Re: [PATCH 3/4] blk-mq: use hw tag for scheduling if hw tag space is big enough

[Ming Lei <ming.lei@redhat.com>]

On Fri, Apr 28, 2017 at 12:22:45PM -0600, Jens Axboe wrote:
> On 04/28/2017 09:15 AM, Ming Lei wrote:  
> > +/*
> > + * If this queue has enough hardware tags and doesn't share tags with
> > + * other queues, just use hw tag directly for scheduling.
> > + */
> > +static inline bool blk_mq_sched_may_use_hw_tag(struct request_queue *q)
> > +{
> > +	if (q->tag_set->flags & BLK_MQ_F_TAG_SHARED)
> > +		return false;
> > +
> > +	if (blk_mq_get_queue_depth(q) < q->nr_requests)
> > +		return false;
> 
> I think we should leave a bigger gap. Ideally, for scheduling, we should
> have a hw queue depth that's around half of what the scheduler has to
> work with. That will always leave us something to schedule with, if the
> hw can't deplete the whole pool.

When .sched_tags and .tags are different, it makes sense to make
nr_requests to be two times of queue_depth.

When we switch to schedule with hw tags directly, the euquation
can't be true at all. The simple policy in this patch can't be worsen
than standalone .sched_tags because lifetime of one sched tag is
actually same with request(from its allocation<io submission> to
freeing<io completion>). When we have bigger queue depth, even we can
schedule more.


Thanks,
Ming