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

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Message-Id: <1244615354.13761.11388.camel@twins>
Date:	Wed, 10 Jun 2009 08:29:14 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Nick Piggin <npiggin@...e.de>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Ingo Molnar <mingo@...e.hu>,
	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>,
	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, 2009-06-09 at 09:26 -0700, Linus Torvalds wrote:
> 
> On Tue, 9 Jun 2009, Nick Piggin wrote:
> > 
> > The idea seems nice but isn't the problem that kmap gives back a
> > basically 1st class kernel virtual memory? (ie. it can then be used
> > by any other CPU at any point without it having to use kmap?).
> 
> No, everybody has to use kmap()/kunmap().
> 
> The "problem" is that you could in theory run out of kmap frames, since if 
> everybody does a kmap() in an interruptible context and you have lots and 
> lots of threads doing different pages, you'd run out. But that has nothing 
> to do with kmap_atomic(), which is basically limited to just the number of 
> CPU's and a (very small) level of nesting.

One of the things I did for -rt back when I rewrote mm/highmem.c for it
was to reserve multiple slots per kmap() user so that if you did 1 you
could always do another.

With everything in task context like rt does 2 seemed enough, but you
cuold ways extend that scheme and reserve enough for the worst case
nesting depth and be done with it.

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