readahead: make context readahead more conservative

readahead: make context readahead more conservative

[Fengguang Wu]

This helps performance on moderately dense random reads on SSD.

Transaction-Per-Second numbers provided by Taobao:

		QPS	case
		-------------------------------------------------------
		7536	disable context readahead totally
w/ patch:	7129	slower size rampup and start RA on the 3rd read
		6717	slower size rampup
w/o patch:	5581	unmodified context readahead

Before, readahead will be started whenever reading page N+1 when it
happen to read N recently. After patch, we'll only start readahead
when *three* random reads happen to access pages N, N+1, N+2. The
probability of this happening is extremely low for pure random reads,
unless they are very dense, which actually deserves some readahead.

Also start with a smaller readahead window. The impact to interleaved
sequential reads should be small, because for a long run stream, the
the small readahead window rampup phase is negletable.

The context readahead actually benefits clustered random reads on HDD
whose seek cost is pretty high. However as SSD is increasingly used
for random read workloads it's better for the context readahead to
concentrate on interleaved sequential reads.

Another SSD rand read test from Miao

        # file size:        2GB
        # read IO amount: 625MB
        sysbench --test=fileio          \
                --max-requests=10000    \
                --num-threads=1         \
                --file-num=1            \
                --file-block-size=64K   \
                --file-test-mode=rndrd  \
                --file-fsync-freq=0     \
                --file-fsync-end=off    run

shows the performance of btrfs grows up from 69MB/s to 121MB/s,
ext4 from 104MB/s to 121MB/s.

Tested-by: Tao Ma <tm@tao.ma>
Tested-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
---
 mm/readahead.c |    8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

--- linux-next.orig/mm/readahead.c	2013-08-08 16:21:29.675286154 +0800
+++ linux-next/mm/readahead.c	2013-08-08 16:21:33.851286019 +0800
@@ -371,10 +371,10 @@ static int try_context_readahead(struct
 	size = count_history_pages(mapping, ra, offset, max);
 
 	/*
-	 * no history pages:
+	 * not enough history pages:
 	 * it could be a random read
 	 */
-	if (!size)
+	if (size <= req_size)
 		return 0;
 
 	/*
@@ -385,8 +385,8 @@ static int try_context_readahead(struct
 		size *= 2;
 
 	ra->start = offset;
-	ra->size = get_init_ra_size(size + req_size, max);
-	ra->async_size = ra->size;
+	ra->size = min(size + req_size, max);
+	ra->async_size = 1;
 
 	return 1;
 }

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Re: readahead: make context readahead more conservative

[Miao Xie]

Hi, everyone

On Thu, 8 Aug 2013 16:54:18 +0800, Fengguang Wu wrote:
> This helps performance on moderately dense random reads on SSD.
> 
> Transaction-Per-Second numbers provided by Taobao:
> 
> 		QPS	case
> 		-------------------------------------------------------
> 		7536	disable context readahead totally
> w/ patch:	7129	slower size rampup and start RA on the 3rd read
> 		6717	slower size rampup
> w/o patch:	5581	unmodified context readahead
> 
> Before, readahead will be started whenever reading page N+1 when it
> happen to read N recently. After patch, we'll only start readahead
> when *three* random reads happen to access pages N, N+1, N+2. The
> probability of this happening is extremely low for pure random reads,
> unless they are very dense, which actually deserves some readahead.
> 
> Also start with a smaller readahead window. The impact to interleaved
> sequential reads should be small, because for a long run stream, the
> the small readahead window rampup phase is negletable.
> 
> The context readahead actually benefits clustered random reads on HDD
> whose seek cost is pretty high. However as SSD is increasingly used
> for random read workloads it's better for the context readahead to
> concentrate on interleaved sequential reads.
> 
> Another SSD rand read test from Miao
> 
>         # file size:        2GB
>         # read IO amount: 625MB
>         sysbench --test=fileio          \
>                 --max-requests=10000    \
>                 --num-threads=1         \
>                 --file-num=1            \
>                 --file-block-size=64K   \
>                 --file-test-mode=rndrd  \
>                 --file-fsync-freq=0     \
>                 --file-fsync-end=off    run
> 
> shows the performance of btrfs grows up from 69MB/s to 121MB/s,
> ext4 from 104MB/s to 121MB/s.

