Re: CFQ iosched praise: good perfomance and better latency

[Pedro Larroy]

On Mon, Apr 19, 2004 at 04:12:56PM +1000, Nick Piggin wrote:
> 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.

Yes, I meant seconds, my mistake. I will be testing elevator=noop this
evening.

> 
> It really seems like a raid problem though, because latency
> measured at the individual devices is under 250ms for AS.

Probably. But I was surprised to find that bonnie gave similar results
with CFQ and with AS when benchmarking the swraid5.

Regards.

-- 
Pedro Larroy Tovar | Linux & Network consultant |  piotr%member.fsf.org 

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