On Tue, Oct 17, 2017 at 08:38:01AM +0200, Hannes Reinecke wrote:

On 10/17/2017 03:29 AM, Ming Lei wrote:

quoted

On Mon, Oct 16, 2017 at 01:30:09PM +0200, Hannes Reinecke wrote:

quoted

On 10/13/2017 07:29 PM, Ming Lei wrote:

quoted

On Fri, Oct 13, 2017 at 05:08:52PM +0000, Bart Van Assche wrote:

quoted

On Sat, 2017-10-14 at 00:45 +0800, Ming Lei wrote:

quoted

On Fri, Oct 13, 2017 at 04:31:04PM +0000, Bart Van Assche wrote:

quoted

On Sat, 2017-10-14 at 00:07 +0800, Ming Lei wrote:

quoted

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