linux-kernel - Re: lmbench lat_mmap slowdown with CONFIG

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Date:	Thu, 22 Jan 2009 14:26:44 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Nick Piggin <npiggin@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>, hpa@...or.com,
	jeremy@...source.com, chrisw@...s-sol.org, zach@...are.com,
	rusty@...tcorp.com.au
Subject: Re: lmbench lat_mmap slowdown with CONFIG_PARAVIRT

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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