Re: CFQ iosched praise: good perfomance and better latency

[Andrew Morton]

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...
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