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

[Andrew Morton]

On Wed, 27 Jul 2011 17:08:28 -0400
Christoph Hellwig <hch@xxxxxxxxxxxxx> 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.

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