On Thu, May 21, 2009 at 02:01:47PM +0800, Hisashi Hifumi wrote:

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At 11:51 09/05/20, Wu Fengguang wrote:

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On Mon, May 18, 2009 at 07:53:00PM +0200, Jens Axboe wrote:

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On Mon, May 18 2009, Hisashi Hifumi wrote:

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Hi.

I wrote a patch that adds blk_run_backing_dev on 

page_cache_async_readahead

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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 <redacted>

diff -Nrup linux-2.6.30-rc6.org/mm/readahead.c 

linux-2.6.30-rc6.unplug/mm/readahead.c

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--- linux-2.6.30-rc6.org/mm/readahead.c	2009-05-18 

10:46:15.000000000 +0900

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+++ linux-2.6.30-rc6.unplug/mm/readahead.c	2009-05-18 

13:00:42.000000000 +0900

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@@ -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.

The possible timing change of this patch is (assuming readahead size=100):

T0   read(100), which triggers readahead(200, 100)
T1   read(101)
T2   read(102)
...
T100 read(200), find_get_page(200) => readahead(300, 100)
               lock_page(200) => implicit unplug

The readahead(200, 100) submitted at time T0 *might* be delayed to the
unplug time of T100.

But that is only a possibility. In normal cases, the read(200) would
be blocking and there will be a lock_page(200) that will immediately
unplug device for readahead(300, 100).

Hi Andrew.
Following patch improves sequential read performance and does not harm
other performance.
Please merge my patch.
Comments?
Thanks.

#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

Signed-off-by: Hisashi Hifumi <redacted>

diff -Nrup linux-2.6.30-rc6.org/mm/readahead.c 

linux-2.6.30-rc6.unplug/mm/readahead.c

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--- 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);

Hi Hisashi,

I wonder if the following updated patch can achieve the same
performance.  Can you try testing this out?

Thanks,
Fengguang
---

diff --git a/mm/readahead.c b/mm/readahead.c
index 133b6d5..fd3df66 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c

@@ -490,5 +490,8 @@ page_cache_async_readahead(struct address_space *mapping,

	/* do read-ahead */
	ondemand_readahead(mapping, ra, filp, true, offset, req_size);
+
+	if (PageUptodate(page))
+		blk_run_backing_dev(mapping->backing_dev_info, NULL);		
}
EXPORT_SYMBOL_GPL(page_cache_async_readahead);