linux-kernel - Re: [PATCH] dio: Fast-path for page-aligned IOs

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Message-ID: <x49vcuns73a.fsf@segfault.boston.devel.redhat.com>
Date:	Wed, 27 Jul 2011 18:15:53 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Dan Ehrenberg <dehrenberg@...gle.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] dio: Fast-path for page-aligned IOs

Andrew Morton <akpm@...ux-foundation.org> writes:

> On Wed, 27 Jul 2011 17:08:28 -0400
> Christoph Hellwig <hch@...radead.org> wrote:
>
>> On Mon, Jun 20, 2011 at 04:17:35PM -0700, Dan Ehrenberg wrote:
>> > The fast path does not apply for operations of the wrong size
>> > or alignmnent, or for operations on raw drives with 512-byte sectors.
>> > It might be possible to make this special case a little more general
>> > while maintaining its performance benefits, but I do not believe that
>> > the full performance benefits can be achieved without resorting to
>> > special handling of simple cases, as is done in this patch.
>> 
>> Did you check how this compares to Andis small optimizations?
>> 
>> Also operations on raw disks are something people with fast devices
>> care about a lot.  We often hear about benchmark regressions due to
>> stupid little things in the direct I/O code.
>> 
>> If we want to special case something that would be a very easy target,
>> with a 1:1 mapping of logical to physical blocks and thus no need
>> to call the allocator first, and no need for any kind of locking
>> or alignment handling.
>
> Ken did this back in 2006
> (http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commitdiff;h=e61c90188b9956edae1105eef361d8981a352fcd)
> but we reverted that shortly afterwards for some reason.

For this reason:

commit b2e895dbd80c420bfc0937c3729b4afe073b3848
Author: Andrew Morton <akpm@...l.org>
Date:   Sat Feb 3 01:14:01 2007 -0800

    [PATCH] revert blockdev direct io back to 2.6.19 version
    
    Andrew Vasquez is reporting as-iosched oopses and a 65% throughput
    slowdown due to the recent special-casing of direct-io against
    blockdevs.  We don't know why either of these things are occurring.
    
    The patch minimally reverts us back to the 2.6.19 code for a 2.6.20
    release.
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