Re: [RFC PATCH 3/3 -v2] x86,smp: auto tune spinlock backoff delayfactor

From: Eric Dumazet
Date: Thu Jan 03 2013 - 08:35:08 EST

On Thu, 2013-01-03 at 08:24 -0500, Steven Rostedt wrote:
> On Thu, 2013-01-03 at 09:05 +0000, Jan Beulich wrote:
> > > How much bus traffic do monitor/mwait cause behind the scenes?
> >
> > I would suppose that this just snoops the bus for writes, but the
> > amount of bus traffic involved in this isn't explicitly documented.
> >
> > One downside of course is that unless a spin lock is made occupy
> > exactly a cache line, false wakeups are possible.
> And that would probably be very likely, as the whole purpose of Rik's
> patches was to lower cache stalls due to other CPUs pounding on spin
> locks that share the cache line of what is being protected (and
> modified).

A monitor/mwait would be an option only if using MCS (or K42 variant)
locks, where each cpu would wait on a private and dedicated cache line.

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