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Message-ID: <4A1EFABA.7070600@goop.org>
Date: Thu, 28 May 2009 13:57:30 -0700
From: Jeremy Fitzhardinge <jeremy@...p.org>
To: Nick Piggin <npiggin@...e.de>
CC: Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
Thomas Gleixner <tglx@...utronix.de>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [benchmark] 1% performance overhead of paravirt_ops on native
kernels
Nick Piggin wrote:
> FWIW, we had to disable paravirt in our default SLES11 kernel.
> (admittedly this was before some of the recent improvements were
> made).
Yes, I think you'll find it worth trying with it enabled again. The
spinlock thing clearly slowed things down, but when that's disabled the
difference to native is very small.
> But OTOH, almost any bit feature is going to cost performance. I don't
> think this is something new (as noted with CONFIG_SECURITY). It is
> just something people have to trade off and decide for themselves.
> If you make it configurable and keep performance as good as reasonably
> possible, then I don't think more can be asked.
>
Yes, that's exactly my point. If I've worked on a feature, I clearly
want people to use that feature. Part of making it useful is to make
the distro/vendor/user decision to enable that feature as easy as
possible, by making the tradeoffs simple.
But tradeoffs are always going to cut both ways: positive (kernel
automatically works in a wider range of environments), and negative
(performance questions, complexity, etc). Ultimately its the distro's
decision to enable a particular feature, and the distro's responsibility
to cope with the consequences of that.
> If performance overhead is too much and/or too few users can take
> advantage of a feature, then distros can always special-case it. I
> think may did for pae...
I think that would be a clear sign of a problem. The whole point of
pvops is to avoid needing multiple kernel builds.
J
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