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

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Date:	Fri, 5 Jun 2009 14:16:18 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...e.hu>, Nick Piggin <npiggin@...e.de>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	"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>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Avi Kivity <avi@...hat.com>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [benchmark] 1% performance overhead of paravirt_ops on native kernels

On Fri, 5 Jun 2009 12:32:14 am Linus Torvalds wrote:
> So I think your minimum and maximum configs should at least _match_ in
> HIGHMEM. Limiting memory to not actually having any (with "mem=880M") will
> avoid the TLB flushing impact of HIGHMEM, which is clearly going to be the
> _bulk_ of the overhead, but HIGHMEM is still going to be noticeable on at
> least some microbenchmarks.

Well, Ingo was ranting because (paraphrase) "no other config option when 
*unused* has as much impact as CONFIG_PARAVIRT!!!!!!!!!!".

That was the point of my mail; facts show it's simply untrue.

> The comparison is ugly and pointless.
(Re: SMP)

Distributions don't ship UP kernels any more; this shows what that costs if 
you're actually on a UP box.  If we really don't care, perhaps we should make 
CONFIG_SMP=n an option under EMBEDDED for x86.  And we can rip out the complex 
patching SMP patching stuff too.

> Something like CONFIG_HIGHMEM* or CONFIG_SMP is not really what I'd ever
> call "optional feature", although I hope to Dog that CONFIG_HIGHMEM can
> some day be considered that some day.

Someone from a distro might know how many deployed machines don't need them.  
Kernel hackers tend to have modern machines; same with "enterprise" sites.  I 
have no idea.

Without those facts, I'll leave further discussion to someone else :)

Thanks,
Rusty.
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