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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Message-ID: <alpine.LFD.2.00.1003231037410.18017@i5.linux-foundation.org>
Date:	Tue, 23 Mar 2010 10:45:08 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ingo Molnar <mingo@...e.hu>
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

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

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.

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.

			Linus
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