Re: [RFC 0/3]block: An IOPS based ioscheduler

[Shaohua Li]

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.

> 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.

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.

Thanks,
Shaohua

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