runtime: remove in-use page count loop from STW
In order to compute the sweep ratio, the runtime needs to know how many pages belong to spans in state _MSpanInUse. Currently it finds this out by looping over all spans during mark termination. However, this takes ~1ms/heap GB, so multi-gigabyte heaps can quickly push our STW time past 10ms. Replace the loop with an actively maintained count of in-use pages. For multi-gigabyte heaps, this reduces max mark termination pause time by 75%–90% relative to tip and by 85%–95% relative to Go 1.5.1. This shifts the longest pause time for large heaps to the sweep termination phase, so it only slightly decreases max pause time, though it roughly halves mean pause time. Here are the results for the garbage benchmark: ---- max mark termination pause ---- Heap Procs after change before change 1.5.1 24GB 12 1.9ms 18ms 37ms 24GB 4 3.7ms 18ms 37ms 4GB 4 920µs 3.8ms 6.9ms Fixes #11484. Change-Id: Ia2d28bb8a1e4f1c3b8ebf79fb203f12b9bf114ac Reviewed-on: https://go-review.googlesource.com/15070Reviewed-by:Rick Hudson <rlh@golang.org> Run-TryBot: Austin Clements <austin@google.com> TryBot-Result: Gobot Gobot <gobot@golang.org>
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