Messages in this thread Patch in this message | | | Date | Fri, 29 May 2009 14:35:55 +0900 | From | Hisashi Hifumi <> | Subject | [RESEND] [PATCH] readahead:add blk_run_backing_dev |
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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)) 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)
Thanks.
Signed-off-by: Hisashi Hifumi <hifumi.hisashi@oss.ntt.co.jp> Acked-by: Wu Fengguang <fengguang.wu@intel.com>
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); } EXPORT_SYMBOL_GPL(page_cache_async_readahead);
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