runtime: remove sweep wait loop in finishsweep_m
In general, finishsweep_m must block until any spans that are concurrently being swept have been swept. It accomplishes this by looping over all spans, which, as in the previous commit, takes ~1ms/heap GB. Unfortunately, we do this during the STW sweep termination phase, so multi-gigabyte heaps can push our STW time past 10ms. However, there's no need to do this wait if the world is stopped because, in effect, stopping the world already had to wait for anything that was sweeping (and if it didn't, the wait in finishsweep_m would deadlock). Hence, we can simply skip this loop if the world is stopped, such as during sweep termination. In fact, currently all calls to finishsweep_m are STW, but this hasn't always been the case and may not be the case in the future, so we keep the logic around. For 24GB heaps, this reduces max pause time by 75% relative to tip and by 90% relative to Go 1.5. Notably, all pauses are now well under 10ms. Here are the results for the garbage benchmark: ------------- max pause ------------ Heap Procs after change before change 1.5.1 24GB 12 3.8ms 16ms 37ms 24GB 4 3.7ms 16ms 37ms 4GB 4 3.7ms 3ms 6.9ms In the 4GB/4P case, it seems the "before change" run got lucky: the max went up, but the 99%ile pause time went down from 3ms to 2.04ms. Change-Id: Ica22189559f231d408ef2815019c9dbb5f38bf31 Reviewed-on: https://go-review.googlesource.com/15071Reviewed-by:Rick Hudson <rlh@golang.org> Run-TryBot: Austin Clements <austin@google.com> TryBot-Result: Gobot Gobot <gobot@golang.org>
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