Re: CFQ iosched praise: good perfomance and better latency

[Nick Piggin <nickpiggin@yahoo.com.au>]

Andrew Morton wrote:
> Nick Piggin <nickpiggin@yahoo.com.au> 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.