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

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Message-ID: <m2oc1q3okc.fsf@firstfloor.org>
Date:	Tue, 21 Jun 2011 13:41:07 -0700
From:	Andi Kleen <andi@...stfloor.org>
To:	Dan Ehrenberg <dehrenberg@...gle.com>
Cc:	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] dio: Fast-path for page-aligned IOs

Dan Ehrenberg <dehrenberg@...gle.com> 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.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only
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