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

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Message-Id: <200906060001.38069.rusty@rustcorp.com.au>
Date:	Sat, 6 Jun 2009 00:01:36 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Gerd Hoffmann <kraxel@...hat.com>
Cc:	Dave McCracken <dcm@...r.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	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 05:01:25 pm Gerd Hoffmann wrote:
>    Hi,
>
> > I think you're missing the point of Rusty's benchmark.  I see his
> > exercise as "compare a kernel configured as a distro would vs a
> > custom-built kernel configured for the exact target environment".  In
> > that light, questions about the CONFIG options Rusty used should be based
> > on whether most distros would use them in their stock kernels as opposed
> > to how necessary they are.
>
> Well.  The test ran on a machine with so much memory that you need
> HIGHMEM to use it all.  I think it also was SMP.  So a custom kernel for
> *that* machine would certainly include SMP and HIGHMEM ...

I have a UP machine with 512M of RAM, but I wasn't going to take it out just 
to prove the point.  Hence I used my test machine with mem=880 maxcpus=1 to 
simulate it, but that's a distraction here.

> While it might be
> interesting by itself to see what the overhead of these config options
> is, it is IMHO quite pointless *in the context of this discussion*.

No, you completely missed the point.
Rusty.
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