linux-kernel - Re: AIM7 40% regression with 2.6.26-rc1

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Date:	Wed, 7 May 2008 10:31:59 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ingo Molnar <mingo@...e.hu>
cc:	Matthew Wilcox <matthew@....cx>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"J. Bruce Fields" <bfields@...i.umich.edu>,
	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Alexander Viro <viro@....linux.org.uk>,
	linux-fsdevel@...r.kernel.org
Subject: Re: AIM7 40% regression with 2.6.26-rc1

On Wed, 7 May 2008, Ingo Molnar wrote:
> 
> There's far more normal mutex fastpath use during an AIM7 run than any 
> BKL use. So if it's due to any direct fastpath overhead and the 
> resulting widening of the window for the real slowdown, we should see a 
> severe slowdown on AIM7 with CONFIG_MUTEX_DEBUG=y. Agreed?

Not agreed.

The BKL is special because it is a *single* lock.

All the "normal" mutex code use fine-grained locking, so even if you slow 
down the fast path, that won't cause the same kind of fastpath->slowpath 
increase.

In order to see the fastpath->slowpath thing, you do need to have many 
threads hitting the same lock: ie the slowdown has to result in real 
contention. 

Almost no mutexes have any potential for contention what-so-ever, except 
for things that very consciously try to hit it (multiple threads doing 
readdir and/or file creation on the *same* directory etc).

The BKL really is special. 

		Linus
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