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

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Date:	Tue, 9 Jun 2009 11:39:18 +0200
From:	Nick Piggin <npiggin@...e.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	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>,
	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 Thu, Jun 04, 2009 at 08:02:14AM -0700, Linus Torvalds wrote:
> 
> 
> On Thu, 4 Jun 2009, Rusty Russell wrote:
> > >
> > > Turn off HIGHMEM64G, please (and HIGHMEM4G too, for that matter - you
> > > can't compare it to a no-highmem case).
> > 
> > Thanks, your point is demonstrated below.  I don't think HIGHMEM4G is 
> > unreasonable for a distro tho, so I turned that on instead.
> 
> Well, I agree that HIGHMEM4G is a _reasonable_ thing to turn on.
> 
> The thing I disagree with is that it's at all valid to then compare to 
> some all-software feature thing. HIGHMEM doesn't expand any esoteric 
> capability that some people might use - it's about regular RAM for regular 
> users.
> 
> And don't get me wrong - I don't like HIGHMEM. I detest the damn thing. I 
> hated having to merge it, and I still hate it. It's a stupid, ugly, and 
> very invasive config option. It's just that it's there to support a 
> stupid, ugly and very annoying fundamental hardware problem.

I was looking forward to be able to get rid of it... unfortunately
other 32-bit architectures are starting to use it again :(

I guess it is not incredibly intrusive for generic mm code. A bit
of kmap sprinkled around which is actually quite a useful delimiter
of where pagecache is addressed via its kernel mapping.

Do you hate more the x86 code? Maybe that can be removed?

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