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

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Message-ID: <20090602195132.GL3914@think>
Date:	Tue, 2 Jun 2009 15:51:32 -0400
From:	Chris Mason <chris.mason@...cle.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Ingo Molnar <mingo@...e.hu>, Nick Piggin <npiggin@...e.de>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	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 09:14:23PM +0200, Thomas Gleixner wrote:
> On Tue, 2 Jun 2009, Chris Mason wrote:
> > I'm not suggesting we should take broken code, or that we should lower
> > standards just for xen.  But, expecting the xen developers to fix the 1%
> > hit on a very specific micro-benchmark is not a way to promote new
> > projects for the kernel, and it isn't a good way to convince people to
> > do continued development in mainline instead of in private trees.
> > 
> > Please reconsider.  Keeping these patches out is only making it harder
> > on the people that want to make them better.
> 
> You are missing one subtle point. 
> 

I'm sure I'm missing many more than one ;)

> I read several times, that A, B and C can not be changed design wise
> to allow newer kernels to run on older hypervisors. That's what
> frightens me:
> 
>    dom0 imposes a kind of ABI which we can not change anymore. 
> 
> So where is the room for the improvements which you expect when dom0
> is merged ? It's not about micro benchmark results, it's about the
> inability to fix the existing design decisions in the near future.
> 
> You can change the internals of btrfs as often as you want, but you
> can not change the on disk layout at will. And while you can invent
> btrfs2 w/o any impact aside of grumpy users and a couple of thousand
> lines self contained code, dom0v2 would just add a different layer of
> intrusiveness into the x86 code base w/o removing the existing one.

Well, if there's a line we want to draw in the sand based on firm and
debatable criteria, great.

The problem I see here is that our line in the sand for the xen
developers is fuzzy and winding (yeah, I saw Linus' reply in the other
thread, full ack on that).

-chris

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