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At the start of a GC cycle, the garbage collector computes the assist ratio based on the total scannable heap size. This was intended to be conservative; after all, this assumes the entire heap may be reachable and hence needs to be scanned. But it only assumes that the *current* entire heap may be reachable. It fails to account for heap allocated during the GC cycle. If the trigger ratio is very low (near zero), and most of the heap is reachable when GC starts (which is likely if the trigger ratio is near zero), then it's possible for the mutator to create new, reachable heap fast enough that the assists won't keep up based on the assist ratio computed at the beginning of the cycle. As a result, the heap can grow beyond the heap goal (by hundreds of megs in stress tests like in issue #11911). We already have some vestigial logic for dealing with situations like this; it just doesn't run often enough. Currently, every 10 ms during the GC cycle, the GC revises the assist ratio. This was put in before we switched to a conservative assist ratio (when we really were using estimates of scannable heap), and it turns out to be exactly what we need now. However, every 10 ms is far too infrequent for a rapidly allocating mutator. This commit reuses this logic, but replaces the 10 ms timer with revising the assist ratio every time the heap is locked, which coincides precisely with when the statistics used to compute the assist ratio are updated. Fixes #11911. Change-Id: I377b231ab064946228378fa10422a46d1b50f4c5 Reviewed-on: https://go-review.googlesource.com/13047 Reviewed-by: Rick Hudson <rlh@golang.org> Reviewed-by: Russ Cox <rsc@golang.org> |
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