Messages in this thread | | | Date | Mon, 08 Nov 2010 16:06:21 +0100 | From | Jens Axboe <> | Subject | Re: [patch 3/3]cfq-iosched: don't idle if a deep seek queue is slow |
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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
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