On 30/05/19 19:54, Bart Van Assche wrote: > As far as I can see the only impact of defining an empty commit_rqs > callback on the queueing behavior is that blk_mq_make_request() will > queue requests for multiple hwqs on the plug list instead of requests > for a single hwq. The plug list is sorted by hwq before it is submitted > to a block driver. If that helps NVMe performance it should also help > SCSI performance. See the comment in blk_mq_make_request(): sorting by hwq helps NVMe because it uses bd->last, and blk_mq_make_request() uses the presence of the ->commit_rqs() as a sign that the driver uses bd->last. The heuristic basically trades latency (with plugging, the driver rings the doorbell a bit later) for throughput (ringing the doorbell is slow, so we want to do it less). If the driver doesn't use bd->last and the doorbell is always rung, plugging adds to the latency without the throughput benefit. All that the duplicate blk_mq_ops do is letting blk_mq_make_request() use the same heuristic for SCSI. This should be beneficial exactly for the reason that you mention: if the heuristic helps non-SCSI block driver performance, it should also help performance of SCSI drivers that have nr_hw_queues > 1 but no commit_rqs (lpfc, qedi, qla2xxx, smartpqi, storvsc). Paolo > How about always setting commit_rqs = scsi_commit_rqs > in scsi_mq_ops?
