Re: [RFC V2 PATCH 4/5] cfq-iosched: enable idling for last queue on priority class

[Jeff Moyer <jmoyer@redhat.com>]

Corrado Zoccolo <czoccolo@gmail.com> writes:

> cfq can disable idling for queues in various circumstances.
> When workloads of different priorities are competing, if the higher
> priority queue has idling disabled, lower priority queues may steal
> its disk share. For example, in a scenario with an RT process
> performing seeky reads vs a BE process performing sequential reads,
> on an NCQ enabled hardware, with low_latency unset,
> the RT process will dispatch only the few pending requests every full
> slice of service for the BE process.
>
> The patch solves this issue by always performing idle on the last
> queue at a given priority class > idle. If the same process, or one
> that can pre-empt it (so at the same priority or higher), submits a
> new request within the idle window, the lower priority queue won't
> dispatch, saving the disk bandwidth for higher priority ones.
>
> Note: this doesn't touch the non_rotational + NCQ case (no hardware
> to test if this is a benefit in that case).
>
> Signed-off-by: Corrado Zoccolo <czoccolo@gmail.com>

This looks OK at first glance.

Cheers,
Jeff