Re: CFQ iosched praise: good perfomance and better latency

[Nick Piggin]

Andrew Morton wrote:
> Nick Piggin <nickpiggin@xxxxxxxxxxxx> wrote:
>
> > Pedro Larroy wrote:
> >
> > > Hi
> > >
> > > I've been trying CFQ ioscheduler in my software raid5 with nice results,
> > > I've observed that a latency pattern still exists, just as in the
> > > anticipatory ioscheduler, but those spikes are now much lower (from
> > > 6ms with AS to 2ms with CFQ as seen in the bottom of
> > > http://pedro.larroy.com/devel/iolat/analisys/),
> > > plus apps seems to get a fair amount of io so they don't get starved.
> > >
> > > Seems a good choice for io loaded boxes. Thanks Jens Axboe.
> >
> > Although AS isn't at its best when behind raid devices (it should
> > probably be in front of them), you could be seeing some problem
> > with the raid code.
> >
> > I'd be interested to see what the graph looks like with elevator=noop
>
>
> This isn't a very surprising result, is it?  AS throws away latency to gain
> throughput.  Pedro is measuring latency...

Well I think Pedro actually means *seconds*, not ms. The URL
shows AS peaks at nearly 10 seconds latency, and CFQ over 2s.

It really seems like a raid problem though, because latency
measured at the individual devices is under 250ms for AS.
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/