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    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.

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      Last update: 2011-06-21 23:25    [W:0.023 / U:18.944 seconds]
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