linux-kernel - Re: [PATCH] readahead:add blk_run_backing

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Date:	Mon, 18 May 2009 19:53:00 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Hisashi Hifumi <hifumi.hisashi@....ntt.co.jp>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] readahead:add blk_run_backing_dev

On Mon, May 18 2009, Hisashi Hifumi wrote:
> Hi.
> 
> I wrote a patch that adds blk_run_backing_dev on page_cache_async_readahead
> so readahead I/O is unpluged to improve throughput.
> 
> Following is the test result with dd.
> 
> #dd if=testdir/testfile of=/dev/null bs=16384
> 
> -2.6.30-rc6
> 1048576+0 records in
> 1048576+0 records out
> 17179869184 bytes (17 GB) copied, 224.182 seconds, 76.6 MB/s
> 
> -2.6.30-rc6-patched
> 1048576+0 records in
> 1048576+0 records out
> 17179869184 bytes (17 GB) copied, 206.465 seconds, 83.2 MB/s
> 
> Sequential read performance on a big file was improved.
> Please merge my patch.
> 
> Thanks.
> 
> Signed-off-by: Hisashi Hifumi <hifumi.hisashi@....ntt.co.jp>
> 
> diff -Nrup linux-2.6.30-rc6.org/mm/readahead.c linux-2.6.30-rc6.unplug/mm/readahead.c
> --- linux-2.6.30-rc6.org/mm/readahead.c	2009-05-18 10:46:15.000000000 +0900
> +++ linux-2.6.30-rc6.unplug/mm/readahead.c	2009-05-18 13:00:42.000000000 +0900
> @@ -490,5 +490,7 @@ page_cache_async_readahead(struct addres
>  
>  	/* do read-ahead */
>  	ondemand_readahead(mapping, ra, filp, true, offset, req_size);
> +
> +	blk_run_backing_dev(mapping->backing_dev_info, NULL);
>  }
>  EXPORT_SYMBOL_GPL(page_cache_async_readahead);

I'm surprised this makes much of a difference. It seems correct to me to
NOT unplug the device, since it will get unplugged when someone ends up
actually waiting for a page. And that will then kick off the remaining
IO as well. For this dd case, you'll be hitting lock_page() for the
readahead page really soon, definitely not long enough to warrant such a
big difference in speed.

So, are these numbers 100% reproducible? Could you capture blktrace data
for both with and without the patch, so we can take a closer look at the
generated IO for each case?

-- 
Jens Axboe

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