From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: VM performance issue in KVM guests. Date: Thu, 15 Apr 2010 11:18:43 +0300 Message-ID: <4BC6CBE3.8020207@redhat.com> References: <4BC0D125.9050108@redhat.com> <4BC2C07B.4040607@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: "Zhang, Xiantao" , "kvm@vger.kernel.org" , Marcelo Tosatti , "Yang, Xiaowei" , "Dong, Eddie" , "Li, Xin" , Ingo Molnar , Peter Zijlstra , Mike Galbraith , Linux Kernel Mailing List , vatsa@linux.vnet.ibm.com To: Srivatsa Vaddagiri Return-path: Received: from mx1.redhat.com ([209.132.183.28]:63223 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756972Ab0DOITK (ORCPT ); Thu, 15 Apr 2010 04:19:10 -0400 In-Reply-To: Sender: kvm-owner@vger.kernel.org List-ID: On 04/15/2010 07:58 AM, Srivatsa Vaddagiri wrote: > On Sun, Apr 11, 2010 at 11:40 PM, Avi Kivity > wrote: > > The current handing of PLE is very suboptimal. With proper > directed yield we should be much better there. > > > > Hi Avi, > By directed yield, do you mean transfer the timeslice of > one thread (which is contending for a lock) to another thread (which > is holding a lock)? It's a priority transfer (in CFS terms, vruntime) (we don't know who holds the lock, so we pick a co-vcpu at random). > If at that point in time, the lock-holder thread/VCPU is actually not > running currently, ie it is at the back of the runqueue, would it help > much? In such case, it will take time for the lock holder to run again > and the default timeslice it would have got could have been sufficient > to release the lock? The idea is to increase the chances to the target vcpu to run, and to decrease the changes of the spinner to run (hopefully they change places). > > I am also working on a prototype for some other technique here - to > avoid preempting guest threads/VCPUs in the middle of their > (spin-lock) critical section. This requires guest to hint host when > there are in such a section. [1] has shown 33% improvement to an > apache benchmark based on this idea. > Certainly that has even greater potential for Linux guests. Note that we spin on mutexes now, so we need to prevent preemption while the lock owner is running. -- I have a truly marvellous patch that fixes the bug which this signature is too narrow to contain.