With the hybrid barrier in place, we can now disable stack rescanning
by default. This commit adds a "gcrescanstacks" GODEBUG variable that
is off by default but can be set to re-enable STW stack rescanning.
The plan is to leave this off but available in Go 1.8 for debugging
and as a fallback.
With this change, worst-case mark termination time at GOMAXPROCS=12
*not* including time spent stopping the world (which is still
unbounded) is reliably under 100 µs, with a 95%ile around 50 µs in
every benchmark I tried (the go1 benchmarks, the x/benchmarks garbage
benchmark, and the gcbench activegs and rpc benchmarks). Including
time spent stopping the world usually adds about 20 µs to total STW
time at GOMAXPROCS=12, but I've seen it add around 150 µs in these
benchmarks when a goroutine takes time to reach a safe point (see
issue #10958) or when stopping the world races with goroutine
switches. At GOMAXPROCS=1, where this isn't an issue, worst case STW
is typically 30 µs.
The go-gcbench activegs benchmark is designed to stress large numbers
of dirty stacks. This commit reduces 95%ile STW time for 500k dirty
stacks by nearly three orders of magnitude, from 150ms to 195µs.
This has little effect on the throughput of the go1 benchmarks or the
x/benchmarks benchmarks.
It reduces the tail latency of the x/benchmarks HTTP benchmark:
name old p50-time new p50-time delta
XHTTP-12 489µs ± 0% 491µs ± 1% +0.54% (p=0.000 n=20+18)
name old p95-time new p95-time delta
XHTTP-12 957µs ± 1% 960µs ± 1% +0.28% (p=0.002 n=20+17)
name old p99-time new p99-time delta
XHTTP-12 1.76ms ± 1% 1.64ms ± 1% -7.20% (p=0.000 n=20+18)
Comparing to the beginning of the hybrid barrier implementation
("runtime: parallelize STW mcache flushing") shows that the hybrid
barrier trades a small performance impact for much better STW latency,
as expected. The magnitude of the performance impact is generally
small:
Updates #17099, since you can't have a rescan list bug if there's no
rescan list. I'm not marking it as fixed, since gcrescanstacks can
still be set to re-enable the rescan lists.
Change-Id: I6e926b4c2dbd4cd56721869d4f817bdbb330b851
Reviewed-on: https://go-review.googlesource.com/31766 Reviewed-by: Rick Hudson <rlh@golang.org>