Re: A quick fio test (was Re: [patch 00/13] Syslets, "Threadlets", generic AIO support, v3)

Jens Axboe
2007-02-27T09:43:58Z

On Tue, Feb 27 2007, Suparna Bhattacharya wrote:
> On Mon, Feb 26, 2007 at 03:45:48PM +0100, Jens Axboe wrote:
> > On Mon, Feb 26 2007, Suparna Bhattacharya wrote:
> > > On Mon, Feb 26, 2007 at 02:57:36PM +0100, Jens Axboe wrote:
> > > >
> > > > Some more results, using a larger number of processes and io depths. A
> > > > repeat of the tests from friday, with added depth 20000 for syslet and
> > > > libaio:
> > > >
> > > > Engine Depth Processes Bw (MiB/sec)
> > > > ----------------------------------------------------
> > > > libaio 1 1 602
> > > > syslet 1 1 759
> > > > sync 1 1 776
> > > > libaio 32 1 832
> > > > syslet 32 1 898
> > > > libaio 20000 1 581
> > > > syslet 20000 1 609
> > > >
> > > > syslet still on top. Measuring O_DIRECT reads (of 4kb size) on ramfs
> > > > with 100 processes each with a depth of 200, reading a per-process
> > > > private file of 10mb (need to fit in my ram...) 10 times each. IOW,
> > > > doing 10,000MiB of IO in total:
> > >
> > > But, why ramfs ? Don't we want to exercise the case where O_DIRECT actually
> > > blocks ? Or am I missing something here ?
> >
> > Just overhead numbers for that test case, lets try something like your
> > described job.
> >
> > Test case is doing random reads from /dev/sdb, in chunks of 64kb:
> >
> > Engine Depth Processes Bw (KiB/sec)
> > ----------------------------------------------------
> > libaio 200 100 2813
> > syslet 200 100 3944
> > libaio 20000 1 2793
> > syslet 20000 1 3854
> > sync (*) 20000 1 2866
> >
> > deadline was used for IO scheduling, to minimize impact. Not sure why
> > syslet actually does so much better here, looing at vmstat the rate is
> > steady and all runs are basically 50/50 idle/wait. One difference is
> > that the submission itself takes a long time on libaio, since the
> > io_submit() will block on request allocation. The generated IO pattern
> > from each process is the same for all runs. The drive is a lousy sata
> > that doesn't even do queuing, FWIW.
>
>
> I tried the latest fio code with syslet v4, and my results are a little
> different - have yet to figure out why or what to make of it.
> I hope I have all the right pieces now.
>
> This is an ext2 filesystem, SCSI AIC7xxx.
>
> I used an iodepth_batch size of 8 to limit the number of ios in a single
> io_submit (thanks for adding that parameter to fio !), like we did in
> aio-stress.
>
> Engine Depth Batch Bw (KiB/sec)
> ----------------------------------------------------
> libaio 64 8 17,226
> syslet 64 8 17,620
> libaio 20000 8 18,552
> syslet 20000 8 14,935
>
>
> Which is not bad, actually.

It's not bad for such a high depth/batch setting, but I still wonder why
are results are so different. I'll look around for an x86 box with some
TCQ/NCQ enabled storage attached for testing. Can you pass me your
command line or job file (whatever you use) so we are on the same page?

> If I do not specify the iodepth_batch (i.e. default to depth), then the
> difference becomes more pronounced at higher depths. However, I doubt
> whether anyone would be using such high batch sizes in practice ...
>
> Engine Depth Batch Bw (KiB/sec)
> ----------------------------------------------------
> libaio 64 default 17,429
> syslet 64 default 16,155
> libaio 20000 default 15,494
> syslet 20000 default 7,971
>
If iodepth_batch isn't set, the syslet queued io will be serialized and
not take advantage of queueing. How does the job file perform with
ioengine=sync?

> Often times it is the application tuning that makes all the difference,
> so am not really sure how much to read into these results.
> That's always been the hard part of async io ...

Yes I agree, it's handy to get an overview though.

--
Jens Axboe