Re: [PATCH V7 4/6] blk-mq: introduce .get_budget and .put_budget in blk_mq_ops

[Ming Lei <ming.lei@redhat.com>]

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