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

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Message-ID: <1271414436.4807.1935.camel@twins>
Date:	Fri, 16 Apr 2010 12:40:36 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	vatsa@...ibm.com
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, 2010-04-15 at 09:43 -0700, Srivatsa Vaddagiri wrote:
> 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?

Except gang scheduling is a scalability nightmare waiting to happen. I
much prefer this hint thing.
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