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@xxxxxx> > Tested-by: Miao Xie <miaox@xxxxxxxxxxxxxx> > Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx> > --- > 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, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>