On Tue, Oct 17, 2017 at 08:38:01AM +0200, Hannes Reinecke wrote: > 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. That is also my initial understanding. But my test showed that actually the max outstanding cmds per LUN is really 3 even though q->queue_depth is 30, that is why I guess the hardware may put a hard limit inside: https://marc.info/?l=linux-block&m=150549401611868&w=2 Also if they were same thing, why does lpfc define different default value for q->queue_depth and .cmd_per_lun? drivers/scsi/lpfc/lpfc_attr.c: 3411 /* # lun_queue_depth: This parameter is used to limit the number of outstanding # commands per FCP LUN. Value range is [1,512]. Default value is 30. # If this parameter value is greater than 1/8th the maximum number of exchanges # supported by the HBA port, then the lun queue depth will be reduced to # 1/8th the maximum number of exchanges. */ LPFC_VPORT_ATTR_R(lun_queue_depth, 30, 1, 512, "Max number of FCP commands we can queue to a specific LUN"); drivers/scsi/lpfc/lpfc.h: 47 #define LPFC_CMD_PER_LUN 3 /* max outstanding cmds per lun */ -- Ming
