linux-kernel - Re: VM performance issue in KVM guests.

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Message-ID: <20100415164313.GF9128@linux.vnet.ibm.com>
Date:	Thu, 15 Apr 2010 09:43:13 -0700
From:	Srivatsa Vaddagiri <vatsa@...ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Avi Kivity <avi@...hat.com>,
	Srivatsa Vaddagiri <vatsa.ml@...il.com>,
	"Zhang, Xiantao" <xiantao.zhang@...el.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	"Yang, Xiaowei" <xiaowei.yang@...el.com>,
	"Dong, Eddie" <eddie.dong@...el.com>, "Li, Xin" <xin.li@...el.com>,
	Ingo Molnar <mingo@...e.hu>, Mike Galbraith <efault@....de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: VM performance issue in KVM guests.

On Thu, Apr 15, 2010 at 03:33:18PM +0200, Peter Zijlstra wrote:
> On Thu, 2010-04-15 at 11:18 +0300, Avi Kivity wrote:
> > 
> > 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. 
> 
> either that, or disable spinning on (para) virt kernels. Para virt
> kernels could possibly extend the thing by also checking to see if the
> owner's vcpu is running.

I suspect we will need a combination of both approaches, given that we will not
be able to avoid preempting guests in their critical section always (too long
critical sections or real-time tasks wanting to preempt). Other idea is to
gang-schedule VCPUs of the same guest as much as possible?

- vatsa
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