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Message-ID: <4978F2A4.8010807@goop.org>
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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