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Make -fprofile-update=atomic opt-in for PGO builds to avoid 34% slowdown - #157026
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builtbyashwin wants to merge 1 commit into
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Make -fprofile-update=atomic opt-in for PGO builds to avoid 34% slowdown#157026builtbyashwin wants to merge 1 commit into
builtbyashwin wants to merge 1 commit into
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builtbyashwin
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AA-Turner,
corona10,
emmatyping,
erlend-aasland and
itamaro
as code owners
September 6, 2026 08:53
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Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool. If this change has little impact on Python users, wait for a maintainer to apply the |
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Please open an issue first, also specify what impact will this have on performance, we want PGO builds to be optimised after all. |
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Problem
The
-fprofile-update=atomicflag inPGO_PROF_GEN_FLAGcauses severe lock contention during PGO builds, making them 34% slower on multi-core machines.Benchmark (on T430, compiling Python/ceval.c)
Fix
Made
-fprofile-update=atomicopt-in via a newAC_ARG_ENABLE([pgo-atomic])option inconfigure.ac. Default is now-fprofile-generateonly, which is significantly faster.Usage
./configure --enable-optimizations(no atomic flag, faster)./configure --enable-optimizations --enable-pgo-atomic(safe for users who need atomic profiling)Compatibility
Backwards-compatible. Users who need the atomic flag can still enable it via the new configure option.
Verification
Ran
make -j2successfully after the change.0 failed on import.