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runtime: execute memory barrier conditionally when changing netpoll timers

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We only need the memory barrier in poll_runtime_pollSetDeadline only
when one of the timers has fired, which is not the expected case.
Memory barrier can be somewhat expensive on some archs,
so execute it only if one of the timers has in fact fired.

name                  old time/op  new time/op  delta
TCP4OneShotTimeout-6  17.0µs ± 0%  17.1µs ± 0%  +0.35%  (p=0.032 n=5+5)
SetReadDeadline-6      232ns ± 0%   230ns ± 0%  -1.03%  (p=0.000 n=4+5)

Update #25729

Change-Id: Ifce6f505b9e7ba3717bad8f454077a2e94ea6e75
Reviewed-on: https://go-review.googlesource.com/c/146343
Reviewed-by: Ian Lance Taylor <iant@golang.org>
This commit is contained in:
Dmitry Vyukov 2018-10-31 17:56:14 +01:00
parent ff51353c38
commit 31e7842f3d
1 changed files with 8 additions and 6 deletions
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14
src/runtime/netpoll.go
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@ -254,12 +254,14 @@ func poll_runtime_pollSetDeadline(pd *pollDesc, d int64, mode int) {
}
// If we set the new deadline in the past, unblock currently pending IO if any.
var rg, wg *g
atomicstorep(unsafe.Pointer(&wg), nil) // full memory barrier between stores to rd/wd and load of rg/wg in netpollunblock
if pd.rd < 0 {
rg = netpollunblock(pd, 'r', false)
}
if pd.wd < 0 {
wg = netpollunblock(pd, 'w', false)
if pd.rd < 0 || pd.wd < 0 {
atomicstorep(unsafe.Pointer(&wg), nil) // full memory barrier between stores to rd/wd and load of rg/wg in netpollunblock
if pd.rd < 0 {
rg = netpollunblock(pd, 'r', false)
}
if pd.wd < 0 {
wg = netpollunblock(pd, 'w', false)
}
}
unlock(&pd.lock)
if rg != nil {