[codex] parallelize release code generation - #27702
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The release profile still serializes code generation within each crate. That setting was introduced with fat LTO to improve optimization and binary size. Releases now use ThinLTO, and code generation dominates the critical-path build. Use four codegen units. On an Apple M4 Max with 16 cores and 128 GiB RAM, using rustc 1.96.0, four and eight units took 507.486 and 505.325 seconds respectively. Four keeps the build-time gain while limiting the stripped codex increase to 14.7%, compared with 21.5% at eight units. The gzip-compressed binary grows only 7.8% at four units. The one-unit build from an empty target directory took 981.150 seconds. That comparison also populated dependency and native build caches, so it is only directional. It agrees with an earlier clean matrix where eight units reduced 671 seconds to 303 seconds: https://gist.github.com/anp/4b88393a0acd35783d9f42156f3243d5 At the local 48% reduction, the current release's 55m22s critical-path macOS Cargo step would save about 26 minutes from the 71m28s workflow: https://github.com/openai/codex/actions/runs/27367405663 The prompt-image medians ranged from 3.9% faster to 0.9% slower. CLI startup shifted by 1-2 ms while user and system CPU time were unchanged.
anp-oai
approved these changes
Jun 12, 2026
tamird
marked this pull request as ready for review
June 12, 2026 02:44
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The release profile still uses one codegen unit, which serializes LLVM code generation within each crate. That setting was selected alongside fat LTO for optimization quality and binary size, but releases now use ThinLTO and code generation dominates the critical-path build.
Use four codegen units. On an Apple M4 Max with 16 cores and 128 GiB RAM, using rustc 1.96.0, four and eight units took 507.486 and 505.325 seconds respectively. Four therefore keeps the build-time gain while limiting the stripped
codexincrease to 14.7%, compared with 21.5% at eight units. The gzip-compressed binary grows 7.8% at four units.The one-unit build from an empty target directory took 981.150 seconds. That comparison also populated dependency and native build caches, so it is directional rather than controlled. It agrees with the earlier clean matrix where eight units reduced 671 seconds to 303 seconds: https://gist.github.com/anp/4b88393a0acd35783d9f42156f3243d5
At the local 48% reduction, the current release's 55m22s critical-path macOS Cargo step would save about 26 minutes from the 71m28s workflow: https://github.com/openai/codex/actions/runs/27367405663
The prompt-image medians ranged from 3.9% faster to 0.9% slower. CLI startup shifted by 1-2 ms while user and system CPU time were unchanged.
This is a draft because the release-latency improvement may not justify the binary-size increase.