I did the same test on the hard disk recently,
for btrfs, there is ~5% regression(10.65MB/s -> 10.09MB/s),
for ext4, the performance grows up a bit.(9.98MB/s -> 10.04MB/s).
(I run the test for 4 times, and the above result is the average of the test.)

Any comment?

Thanks
Miao

> 
> Tested-by: Tao Ma <tm@tao.ma>
> Tested-by: Miao Xie <miaox@cn.fujitsu.com>
> Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
> ---
>  mm/readahead.c |    8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> --- linux-next.orig/mm/readahead.c	2013-08-08 16:21:29.675286154 +0800
> +++ linux-next/mm/readahead.c	2013-08-08 16:21:33.851286019 +0800
> @@ -371,10 +371,10 @@ static int try_context_readahead(struct
>  	size = count_history_pages(mapping, ra, offset, max);
>  
>  	/*
> -	 * no history pages:
> +	 * not enough history pages:
>  	 * it could be a random read
>  	 */
> -	if (!size)
> +	if (size <= req_size)
>  		return 0;
>  
>  	/*
> @@ -385,8 +385,8 @@ static int try_context_readahead(struct
>  		size *= 2;
>  
>  	ra->start = offset;
> -	ra->size = get_init_ra_size(size + req_size, max);
> -	ra->async_size = ra->size;
> +	ra->size = min(size + req_size, max);
> +	ra->async_size = 1;
>  
>  	return 1;
>  }
> 

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Re: readahead: make context readahead more conservative

[Fengguang Wu]

On Mon, Aug 19, 2013 at 09:59:09AM +0800, Miao Xie wrote:
> Hi, everyone
> 
> On Thu, 8 Aug 2013 16:54:18 +0800, Fengguang Wu wrote:
> > This helps performance on moderately dense random reads on SSD.
> > 
> > Transaction-Per-Second numbers provided by Taobao:
> > 
> > 		QPS	case
> > 		-------------------------------------------------------
> > 		7536	disable context readahead totally
> > w/ patch:	7129	slower size rampup and start RA on the 3rd read
> > 		6717	slower size rampup
> > w/o patch:	5581	unmodified context readahead
> > 
> > Before, readahead will be started whenever reading page N+1 when it
> > happen to read N recently. After patch, we'll only start readahead
> > when *three* random reads happen to access pages N, N+1, N+2. The
> > probability of this happening is extremely low for pure random reads,
> > unless they are very dense, which actually deserves some readahead.
> > 
> > Also start with a smaller readahead window. The impact to interleaved
> > sequential reads should be small, because for a long run stream, the
> > the small readahead window rampup phase is negletable.
> > 
> > The context readahead actually benefits clustered random reads on HDD
> > whose seek cost is pretty high. However as SSD is increasingly used
> > for random read workloads it's better for the context readahead to
> > concentrate on interleaved sequential reads.
> > 
> > Another SSD rand read test from Miao
> > 
> >         # file size:        2GB
> >         # read IO amount: 625MB
> >         sysbench --test=fileio          \
> >                 --max-requests=10000    \
> >                 --num-threads=1         \
> >                 --file-num=1            \
> >                 --file-block-size=64K   \
> >                 --file-test-mode=rndrd  \
> >                 --file-fsync-freq=0     \
> >                 --file-fsync-end=off    run
> > 
> > shows the performance of btrfs grows up from 69MB/s to 121MB/s,
> > ext4 from 104MB/s to 121MB/s.
> 
> I did the same test on the hard disk recently,
> for btrfs, there is ~5% regression(10.65MB/s -> 10.09MB/s),
> for ext4, the performance grows up a bit.(9.98MB/s -> 10.04MB/s).
> (I run the test for 4 times, and the above result is the average of the test.)
> 
> Any comment?

Thanks for the tests! Minor regressions on the HDD cases are expected.

Since random read workloads are migrating to SSD as it becomes cheaper
and larger, it seems a good tradeoff to optimize for random read
performance on SSD.

Thanks,
Fengguang

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