From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: [RFC PATCH 0/3] directed yield for Pause Loop Exiting Date: Sun, 05 Dec 2010 15:02:08 +0200 Message-ID: <4CFB8D50.6050109@redhat.com> References: <20101202144129.4357fe00@annuminas.surriel.com> <20101202224122.GT10050@sequoia.sous-sol.org> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Rik van Riel , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Srivatsa Vaddagiri , Peter Zijlstra , Ingo Molnar , Anthony Liguori To: Chris Wright Return-path: In-Reply-To: <20101202224122.GT10050@sequoia.sous-sol.org> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On 12/03/2010 12:41 AM, Chris Wright wrote: > * Rik van Riel (riel@redhat.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