diff options
authorHisashi Hifumi <>2009-12-17 15:27:26 -0800
committerLinus Torvalds <>2009-12-17 15:45:32 -0800
commit65a80b4c61f5b5f6eb0f5669c8fb120893bfb388 (patch)
parent26b3c01f7debc1bbc3117bc9c9e016ca6f2e41d5 (diff)
readahead: add blk_run_backing_dev
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 [ unbust comment layout] [ "fix" CONFIG_BLOCK=n] Signed-off-by: Hisashi Hifumi <> Acked-by: Wu Fengguang <> Cc: Jens Axboe <> Cc: KOSAKI Motohiro <> Tested-by: Ronald <> Cc: Bart Van Assche <> Cc: Vladislav Bolkhovitin <> Cc: Randy Dunlap <> Signed-off-by: Andrew Morton <> Signed-off-by: Linus Torvalds <>
1 files changed, 12 insertions, 0 deletions
diff --git a/mm/readahead.c b/mm/readahead.c
index aa1aa2345235..033bc135a41f 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -547,5 +547,17 @@ page_cache_async_readahead(struct address_space *mapping,
/* 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);