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

[Daniel Ehrenberg]

On Tue, Jun 21, 2011 at 1:41 PM, Andi Kleen <andi@xxxxxxxxxxxxxx> wrote:
> Dan Ehrenberg <dehrenberg@xxxxxxxxxx> writes:
>
>> This code introduces a fast-path variant of __blockdev_direct_IO
>> for the special case where the request size is a multiple of the page
>> size, the inode block size is a page, the user memory is page-aligned,
>> the underlying storage is contiguous on disk and the file location is
>> already initialized. The special case decreases the amount of
>> bookkeeping required, which saves a significant amount of CPU time on
>> a fast device such as a ramdisk or an SSD.  The patch is inspired by
>> earlier code by Ken Chen.
>
> Is it understood why your fast path is that much faster?
> i.e. what's the slow part in the normal path that it avoids?
>
> I am wondering if some of the improvements could be gotten even for less
> rigid pre conditions.

I should start by saying that I really should've submitted this with
an [RFC] tag. I'm eager for feedback on my first Linux kernel patch,
and I'm really glad you responded.

The slowness in the dio code that I have observed is not in any
particular place, but rather a death of a thousand cuts. Lines like
        memset(dio, 0, offsetof(struct dio, pages));
show up as significant in the CPU profile, but so do other random
lines that manipulate the struct dio.

In an earlier version of the patch, I restricted the change to only
page-sized operations. This was criticized for being insufficiently
general. In generalizing to page-multiple operations, I noticed a
minor regression, which seems to be from the IS_ALIGNED calls.

You're right that these preconditions are rather rigid, though. If you
have a suggestion for a more general precondition, I can try it out
and see if it maintains the performance properties I want.
>
>> +             /*
>> +              * The i_alloc_sem will be released at I/O completion,
>> +              * possibly in a different thread.
>> +              */
>> +             down_read_non_owner(&inode->i_alloc_sem);
>
> There's just a patch kit posted from hch which removes that semaphore.
>
> -Andi

Once this patch is finalized and merged, I can make a new version of
the patch based on the new synchronization mechanism.

Dan
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