[lkml]   [2011]   [Jun]   [21]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
SubjectRe: [PATCH] dio: Fast-path for page-aligned IOs
On Tue, Jun 21, 2011 at 1:41 PM, Andi Kleen <> wrote:
> Dan Ehrenberg <> 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.

To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to
More majordomo info at
Please read the FAQ at

 \ /
  Last update: 2011-06-21 23:25    [W:0.064 / U:0.148 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site