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The mark 2 phase was originally introduced as a way to reduce the chance of entering STW mark termination while there was still marking work to do. It works by flushing and disabling all local work caches so that all enqueued work becomes immediately globally visible. However, mark 2 is not only slow–disabling caches makes marking and the write barrier both much more expensive–but also imperfect. There is still a rare but possible race (~once per all.bash) that can cause GC to enter mark termination while there is still marking work. This race is detailed at https://github.com/golang/proposal/blob/master/design/17503-eliminate-rescan.md#appendix-mark-completion-race The effect of this is that mark termination must still cope with the possibility that there may be work remaining after a concurrent mark phase. Dealing with this increases STW pause time and increases the complexity of mark termination. Furthermore, a similar but far more likely race can cause early transition from mark 1 to mark 2. This is unfortunate because it causes performance instability because of the cost of mark 2. This CL fixes this by replacing mark 2 with a distributed termination detection algorithm. This algorithm is correct, so it eliminates the mark termination race, and doesn't require disabling local caches. It ensures that there are no grey objects upon entering mark termination. With this change, we're one step closer to eliminating marking from mark termination entirely (it's still used by STW GC and checkmarks mode). This CL does not eliminate the gcBlackenPromptly global flag, though it is always set to false now. It will be removed in a cleanup CL. This led to only minor variations in the go1 benchmarks (https://perf.golang.org/search?q=upload:20180909.1) and compilebench benchmarks (https://perf.golang.org/search?q=upload:20180910.2). This significantly improves performance of the garbage benchmark, with no impact on STW times: name old time/op new time/op delta Garbage/benchmem-MB=64-12 2.21ms ± 1% 2.05ms ± 1% -7.38% (p=0.000 n=18+19) Garbage/benchmem-MB=1024-12 2.30ms ±16% 2.20ms ± 7% -4.51% (p=0.001 n=20+20) name old STW-ns/GC new STW-ns/GC delta Garbage/benchmem-MB=64-12 138k ±44% 141k ±23% ~ (p=0.309 n=19+20) Garbage/benchmem-MB=1024-12 159k ±25% 178k ±98% ~ (p=0.798 n=16+18) name old STW-ns/op new STW-ns/op delta Garbage/benchmem-MB=64-12 4.42k ±44% 4.24k ±23% ~ (p=0.531 n=19+20) Garbage/benchmem-MB=1024-12 591 ±24% 636 ±111% ~ (p=0.309 n=16+18) (https://perf.golang.org/search?q=upload:20180910.1) Updates #26903. Updates #17503. Change-Id: Icbd1e12b7a12a76f423c9bf033b13cb363e4cd19 Reviewed-on: https://go-review.googlesource.com/c/134318 Run-TryBot: Austin Clements <austin@google.com> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Rick Hudson <rlh@golang.org> |
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