Re: scsi-mq - tag# and can_queue, performance.

[Bart Van Assche <Bart.VanAssche@sandisk.com>]

On Mon, 2017-04-03 at 09:29 +0200, Hannes Reinecke wrote:
> On 04/03/2017 08:37 AM, Arun Easi wrote:
> > If the above is true, then for a LLD to get tag# within it's max-tasks 
> > range, it has to report max-tasks / number-of-hw-queues in can_queue, and 
> > in the I/O path, use the tag and hwq# to arrive at a index# to use. This, 
> > though, leads to a poor use of tag resources -- queue reaching it's 
> > capacity while LLD can still take it.
>
> Shared tag sets continue to dog the block-mq on 'legacy' (ie non-NVMe)
> HBAs. ATM the only 'real' solution to this problem is indeed have a
> static split of the entire tag space by the number of hardware queues.
> With the mentioned tag-starvation problem.

Hello Arun and Hannes,

Apparently the current blk_mq_alloc_tag_set() implementation is well suited
for drivers like NVMe and ib_srp but not for traditional SCSI HBA drivers.
How about adding a BLK_MQ_F_* flag that tells __blk_mq_alloc_rq_maps() to
allocate a single set of tags for all hardware queues and also to add a flag
to struct scsi_host_template such that SCSI LLDs can enable this behavior?

Bart.