Device or HBA QD throttling creates holes in Sequential work load

[Kashyap Desai <kashyap.desai@broadcom.com>]

Hi,

I am doing some performance tuning in MR driver to understand how sdev
queue depth and hba queue depth play role in IO submission from above
layer.

I have 24 JBOD connected to MR 12GB controller and I can see performance
for 4K Sequential work load as below.

HBA QD for MR controller is 4065
Per device QD is set to 32

queue depth from <fio> 256 reports 300K IOPS
queue depth from <fio> 128 reports 330K IOPS
queue depth from <fio> 64 reports 360K IOPS
queue depth from <fio> 32 reports 510K IOPS


In MR driver I added debug print and confirm that more IO come to driver
as random IO whenever I have <fio> queue depth more than 32.
I have debug using scsi logging level and blktrace as well. Below is
snippet of logs using scsi logging level.  In summary, if SML do flow
control of IO due to Device QD or HBA QD,  IO coming to LLD is more random
pattern.

I see IO coming to driver is not sequential.

[79546.912041] sd 18:2:21:0: [sdy] tag#854 CDB: Write(10) 2a 00 00 03 c0
3b 00 00 01 00
[79546.912049] sd 18:2:21:0: [sdy] tag#855 CDB: Write(10) 2a 00 00 03 c0
3c 00 00 01 00
[79546.912053] sd 18:2:21:0: [sdy] tag#886 CDB: Write(10) 2a 00 00 03 c0
5b 00 00 01 00 <- After 3c it jumps to 5b. Sequence are overlapped. Due to
sdev QD throttling.
[79546.912056] sd 18:2:21:0: [sdy] tag#887 CDB: Write(10) 2a 00 00 03 c0
5c 00 00 01 00
[79546.912250] sd 18:2:21:0: [sdy] tag#856 CDB: Write(10) 2a 00 00 03 c0
3d 00 00 01 00
[79546.912257] sd 18:2:21:0: [sdy] tag#888 CDB: Write(10) 2a 00 00 03 c0
5d 00 00 01 00
[79546.912259] sd 18:2:21:0: [sdy] tag#857 CDB: Write(10) 2a 00 00 03 c0
3e 00 00 01 00
[79546.912268] sd 18:2:21:0: [sdy] tag#858 CDB: Write(10) 2a 00 00 03 c0
3f 00 00 01 00


If scsi_request_fn() breaks due to unavailability of device queue (due to
below check), will there be any side defect as I observe ?

                if (!scsi_dev_queue_ready(q, sdev))
                        break;

If I reduce HBA QD and make sure IO from above layer is throttled due to
HBA QD, there is a same impact.  MR driver use host wide shared tag map.

Can someone help me if this can be tunable in LLD providing additional
settings or it is expected behavior ? Problem I am facing is, I am not
able to figure out optimal device queue depth for different configuration
and work load.

` Kashyap