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Austin Clements d35dfd405e runtime: make assists perform root jobs
Currently, assists can only perform heap marking jobs. However, at the
beginning of GC, there are only root jobs and no heap marking jobs. As
a result, there's often a period at the beginning of a GC cycle where
no goroutine has accumulated assist credit, but at the same time it
can't get any credit because there are no heap marking jobs for it to
do yet. As a result, many goroutines often block on the assist queue
at the very beginning of the GC cycle.

This commit fixes this by allowing assists to perform root marking
jobs. The tricky part of this (and the reason we haven't done this
before) is that stack scanning jobs can lead to deadlocks if the
goroutines performing the stack scanning are themselves
non-preemptible, since two non-preemptible goroutines may try to scan
each other. To address this, we use the same insight d6625ca used to
simplify the mark worker stack scanning: as long as we're careful with
the stacks and only drain jobs while on the system stack, we can put
the goroutine into a preemptible state while we drain jobs. This means
an assist's user stack can be scanned while it continues to do work.

This reduces the rate of assist blocking in the x/benchmarks HTTP
benchmark by a factor of 3 and all remaining blocking happens towards
the *end* of the GC cycle, when there may genuinely not be enough work
to go around.

Ideally, assists would get credit for working on root jobs. Currently
they do not; however, this change prioritizes heap work over root jobs
in assists, so they're likely to mostly perform heap work. In contrast
with mark workers, for assists, the root jobs act only as a backstop
to create heap work when there isn't enough heap work.

Fixes #15361.

Change-Id: If6e169863e4ad75710b0c8dc00f6125b41e9a595
Reviewed-on: https://go-review.googlesource.com/32432
Reviewed-by: Rick Hudson <rlh@golang.org>
2016-10-31 21:58:17 +00:00
.github doc: improve issue template 2016-08-29 03:33:28 +00:00
api syscall: remove X__cmsg_data from Cmsghdr 2016-10-30 19:04:09 +00:00
doc doc: replace code.google.com/p/sitesearch with its new destination 2016-10-31 20:36:31 +00:00
lib/time lib/time: update to IANA release 2016g (September 2016) 2016-10-20 08:46:49 +00:00
misc cmd/link, plugin: use full plugin path for symbols 2016-10-31 04:48:42 +00:00
src runtime: make assists perform root jobs 2016-10-31 21:58:17 +00:00
test cmd/compile: initialize Decldepth in all cases 2016-10-31 19:48:17 +00:00
.gitattributes
.gitignore cmd/go: fail with nice error message on bad GOOS/GOARCH pair 2016-05-06 01:18:07 +00:00
AUTHORS A+C: automated updates 2016-06-24 02:21:02 +00:00
CONTRIBUTING.md doc: use new Gerrit URL and mention our instance in CONTRIBUTING.md 2016-03-09 00:52:42 +00:00
CONTRIBUTORS A+C: automated updates 2016-06-24 02:21:02 +00:00
favicon.ico website: recreate 16px and 32px favicon 2016-08-25 15:43:32 +00:00
LICENSE doc: revert copyright date to 2009 2016-06-01 22:40:04 +00:00
PATENTS
README.md readme: emphasize issue tracker is for bugs/proposals 2015-10-02 18:44:14 +00:00
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