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    SubjectRe: [PATCH] readahead:add blk_run_backing_dev
    On Thu, May 21, 2009 at 02:01:47PM +0800, Hisashi Hifumi wrote:
    >
    > 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);
    >
    >

    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);

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