Messages in this thread | | | Subject | Re: [RFC 0/3]block: An IOPS based ioscheduler | From | Shaohua Li <> | Date | Mon, 16 Jan 2012 15:55:41 +0800 |
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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.
> > > > In summary, CFQ isn't both fair and good performance. FIOPS is trying to > > be fair and have good performance. I didn't think any time based > > accounting can make the goal happen for NCQ and SSD (even cfq cgroup > > code has iops mode, so suppose you should already know this well). > > > > Surely you can change CFQ to make it IOPS based, but this will mess the > > code a lot, and FIOPS shares a lot of code with CFQ. So I'd like to have > > a separate ioscheduler which is IOPS based. > > I think writing a separate IO scheduler just to do accouting in IOPS while > retaining rest of the CFQ code is not a very good idea. Modifying CFQ code > to be able to deal with both time based as well as IOPS accounting might > turn out to be simpler. changing CFQ works, but I really want to avoid having something like if (iops) xxx else xxx I plan adding scales for read/write, request size, etc, because read/write cost is different and request with different size has different cost in SSD. This can be added to CFQ too with pain. That said I didn't completely object to make CFQ support IOPS accounting, but my feeling is a separate ioscheduler is more clean.
Thanks, Shaohua
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