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

[Bart Van Assche]

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?

> Even SRP/IB isn't big too, just 32.

The default value for ib_srp for cmd_per_lun is 62 but that value can be
overridden by selecting another value in /etc/default/srp_daemon.conf. Note:
a lower value is selected if after SRP login it becomes clear that the target
queue depth is lower than the cmd_per_lun value requested by the user. This
is a performance optimization and avoids that the SRP target system has to
send back BUSY responses to the initiator.

Bart.