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Proportional concurrent sweep is currently based on a ratio of spans to be swept per bytes of object allocation. However, proportional sweeping is performed during span allocation, not object allocation, in order to minimize contention and overhead. Since objects are allocated from spans after those spans are allocated, the system tends to operate in debt, which means when the next GC cycle starts, there is often sweep debt remaining, so GC has to finish the sweep, which delays the start of the cycle and delays enabling mutator assists. For example, it's quite likely that many Ps will simultaneously refill their span caches immediately after a GC cycle (because GC flushes the span caches), but at this point, there has been very little object allocation since the end of GC, so very little sweeping is done. The Ps then allocate objects from these cached spans, which drives up the bytes of object allocation, but since these allocations are coming from cached spans, nothing considers whether more sweeping has to happen. If the sweep ratio is high enough (which can happen if the next GC trigger is very close to the retained heap size), this can easily represent a sweep debt of thousands of pages. Fix this by making proportional sweep proportional to the number of bytes of spans allocated, rather than the number of bytes of objects allocated. Prior to allocating a span, both the small object path and the large object path ensure credit for allocating that span, so the system operates in the black, rather than in the red. Combined with the previous commit, this should eliminate all sweeping from GC start up. On the stress test in issue #11911, this reduces the time spent sweeping during GC (and delaying start up) by several orders of magnitude: mean 99%ile max pre fix 1 ms 11 ms 144 ms post fix 270 ns 735 ns 916 ns Updates #11911. Change-Id: I89223712883954c9d6ec2a7a51ecb97172097df3 Reviewed-on: https://go-review.googlesource.com/13044 Reviewed-by: Rick Hudson <rlh@golang.org> Reviewed-by: Russ Cox <rsc@golang.org> |
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