Re: readahead: make context readahead more conservative
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 > > --- > > 7536disable context readahead totally > > w/ patch: 7129slower size rampup and start RA on the 3rd read > > 6717slower size rampup > > w/o patch: 5581unmodified 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=1\ > > --num-threads=1 \ > > --file-num=1\ > > --file-block-size=64K \ > > --file-test-mode=rndrd \ > > --file-fsync-freq=0 \ > > --file-fsync-end=offrun > > > > 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Re: readahead: make context readahead more conservative
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 > --- > 7536disable context readahead totally > w/ patch: 7129slower size rampup and start RA on the 3rd read > 6717slower size rampup > w/o patch:5581unmodified 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=1\ > --num-threads=1 \ > --file-num=1\ > --file-block-size=64K \ > --file-test-mode=rndrd \ > --file-fsync-freq=0 \ > --file-fsync-end=offrun > > 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 > Tested-by: Miao Xie > Signed-off-by: Wu Fengguang > --- > mm/readahead.c |8 > 1 file changed, 4 insertions(+), 4 deletions(-) > > --- linux-next.orig/mm/readahead.c2013-08-08 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; > } > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
readahead: make context readahead more conservative
This helps performance on moderately dense random reads on SSD. Transaction-Per-Second numbers provided by Taobao: QPS case --- 7536disable context readahead totally w/ patch: 7129slower size rampup and start RA on the 3rd read 6717slower size rampup w/o patch: 5581unmodified 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=1\ --num-threads=1 \ --file-num=1\ --file-block-size=64K \ --file-test-mode=rndrd \ --file-fsync-freq=0 \ --file-fsync-end=offrun shows the performance of btrfs grows up from 69MB/s to 121MB/s, ext4 from 104MB/s to 121MB/s. Tested-by: Tao Ma Tested-by: Miao Xie Signed-off-by: Wu Fengguang --- 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; } -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
