On Fri, 2017-04-28 at 12:22 -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.
Hello Jens,
The scsi-mq core allocates exactly the same number of tags per hardware
queue as the SCSI queue depth. Requiring that there is a gap would cause
BLK_MQ_F_SCHED_USE_HW_TAG not to be enabled for any scsi-mq LLD. I'm not
sure that changing the tag allocation strategy in scsi-mq would be the best
solution. How about changing blk_mq_sched_may_use_hw_tag() into something
like the below to guarantee that the scheduler has sufficient tags available?
static bool blk_mq_sched_may_use_hw_tag(struct request_queue *q)
{
return blk_mq_get_queue_depth(q) >= max(q->nr_requests, 16);
}
Thanks,
Bart.
