Re: [PATCH 1/3] cfq-iosched: Implment IOPS mode

[Vivek Goyal <vgoyal@redhat.com>]

On Wed, Jul 21, 2010 at 04:33:04PM -0400, Jeff Moyer wrote:
> Vivek Goyal <vgoyal@redhat.com> writes:
> 
> > o Implement another CFQ mode where we charge queue/group in terms of number
> >   of requests dispatched instead of measuring the time. Measuring in terms
> >   of time is not possible when we are driving deeper queue depths and there
> >   are requests from multiple cfq queues in the request queue.
> >
> > o This mode currently gets activated if one sets slice_idle=0 and associated
> >   disk supports NCQ. Again the idea is that on an NCQ disk with idling disabled
> >   most of the queues will dispatch 1 or more requests and then cfq queue
> >   expiry happens and we don't have a way to measure time. So start providing
> >   fairness in terms of IOPS.
> >
> > o Currently this primarily is beneficial with cfq group scheduling where one
> >   can disable slice idling so that we don't idle on queue and drive deeper
> >   request queue deptsh (achieving better throughput), at the same time group
> >   idle is enabled so one should get service differentiation among groups.
> 
> I like that this is more isolated now.  I'm slowly warming up to it.  I
> have one question--just a curiosity, really.  What do you see now for
> the reported sl_used in blktrace when slice_idle is zero and the
> hardware supports command queueing?

sl_used, still shows amount of time elapsed since we started dispatch from
the queue. I retained that info because we export that info through cgroup
interface.

Just that charging logic to the group changed where in IOPS mode instead
of charging sl_used, we charge iops. Following is sample output of
blktrace after the patches.

253,0    0        0     0.014157613     0  m   N cfq19226S /cgrp7 sl_used=3 disp=1 charge=1 iops=1 sect=8

Here we slice used since dispatch start is 3 jiffies, we dispatched 1
request in this duration. Because we are iops mode (iops=1), we charge
the group for 1 rq and no 3 jiffies. (charge=1). sect shows we dispatched
8 sectors in this duration.

Vivek




> 
> Cheers,
> Jeff