Re: [PATCH 8/5] llist: Remove cpu_relax() usage in cmpxchg loops

Avi Kivity
2011-09-13T11:43:59Z

On 09/12/2011 05:32 PM, Peter Zijlstra wrote:
> On Mon, 2011-09-12 at 17:26 +0300, Avi Kivity wrote:
> > On 09/12/2011 05:05 PM, Peter Zijlstra wrote:
> > > Subject: llist: Remove cpu_relax() usage in cmpxchg loops
> > > From: Peter Zijlstra<a.p.zijlstra@chello.nl>
> > > Date: Mon Sep 12 15:50:49 CEST 2011
> > >
> > > Initial benchmarks show they're a net loss (2 socket wsm):
> > >
> > > $ for i in /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor ; do echo performance> $i; done
> > > $ echo 4096 32000 64 128> /proc/sys/kernel/sem
> > > $ ./sembench -t 2048 -w 1900 -o 0
> > >
> >
> > We hyperthreading enabled, and were all threads loaded? cpu_relax
> > allows the other thread to make more progress while the spinner relaxes.
>
> Yeah, with HT enabled, the benchmark runs 2048 tasks and does 1900 task
> bulk wakeups or so. Forgot the details, but it basically stresses the
> sleep+wakeup paths like nobodies business.

Ok.

Another issue is that hypervisors use PAUSE to detect a spinning guest
and issue a directed yield to another vcpu. But for cmpxchg loops, the
"spinner" would just commit on the next loop, no? So I think there's no
objection from that front.

--
error compiling committee.c: too many arguments to function