linux-kernel - Re: [Bugme-new] [Bug 15618] New: 2.6.18->2.6.32->2.6.33 huge regression in performance

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Date:	Tue, 23 Mar 2010 19:00:02 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, bugzilla-daemon@...zilla.kernel.org,
	bugme-daemon@...zilla.kernel.org, ant.starikov@...il.com,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [Bugme-new] [Bug 15618] New: 2.6.18->2.6.32->2.6.33 huge
 regression in performance


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Tue, 23 Mar 2010, Ingo Molnar wrote:
> > 
> > It shows a very brutal amount of page fault invoked mmap_sem spinning 
> > overhead.
> 
> Isn't this already fixed? It's the same old "x86-64 rwsemaphores are using 
> the shit-for-brains generic version" thing, and it's fixed by
> 
> 	1838ef1 x86-64, rwsem: 64-bit xadd rwsem implementation
> 	5d0b723 x86: clean up rwsem type system
> 	59c33fa x86-32: clean up rwsem inline asm statements

Ah, indeed!

> NOTE! None of those are in 2.6.33 - they were merged afterwards. But they 
> are in 2.6.34-rc1 (and obviously current -git). So Anton would have to 
> compile his own kernel to test his load.

another option is to run the rawhide kernel via something like:

	yum update --enablerepo=development kernel

this will give kernel-2.6.34-0.13.rc1.git1.fc14.x86_64, which has those 
changes included.

OTOH that kernel has debugging [lockdep] enabled so it might not be 
comparable.

> We could mark them as stable material if the load in question is a real load 
> rather than just a test-case. On one of the random page-fault benchmarks the 
> rwsem fix was something like a 400% performance improvement, and it was 
> apparently visible in real life on some crazy SGI "initialize huge heap 
> concurrently on lots of threads" load.
> 
> Side note: the reason the spinlock sucks is because of the fair ticket 
> locks, it really does all the wrong things for the rwsem code. That's why 
> old kernels don't show it - the old unfair locks didn't show the same kind 
> of behavior.

Yeah.

	Ingo
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