Re: [patch 3/3]cfq-iosched: don't idle if a deep seek queue is slow

[Jens Axboe <jaxboe@fusionio.com>]

On 2010-11-08 15:20, Vivek Goyal wrote:
> On Mon, Nov 08, 2010 at 10:07:25AM +0800, Shaohua Li wrote:
>> If a deep seek queue slowly deliver requests but disk is much faster, idle
>> for the queue just wastes disk throughput. If the queue delevers all requests
>> before half its slice is used, the patch disable idle for it.
>> In my test, application delivers 32 requests one time, the disk can accept
>> 128 requests at maxium and disk is fast. without the patch, the throughput
>> is just around 30m/s, while with it, the speed is about 80m/s. The disk is
>> a SSD, but is detected as a rotational disk. I can configure it as SSD, but
>> I thought the deep seek queue logic should be fixed too, for example,
>> considering a fast raid.
>>
> 
> Hi Shaohua,
> 
> So looks like you are trying to cut down queue idling in the case when
> device is fast and idling hurts. That's a noble goal, just that detetction
> of this condition only for deep queues does not seem to cover lots of
> cases. Manually one can set slice_idle=0 to handle this situation.
> 
> What about if you have lots of sequential queues (not deep) and they all
> will still idle.
> 
> Secondly, what if driver is just buffering lots of requests in its device
> queue and not necessarily device is processing the reuqests faster.

That is not a valid concern, a driver should never extract more than it
can process (pretty much) immediately.

> So I think it is a good idea to cut down on idling if we can find that
> underlying device is fast and idling on queue might hurt more. But
> discovering this only using deep queues does not sound very appleaing to
> me. This is help only a particular workload which is driving deep queues.
> So if there was a generic mechanism to tackle this, that would be much
> better.

Agree, we could use better metrics for this.

-- 
Jens Axboe