linux-kernel - Re: [PATCH] Introduce a method to catch mmap_region (was: Recent kernel "mount" slow)

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Message-ID: <Pine.LNX.4.64.1211281705020.5283@file.rdu.redhat.com>
Date:	Wed, 28 Nov 2012 17:10:47 -0500 (EST)
From:	Mikulas Patocka <mpatocka@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Jens Axboe <axboe@...nel.dk>,
	Jeff Chua <jeff.chua.linux@...il.com>,
	Lai Jiangshan <laijs@...fujitsu.com>, Jan Kara <jack@...e.cz>,
	lkml <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH] Introduce a method to catch mmap_region (was: Recent
 kernel "mount" slow)



On Wed, 28 Nov 2012, Linus Torvalds wrote:

> On Wed, Nov 28, 2012 at 12:32 PM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > Here is a *COMPLETELY* untested patch. Caveat emptor. It will probably
> > do unspeakable things to your family and pets.
> 
> Btw, *if* this approach works, I suspect we could just switch the
> bd_block_size_semaphore semaphore to be a regular rw-sem.
> 
> Why? Because now it's no longer ever gotten in the cached IO paths, we
> only get it when we're doing much more expensive things (ie actual IO,
> and buffer head allocations etc etc). As long as we just work with the
> page cache, we never get to the whole lock at all.
> 
> Which means that the whole percpu-optimized thing is likely no longer
> all that relevant.

Using normal semaphore degrades direct-IO performance on raw devices, I 
measured that (45.1s with normal rw-semaphore vs. 42.8s with 
percpu-rw-semaphore). It could be measured on ramdisk or high-performance 
SSDs.

Mikulas
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