Date:	Mon, 16 Jan 2012 15:55:41 +0800
From:	Shaohua Li <shaohua.li@...el.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	Dave Chinner <david@...morbit.com>, linux-kernel@...r.kernel.org,
	axboe@...nel.dk, jmoyer@...hat.com
Subject: Re: [RFC 0/3]block: An IOPS based ioscheduler

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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