On 10/17/2017 03:29 AM, Ming Lei wrote: > 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. > No, this is purely a linux abstraction. Nothing to do with the hardware. > 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. > Again, these are just some pre-defined values to avoid I/O starvation when having several LUNs. _if_ we can guarantee I/O fairness between several (hundreds!) devices all sharing the same tagspace we wouldn't need these variables. Cheers, Hannes -- Dr. Hannes Reinecke Teamlead Storage & Networking hare@xxxxxxx +49 911 74053 688 SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg GF: F. Imendörffer, J. Smithard, J. Guild, D. Upmanyu, G. Norton HRB 21284 (AG Nürnberg)
