linux-kernel - Re: [benchmark] 1% performance overhead of paravirt

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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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