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

From: Andi Kleen
Date: Tue Jun 21 2011 - 16:42:45 EST

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.

> + /*
> + * 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.


ak@xxxxxxxxxxxxxxx -- Speaking for myself only
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