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    SubjectRe: [PATCH] readahead:add blk_run_backing_dev

    At 10:05 09/05/22, Wu Fengguang wrote:
    >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);

    Hi.
    I tested above patch, and I got same performance number.
    I wonder why if (PageUptodate(page)) check is there...




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