Messages in this thread | | | Subject | Re: [RFC 0/3]block: An IOPS based ioscheduler | From | Shaohua Li <> | Date | Tue, 17 Jan 2012 09:06:28 +0800 |
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On Mon, 2012-01-16 at 03:29 -0500, Vivek Goyal wrote: > On Mon, Jan 16, 2012 at 03:55:41PM +0800, Shaohua Li wrote: > > On Mon, 2012-01-16 at 02:11 -0500, Vivek Goyal wrote: > > > On Mon, Jan 16, 2012 at 12:36:30PM +0800, Shaohua Li wrote: > > > > On Sun, 2012-01-15 at 17:45 -0500, Vivek Goyal wrote: > > > > > On Mon, Jan 09, 2012 at 09:09:35AM +0800, Shaohua Li wrote: > > > > > > > > > > [..] > > > > > > > You need to present raw numbers and give us some idea of how close > > > > > > > those numbers are to raw hardware capability for us to have any idea > > > > > > > what improvements these numbers actually demonstrate. > > > > > > Yes, your guess is right. The hardware has limitation. 12 SSD exceeds > > > > > > the jbod capability, for both throughput and IOPS, that's why only > > > > > > read/write mixed workload impacts. I'll use less SSD in later tests, > > > > > > which will demonstrate the performance better. I'll report both raw > > > > > > numbers and fiops/cfq numbers later. > > > > > > > > > > If fiops number are better please explain why those numbers are better. > > > > > If you cut down on idling, it is obivious that you will get higher > > > > > throughput on these flash devices. CFQ does disable queue idling for > > > > > non rotational NCQ devices. If higher throughput is due to driving > > > > > deeper queue depths, then CFQ can do that too just by changing quantum > > > > > and disabling idling. > > > > it's because of quantum. Surely you can change the quantum, and CFQ > > > > performance will increase, but you will find CFQ is very unfair then. > > > > > > Why increasing quantum leads to CFQ being unfair? In terms of time it > > > still tries to be fair. > > we can dispatch a lot of requests to NCQ SSD with very small time > > interval. The disk can finish a lot of requests in small time interval > > too. The time is much smaller than 1 jiffy. Increasing quantum can lead > > a task dispatches request more faster and makes the accounting worse, > > because with small quantum the task needs wait to dispatch. you can > > easily verify this with a simple fio test. > > > > > That's a different thing that with NCQ, right > > > time measurement is not possible with requests from multiple queues > > > being in the driver/disk at the same time. So accouting in terms of > > > iops per queue might make sense. > > yes. > > > > > > > So I really don't understand that what are you doing fundamentally > > > > > different in FIOPS ioscheduler. > > > > > > > > > > The only thing I can think of more accurate accounting per queue in > > > > > terms of number of IOs instead of time. Which can just serve to improve > > > > > fairness a bit for certain workloads. In practice, I think it might > > > > > not matter much. > > > > If quantum is big, CFQ will have better performance, but it actually > > > > fallbacks to Noop, no any fairness. fairness is important and is why we > > > > introduce CFQ. > > > > > > It is not exactly noop. It still preempts writes and prioritizes reads > > > and direct writes. > > sure, I mean fairness mostly here. > > > > > Also, what's the real life workload where you face issues with using > > > say deadline with these flash based storage. > > deadline doesn't provide fairness. mainly cgroup workload. workload with > > different ioprio has issues too, but I don't know which real workload > > uses ioprio. > > Personally I have not run into any workload which provides deep queue depths > constantly for a very long time. I had to run fio to create such > scnearios. > > Not running deep queue depths will lead to expiration of queue (Otherwise > idling will kill performance on these fast devices). And without idling > most of the logic of slice and accounting does not help. A queue > dispatches some requests and expires (irrespective of what time slice > you had allocated it based on ioprio). That's true, if workload doesn't drive deep queue depths, any accounting can't help for NCQ disks as far as I tried. Idling is the only method to make accounting correct, but it impacts performance too much.
> That's why I am insisting that it would be nice that any move in this > direction should be driven by some real workload instead of just coming > up with synthetic workloads. I thought yanhai from taobao (cc-ed) has real workload and he found cfq performance suffers a lot.
Thanks, Shaohua
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