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

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Message-Id: <20080506102153.5484c6ac.akpm@linux-foundation.org>
Date:	Tue, 6 May 2008 10:21:53 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Matthew Wilcox <matthew@....cx>
Cc:	Ingo Molnar <mingo@...e.hu>,
	"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>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-fsdevel@...r.kernel.org
Subject: Re: AIM7 40% regression with 2.6.26-rc1

On Tue, 6 May 2008 10:23:32 -0600 Matthew Wilcox <matthew@....cx> wrote:

> On Tue, May 06, 2008 at 06:09:34AM -0600, Matthew Wilcox wrote:
> > So the only likely things I can see are:
> > 
> >  - file locks
> >  - fasync
> 
> I've wanted to fix file locks for a while.  Here's a first attempt.

Do we actually know that the locks code is implicated in this regression?

I'd initially thought "lseek" but afaict tmpfs doesn't hit default_llseek()
or remote_llseek().

tmpfs tends to do weird stuff - it would be interesting to know if the
regression is also present on ramfs or ext2/ext3/xfs/etc.

It would be interesting to see if the context switch rate has increased.

Finally: how come we regressed by swapping the semaphore implementation
anyway?  We went from one sleeping lock implementation to another - I'd
have expected performance to be pretty much the same.

<looks at the implementation>

down(), down_interruptible() and down_try() should use spin_lock_irq(), not
irqsave.

up() seems to be doing wake-one, FIFO which is nice.  Did the
implementation which we just removed also do that?  Was it perhaps
accidentally doing LIFO or something like that?

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