Re: [PATCH] readahead:add blk_run_backing_dev

[Hisashi Hifumi <hifumi.hisashi@oss.ntt.co.jp>]

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...