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    SubjectRe: [PATCH] readahead:add blk_run_backing_dev

    At 11:51 09/05/20, Wu Fengguang wrote:
    >On Mon, May 18, 2009 at 07:53:00PM +0200, Jens Axboe wrote:
    >> On Mon, May 18 2009, Hisashi Hifumi wrote:
    >> > Hi.
    >> >
    >> > I wrote a patch that adds blk_run_backing_dev on page_cache_async_readahead
    >> > 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 <hifumi.hisashi@oss.ntt.co.jp>
    >> >
    >> > diff -Nrup linux-2.6.30-rc6.org/mm/readahead.c
    >linux-2.6.30-rc6.unplug/mm/readahead.c
    >> > --- 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);
    >>
    >> 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 <hifumi.hisashi@oss.ntt.co.jp>

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




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