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Subject[RESEND] [PATCH] readahead:add blk_run_backing_dev
Hi Andrew.

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))
if (!PageUptodate(page))
goto page_not_up_to_date;

Therefore explicit unplugging can help.

Following is the test result with dd.

#dd if=testdir/testfile of=/dev/null bs=16384

1048576+0 records in
1048576+0 records out
17179869184 bytes (17 GB) copied, 224.182 seconds, 76.6 MB/s

1048576+0 records in
1048576+0 records out
17179869184 bytes (17 GB) copied, 206.465 seconds, 83.2 MB/s

(7Disks RAID-0 Array)

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

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

(7Disks RAID-5 Array)


Signed-off-by: Hisashi Hifumi <>
Acked-by: Wu Fengguang <>

mm/readahead.c | 10 ++++++++++
1 file changed, 10 insertions(+)

--- linux.orig/mm/readahead.c
+++ linux/mm/readahead.c
@@ -490,5 +490,15 @@ page_cache_async_readahead(struct addres

/* do read-ahead */
ondemand_readahead(mapping, ra, filp, true, offset, req_size);
+ /*
+ * 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);

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