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

Jens Axboe
2007-02-27T19:26:08Z

On Tue, Feb 27 2007, Evgeniy Polyakov wrote:
> On Tue, Feb 27, 2007 at 07:45:41PM +0100, Jens Axboe (jens.axboe@oracle.com) wrote:
> > > Deadline shows this:
> > >
> > > sync:
> > > READ: io=1,024MiB, aggrb=38,212KiB/s, minb=38,212KiB/s,
> > > maxb=38,212KiB/s, mint=28099msec, maxt=28099msec
> > >
> > > libaio:
> > > READ: io=1,024MiB, aggrb=37,933KiB/s, minb=37,933KiB/s,
> > > maxb=37,933KiB/s, mint=28306msec, maxt=28306msec
> > >
> > > syslet-rw:
> > > READ: io=1,024MiB, aggrb=34,759KiB/s, minb=34,759KiB/s,
> > > maxb=34,759KiB/s, mint=30891msec, maxt=30891msec
> > >
> > > There were about 10k async schedulings.
> >
> > I think the issue here is pretty simple - when fio gets a queue full
> > like condition (it reaches the depth you set, 32), it commits them and
> > starts queuing again. Since that'll likely block, it'll get issued by
> > another process. So you suddenly have a nice sequence of reads from one
> > process (pending, only one is actually committed since it's serialized),
> > and then a read further down the line that goes behind those you already
> > committed. Then result is seeky, where it should have been sequential.
> >
> > Do you get expected results if you set iodepth_low=1? That'll make fio
> > drain the queue before building it up again, should get you a sequential
> > access pattern with syslets.
>
> With such a change results should be better - not only because seek is
> removed with sequential read, but also number of working threads
> decreases with time - until queue is filled again.

Yep, although it probably doesn't matter for such a low bandwidth test
anyway.

> So, syslet-rw has increased to 37mb/sec out of 39/sync and 38/libaio,
> the latter two did not changed.

I wonder why all three aren't doing 39mb/sec flat here, it's a pretty
trivial case...

> With iodepth of 10k, I get the same performance for
> libaio and syslets - about 36mb/sec, it does not depend on iodepth_low
> being set to 1 or default (full).

Yep, the larger the iodepth, the less costly a seek on new queue buildup
gets. So that is as expected.

> So syslets have small problems with small number of iodepth - its
> performance is about 34mb/sec and then increases to 36 with iodepth
> grow. While libaio decreases from 38 down to 36 mb/sec.

Using your job file and fio HEAD (forces iodepth_low=1 for syslet if
iodepth_low isn't specified), I get:

Engine Depth Bw (kb/sec)
-----------------------------------
syslet 1 37163
syslet 32 37197
syslet 10000 36577
libaio 1 37144
libaio 32 37159
libaio 10000 36463
sync 1 37154

Results are highly stable. Note that this test case isn't totally fair,
since libaio isn't really async when you do buffered file IO.

--
Jens Axboe