Re: [PATCH] readahead:add blk_run_backing_dev

Hisashi Hifumi
2009-05-22T01:48:51Z

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...