f8b2314c56
This optimizes deferproc and deferreturn in various ways. The most important optimization is that it more carefully arranges to prevent preemption or stack growth. Currently we do this by switching to the system stack on every deferproc and every deferreturn. While we need to be on the system stack for the slow path of allocating and freeing defers, in the common case we can fit in the nosplit stack. Hence, this change pushes the system stack switch down into the slow paths and makes everything now exposed to the user stack nosplit. This also eliminates the need for various acquirem/releasem pairs, since we are now preventing preemption by preventing stack split checks. As another smaller optimization, we special case the common cases of zero-sized and pointer-sized defer frames to respectively skip the copy and perform the copy in line instead of calling memmove. This speeds up the runtime defer benchmark by 42%: name old time/op new time/op delta Defer-4 75.1ns ± 1% 43.3ns ± 1% -42.31% (p=0.000 n=8+10) In reality, this speeds up defer by about 2.2X. The two benchmarks below compare a Lock/defer Unlock pair (DeferLock) with a Lock/Unlock pair (NoDeferLock). NoDeferLock establishes a baseline cost, so these two benchmarks together show that this change reduces the overhead of defer from 61.4ns to 27.9ns. name old time/op new time/op delta DeferLock-4 77.4ns ± 1% 43.9ns ± 1% -43.31% (p=0.000 n=10+10) NoDeferLock-4 16.0ns ± 0% 15.9ns ± 0% -0.39% (p=0.000 n=9+8) This also shaves 34ns off cgo calls: name old time/op new time/op delta CgoNoop-4 122ns ± 1% 88.3ns ± 1% -27.72% (p=0.000 n=8+9) Updates #14939, #16051. Change-Id: I2baa0dea378b7e4efebbee8fca919a97d5e15f38 Reviewed-on: https://go-review.googlesource.com/29656 Reviewed-by: Keith Randall <khr@golang.org> |
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