linux-kernel - Re: [RFC PATCH v1 3/5] KVM: Add paravirt kvm_flush_tlb

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Message-ID: <4FA00234.1020006@redhat.com>
Date:	Tue, 01 May 2012 18:33:08 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	"Nikunj A. Dadhania" <nikunj@...ux.vnet.ibm.com>, mingo@...e.hu,
	jeremy@...p.org, mtosatti@...hat.com, kvm@...r.kernel.org,
	x86@...nel.org, vatsa@...ux.vnet.ibm.com,
	linux-kernel@...r.kernel.org, hpa@...or.com
Subject: Re: [RFC PATCH v1 3/5] KVM: Add paravirt kvm_flush_tlb_others

On 05/01/2012 06:11 PM, Peter Zijlstra wrote:
> On Tue, 2012-05-01 at 15:12 +0300, Avi Kivity wrote:
> > We're now moving the freeing of kvm shadow page tables from using rcu to
> > using an irq-protected scheme like gup_fast(), because of the
> > performance differences.  We didn't track it down but I expect the cause
> > is less reuse of cache-hot pages. 
>
> It would be good to understand these things.. just doing things because
> tends to come back and bite you :-)

Agree.  Anyway that was only part of the motivation, the other part was
establishing a bound on the number of pages under deferred freeing.

> If its cache behaviour you should be able to clearly see that using perf
> stat.

It's hard to see it clearly because the changes are in the order of 1-2%
in a very noisy benchmark.

-- 
error compiling committee.c: too many arguments to function

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