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

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Message-ID: <4BC6CBE3.8020207@redhat.com>
Date:	Thu, 15 Apr 2010 11:18:43 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Srivatsa Vaddagiri <vatsa.ml@...il.com>
CC:	"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>,
	Peter Zijlstra <peterz@...radead.org>,
	Mike Galbraith <efault@....de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	vatsa@...ux.vnet.ibm.com
Subject: Re: VM performance issue in KVM guests.

On 04/15/2010 07:58 AM, Srivatsa Vaddagiri wrote:
> On Sun, Apr 11, 2010 at 11:40 PM, Avi Kivity <avi@...hat.com 
> <mailto:avi@...hat.com>> 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.

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