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

[Hannes Reinecke]

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.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		   Teamlead Storage & Networking
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