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

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Message-Id: <20090922135838.33ebe36b.akpm@linux-foundation.org>
Date:	Tue, 22 Sep 2009 13:58:38 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Hisashi Hifumi <hifumi.hisashi@....ntt.co.jp>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org, Wu Fengguang <fengguang.wu@...el.com>
Subject: Re: [RESEND] [PATCH] readahead:add blk_run_backing_dev

On Fri, 29 May 2009 14:35:55 +0900
Hisashi Hifumi <hifumi.hisashi@....ntt.co.jp> wrote:

> I added blk_run_backing_dev on page_cache_async_readahead
> so readahead I/O is unpluged to improve throughput on 
> especially RAID environment. 

I still haven't sent this upstream.  It's unclear to me that we've
decided that it merits merging?



From: Hisashi Hifumi <hifumi.hisashi@....ntt.co.jp>

I added blk_run_backing_dev on page_cache_async_readahead so readahead I/O
is unpluged to improve throughput on especially RAID environment.

The normal case is, if page N become uptodate at time T(N), then T(N) <=
T(N+1) holds.  With RAID (and NFS to some degree), there is no strict
ordering, the data arrival time depends on runtime status of individual
disks, which breaks that formula.  So in do_generic_file_read(), just
after submitting the async readahead IO request, the current page may well
be uptodate, so the page won't be locked, and the block device won't be
implicitly unplugged:

               if (PageReadahead(page))
                        page_cache_async_readahead()
                if (!PageUptodate(page))
                                goto page_not_up_to_date;
                //...
page_not_up_to_date:
                lock_page_killable(page);

Therefore explicit unplugging can help.

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

(7Disks RAID-0 Array)

-2.6.30-rc6
1054976+0 records in
1054976+0 records out
17284726784 bytes (17 GB) copied, 212.233 seconds, 81.4 MB/s

-2.6.30-rc6-patched
1054976+0 records out
17284726784 bytes (17 GB) copied, 198.878 seconds, 86.9 MB/s

(7Disks RAID-5 Array)

The patch was found to improve performance with the SCST scsi target
driver.  See
http://sourceforge.net/mailarchive/forum.php?thread_name=a0272b440906030714g67eabc5k8f847fb1e538cc62%40mail.gmail.com&forum_name=scst-devel

[akpm@...ux-foundation.org: unbust comment layout]
[akpm@...ux-foundation.org: "fix" CONFIG_BLOCK=n]
Signed-off-by: Hisashi Hifumi <hifumi.hisashi@....ntt.co.jp>
Acked-by: Wu Fengguang <fengguang.wu@...el.com>
Cc: Jens Axboe <jens.axboe@...cle.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Tested-by: Ronald <intercommit@...il.com>
Cc: Bart Van Assche <bart.vanassche@...il.com>
Cc: Vladislav Bolkhovitin <vst@...b.net>
Cc: Randy Dunlap <randy.dunlap@...cle.com>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
---

 mm/readahead.c |   12 ++++++++++++
 1 file changed, 12 insertions(+)

diff -puN mm/readahead.c~readahead-add-blk_run_backing_dev mm/readahead.c
--- a/mm/readahead.c~readahead-add-blk_run_backing_dev
+++ a/mm/readahead.c
@@ -547,5 +547,17 @@ page_cache_async_readahead(struct addres
 
 	/* do read-ahead */
 	ondemand_readahead(mapping, ra, filp, true, offset, req_size);
+
+#ifdef CONFIG_BLOCK
+	/*
+	 * Normally the current page is !uptodate and lock_page() will be
+	 * immediately called to implicitly unplug the device. However this
+	 * is not always true for RAID conifgurations, where data arrives
+	 * not strictly in their submission order. In this case we need to
+	 * explicitly kick off the IO.
+	 */
+	if (PageUptodate(page))
+		blk_run_backing_dev(mapping->backing_dev_info, NULL);
+#endif
 }
 EXPORT_SYMBOL_GPL(page_cache_async_readahead);
_


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