From ae1fa08e4138c49c8e7fa10c3eadbfca0233842b Mon Sep 17 00:00:00 2001 From: Josh Bleecher Snyder Date: Fri, 29 Jan 2021 15:41:18 -0800 Subject: context: reduce contention in cancelCtx.Done MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Use an atomic.Value to hold the done channel. Conveniently, we have a mutex handy to coordinate writes to it. name old time/op new time/op delta ContextCancelDone-8 67.5ns ±10% 2.2ns ±11% -96.74% (p=0.000 n=30+28) Fixes #42564 Change-Id: I5d72e0e87fb221d4e230209e5fb4698bea4053c6 Reviewed-on: https://go-review.googlesource.com/c/go/+/288193 Trust: Josh Bleecher Snyder Trust: Sameer Ajmani Run-TryBot: Josh Bleecher Snyder TryBot-Result: Go Bot Reviewed-by: Bryan C. Mills --- src/context/context.go | 30 +++++++++++++++++------------- 1 file changed, 17 insertions(+), 13 deletions(-) (limited to 'src/context/context.go') diff --git a/src/context/context.go b/src/context/context.go index b3fdb8277a..733c5f56d9 100644 --- a/src/context/context.go +++ b/src/context/context.go @@ -303,10 +303,8 @@ func parentCancelCtx(parent Context) (*cancelCtx, bool) { if !ok { return nil, false } - p.mu.Lock() - ok = p.done == done - p.mu.Unlock() - if !ok { + pdone, _ := p.done.Load().(chan struct{}) + if pdone != done { return nil, false } return p, true @@ -345,7 +343,7 @@ type cancelCtx struct { Context mu sync.Mutex // protects following fields - done chan struct{} // created lazily, closed by first cancel call + done atomic.Value // of chan struct{}, created lazily, closed by first cancel call children map[canceler]struct{} // set to nil by the first cancel call err error // set to non-nil by the first cancel call } @@ -358,13 +356,18 @@ func (c *cancelCtx) Value(key interface{}) interface{} { } func (c *cancelCtx) Done() <-chan struct{} { + d := c.done.Load() + if d != nil { + return d.(chan struct{}) + } c.mu.Lock() - if c.done == nil { - c.done = make(chan struct{}) + defer c.mu.Unlock() + d = c.done.Load() + if d == nil { + d = make(chan struct{}) + c.done.Store(d) } - d := c.done - c.mu.Unlock() - return d + return d.(chan struct{}) } func (c *cancelCtx) Err() error { @@ -401,10 +404,11 @@ func (c *cancelCtx) cancel(removeFromParent bool, err error) { return // already canceled } c.err = err - if c.done == nil { - c.done = closedchan + d, _ := c.done.Load().(chan struct{}) + if d == nil { + c.done.Store(closedchan) } else { - close(c.done) + close(d) } for child := range c.children { // NOTE: acquiring the child's lock while holding parent's lock. -- cgit v1.3-5-g9baa