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

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Message-ID: <84144f020906021113j12c3a8fbt40dd37ab780847ca@mail.gmail.com>
Date:	Tue, 2 Jun 2009 21:13:37 +0300
From:	Pekka Enberg <penberg@...helsinki.fi>
To:	Chris Mason <chris.mason@...cle.com>,
	Ulrich Drepper <drepper@...il.com>,
	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

Hi Chris,

On Tue, Jun 2, 2009 at 8:03 AM, Chris Mason <chris.mason@...cle.com> wrote:
>>> The best place to fix xen is in the kernel.

On Tue, Jun 02, 2009 at 08:22:57AM -0700, Ulrich Drepper wrote:
>> No.  The best way to fix things is _on the way into the kernel_.

On Tue, Jun 2, 2009 at 7:20 PM, Chris Mason <chris.mason@...cle.com> wrote:
> 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.

The fact that CONFIG_PARAVIRT can be disabled doesn't really help. As
a matter of fact, I'd argue that one of the primary reasons
CONFIG_SLUB regression is still there is because people can just
disable it and use CONFIG_SLAB instead.

So I think we have some evidence to suggest that people have less
incentive to fix things once something is merged to the kernel. And I
don't mean the authors of the code here but basically _everyone_
involved in kernel development. It usually takes effort from variety
of people to get everything ironed out because, lets face it, we can't
expect a handful of people to test out every configuration let alone
fix them.

                        Pekka
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