linux-kernel - Re: Re-tune x86 uaccess code for PREEMPT

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Message-ID: <1376194657.7006.11.camel@marge.simpson.net>
Date:	Sun, 11 Aug 2013 06:17:37 +0200
From:	Mike Galbraith <bitbucket@...ine.de>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
	x86@...nel.org, mingo@...nel.org, torvalds@...ux-foundation.org
Subject: Re: Re-tune x86 uaccess code for PREEMPT_VOLUNTARY

On Sat, 2013-08-10 at 09:09 -0700, H. Peter Anvin wrote: 
> On 08/09/2013 10:55 PM, Mike Galbraith wrote:
> >>
> >> Now, here is a bigger question: shouldn't we be deprecating/getting rid
> >> of PREEMPT_VOUNTARY in favor of PREEMPT?
> > 
> > I sure hope not, PREEMPT munches throughput.  If you need PREEMPT, seems
> > to me what you _really_ need is PREEMPT_RT (the real deal), so
> > eventually depreciating PREEMPT makes more sense to me.
> > 
> 
> Do you have any quantification of "munches throughput?"  It seems odd
> that it would be worse than polling for preempt all over the kernel, but
> perhaps the additional locking is what costs.

I hadn't compared in ages, so made some fresh samples.

Q6600 3.11-rc4

vmark
voluntary     169808     155826     154741     1.000
preempt       149354     124016     128436      .836

That should be ~worst case, it hates preemption. 

tbench 8
voluntary    1027.96    1028.76    1044.60     1.000
preempt       929.06     935.01     928.64      .900

hackbench -l 10000
voluntary     23.146     23.124     23.230     1.000
preempt       25.065     24.633     24.789     1.071

kbuild vmlinux
voluntary  3m44.842s  3m42.975s  3m42.954s     1.000
preempt    3m46.141s  3m45.835s  3m45.953s     1.010

Compute load comparisons are boring 'course.

	-Mike

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