On Mon, Oct 16, 2017 at 01:30:09PM +0200, Hannes Reinecke wrote: > On 10/13/2017 07:29 PM, Ming Lei wrote: > > On Fri, Oct 13, 2017 at 05:08:52PM +0000, Bart Van Assche wrote: > >> On Sat, 2017-10-14 at 00:45 +0800, Ming Lei wrote: > >>> On Fri, Oct 13, 2017 at 04:31:04PM +0000, Bart Van Assche wrote: > >>>> On Sat, 2017-10-14 at 00:07 +0800, Ming Lei wrote: > >>>>> Actually it is in hot path, for example, lpfc and qla2xx's queue depth is 3, > >>>> > >>>> Sorry but I doubt whether that is correct. More in general, I don't know any modern > >>>> storage HBA for which the default queue depth is so low. > >>> > >>> You can grep: > >>> > >>> [ming@ming linux]$ git grep -n cmd_per_lun ./drivers/scsi/ | grep -E "qla2xxx|lpfc" > >> > >> Such a low queue depth will result in suboptimal performance for adapters > >> that communicate over a storage network. I think that's a bug and that both > >> adapters support much higher cmd_per_lun values. > >> > >> (+James Smart) > >> > >> James, can you explain us why commit 445cf4f4d2aa decreased LPFC_CMD_PER_LUN > >> from 30 to 3? Was that perhaps a workaround for a bug in a specific target > >> implementation? > >> > >> (+Himanshu Madhani) > >> > >> Himanshu, do you perhaps know whether it is safe to increase cmd_per_lun for > >> the qla2xxx initiator driver to the scsi_host->can_queue value? > > > > ->can_queue is size of the whole tag space shared by all LUNs, looks it isn't > > reasonable to increase cmd_per_lun to .can_queue. > > > '3' is just a starting point; later on it'll be adjusted via > scsi_change_depth(). > Looks like it's not working correctly with blk-mq, though. At default, in scsi_alloc_sdev(), q->queue_depth is set as host->cmd_per_lun. You are right, q->queue_depth can be adjusted later too. q->queue_depth is respected in scsi_dev_queue_ready(). .cmd_per_lun defines the max outstanding cmds for each lun, I guess it is respected by some hardware inside. For example, I remembered that on lpfc q->queue_depth is 30 because the default 'lpfc_lun_queue_depth' is 30. And its .cmd_per_lun is 3. Per my observation, this .cmd_per_lun limit is still workable. -- Ming
