in many cases. Since this library is used internally in Cloudflare Workers, such performance improvement can be valuable.

Further steps

I can suggest the following action points:

• Perform more PGO benchmarks on lol-html. If it shows improvements - add a note to the documentation about possible improvements in lol-html 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 lol-html according to their workloads.

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.

Here are some examples of how PGO optimization is integrated into 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