Messages in this thread | | | Subject | Re: [patch 3/3]cfq-iosched: don't idle if a deep seek queue is slow | From | Shaohua Li <> | Date | Tue, 09 Nov 2010 09:36:42 +0800 |
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On Mon, 2010-11-08 at 23:06 +0800, Jens Axboe wrote: > 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. Agree we'd better have a better method to measure device speed, but this seems not easy. Even in a fast device, a request might take long time to finish when NCQ is enabled. Before we have generic mechanism, we still need fix some particular cases.
Thanks, Shaohua
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