8f34b25318
Currently, there are three ways to satisfy a GC assist: 1) the mutator steals credit from background GC, 2) the mutator actually does GC work, and 3) there is no more work available. 3 was never really intended as a way to satisfy an assist, and it causes problems: there are periods when it's expected that the GC won't have any work, such as when transitioning from mark 1 to mark 2 and from mark 2 to mark termination. During these periods, there's no back-pressure on rapidly allocating mutators, which lets them race ahead of the heap goal. For example, test/init1.go and the runtime/trace test both have small reachable heaps and contain loops that rapidly allocate large garbage byte slices. This bug lets these tests exceed the heap goal by several orders of magnitude. Fix this by forcing the assist (and hence the allocation) to block until it can satisfy its debt via either 1 or 2, or the GC cycle terminates. This fixes one the causes of #11677. It's still possible to overshoot the GC heap goal, but with this change the overshoot is almost exactly by the amount of allocation that happens during the concurrent scan phase, between when the heap passes the GC trigger and when the GC enables assists. Change-Id: I5ef4edcb0d2e13a1e432e66e8245f2bd9f8995be Reviewed-on: https://go-review.googlesource.com/12671 Reviewed-by: Russ Cox <rsc@golang.org> |
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