linux-kernel - Re: [RFC PATCH 0/3] directed yield for Pause Loop Exiting

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Message-ID: <4CFB8D50.6050109@redhat.com>
Date:	Sun, 05 Dec 2010 15:02:08 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Chris Wright <chrisw@...s-sol.org>
CC:	Rik van Riel <riel@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>,
	Anthony Liguori <aliguori@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH 0/3] directed yield for Pause Loop Exiting

On 12/03/2010 12:41 AM, Chris Wright wrote:
> * Rik van Riel (riel@...hat.com) wrote:
> >  When running SMP virtual machines, it is possible for one VCPU to be
> >  spinning on a spinlock, while the VCPU that holds the spinlock is not
> >  currently running, because the host scheduler preempted it to run
> >  something else.
> >
> >  Both Intel and AMD CPUs have a feature that detects when a virtual
> >  CPU is spinning on a lock and will trap to the host.
> >
> >  The current KVM code sleeps for a bit whenever that happens, which
> >  results in eg. a 64 VCPU Windows guest taking forever and a bit to
> >  boot up.  This is because the VCPU holding the lock is actually
> >  running and not sleeping, so the pause is counter-productive.
>
> Seems like simply increasing the spin window help in that case?  Or is
> it just too contended a lock (I think they use mcs locks, so I can see a
> single wrong sleep causing real contention problems).

It may, but that just pushes the problem to a more contended lock or to 
a higher vcpu count.  We want something that works after PLE threshold 
tuning has failed.

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

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