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SubjectRe: 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
> > -------------------------------------------------------
> > 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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