Re: [PATCH] readahead:add blk_run_backing_dev

[Wu Fengguang]

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@xxxxxxxxxxxxx>
> >> > 
> >> > 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@xxxxxxxxxxxxx>
> 
> 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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