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

[Jeff Moyer <jmoyer@redhat.com>]

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?

Cheers,
Jeff