Fix GC pacing problem exacerbated by mark-delay #3331
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mshinwell
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Dec 3, 2024
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This seems to be causing a flaky test failure e.g. https://github.com/ocaml-flambda/flambda-backend/actions/runs/12143046690/job/33858963003 . Reverting for now, I've pinged Nick separately. |
mshinwell
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This fixes a problem detected by CI when upstreaming #2348 / #2358 / #3029 to ocaml/ocaml#13580. It is a pacing problem in which major GC work performed can, in some circumstances, fall further and further behind the estimated work required. See the discussion on ocaml/ocaml#13580 for more details. In a long-running process this could be problematic, leading to longer GC pauses. On 32-bit platforms the flawed estimation can lead to an overflow which stops GC altogether, fatally. The short-term fix in this PR applies a "catch-up" adjustment when it may be necessary. Work is in progress on replacing all the pacing code, which should result in a longer-term fix.
This is one of the three fixes in #3297, which @mshinwell asked to be separated out.