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

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Date:	Tue, 2 Jun 2009 12:20:08 -0400
From:	Chris Mason <chris.mason@...cle.com>
To:	Ulrich Drepper <drepper@...il.com>
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>,
	Linus Torvalds <torvalds@...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 Tue, Jun 02, 2009 at 08:22:57AM -0700, Ulrich Drepper wrote:
> On Tue, Jun 2, 2009 at 8:03 AM, Chris Mason <chris.mason@...cle.com> wrote:
> > The idea that people shipping xen aren't interested in performance
> > regressions is really strange to me.
> 
> Why?  They have a different base line.  For them any regression to
> bare hardware performance is even a positive (since it means the gap
> between hardware and virt shrinks).

And we would have gotten away with it too if it weren't for you meddling
kids!

> 
> 
> > Dynamic patching is a big wad of duct tape over the problem.
> 
> And what do you call the Xen model?  It's a perfect fit IMO.
> 
> > I'm not saying to take harmful code, I'm saying to take code with a
> > small performance regression under a specific CONFIG_.  Slub regresses
> > more than 1% on database loads, CONFIG_SCHED_GROUPS, the list goes on
> > and on.
> 
> None of those have to be enabled in default kernels.
> 
> 
> > The best place to fix xen is in the kernel.
> 
> No.  The best way to fix things is _on the way into the kernel_.

It all depends on which parts are causing problems.  A 1% performance
hit, under a CONFIG_ that can be disabled?  If maintainers are focusing
on details like this for long term and active projects, we're doing
something very wrong.

-chris

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