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Message-ID: <1357227169.10284.32.camel@gandalf.local.home> Date: Thu, 03 Jan 2013 10:32:49 -0500 From: Steven Rostedt <rostedt@...dmis.org> To: Eric Dumazet <eric.dumazet@...il.com> Cc: Jan Beulich <JBeulich@...e.com>, Rik van Riel <riel@...hat.com>, therbert@...gle.com, walken@...gle.com, jeremy@...p.org, tglx@...utronix.de, aquini@...hat.com, lwoodman@...hat.com, linux-kernel@...r.kernel.org Subject: Re: [RFC PATCH 3/3 -v2] x86,smp: auto tune spinlock backoff delay factor 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? -- Steve -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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