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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