Re: [RFC PATCH 3/3 -v2] x86,smp: auto tune spinlock backoff delayfactor
From: Steven Rostedt
Date: Thu Jan 03 2013 - 10:32:40 EST
On Thu, 2013-01-03 at 05:35 -0800, Eric Dumazet wrote:
> 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.
But then would the problem even exist? If the lock is on its own cache
line, it shouldn't cause a performance issue if other CPUs are spinning
on it. Would it?
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