Message-ID: <4978F2A4.8010807@goop.org>
Date: Thu, 22 Jan 2009 14:26:44 -0800
From: Jeremy Fitzhardinge <jeremy@goop.org>
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To: Ingo Molnar <mingo@elte.hu>
CC: Nick Piggin <npiggin@suse.de>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       Linus Torvalds <torvalds@linux-foundation.org>, hpa@zytor.com,
       jeremy@xensource.com, chrisw@sous-sol.org, zach@vmware.com,
       rusty@rustcorp.com.au
Subject: Re: lmbench lat_mmap slowdown with CONFIG_PARAVIRT
References: <20090120110542.GE19505@wotan.suse.de> <20090120112634.GA20858@elte.hu>
In-Reply-To: <20090120112634.GA20858@elte.hu>
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Ingo Molnar wrote:
> Ouch, that looks unacceptably expensive. All the major distros turn 
> CONFIG_PARAVIRT on. paravirt_ops was introduced in x86 with the express 
> promise to have no measurable runtime overhead.
>
> ( And i suspect the real life mmap cost is probably even more expensive,
>   as on a Barcelona all of lmbench fits into the cache hence we dont see
>   any real $cache overhead. )
>
> Jeremy, any ideas where this slowdown comes from and how it could be 
> fixed?
>   

I just posted a couple of patches to pick some low-hanging fruit.  It 
turns out that we don't need to do any pvops calls to do pte flag 
manipulations.  I'd be interested to see how much of a difference it 
makes (it reduces the static code size by a few k).

    J
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