Re: Device or HBA level QD throttling creates randomness in sequetial workload

[Omar Sandoval]

On Fri, Oct 21, 2016 at 05:43:35PM +0530, Kashyap Desai wrote:
> Hi -
> 
> I found below conversation and it is on the same line as I wanted some
> input from mailing list.
> 
> http://marc.info/?l=linux-kernel&m=147569860526197&w=2
> 
> I can do testing on any WIP item as Omar mentioned in above discussion.
> https://github.com/osandov/linux/tree/blk-mq-iosched

Are you using blk-mq for this disk? If not, then the work there won't
affect you.

> Is there any workaround/alternative in latest upstream kernel, if user
> wants to see limited penalty  for Sequential Work load on HDD ?
> 
> ` Kashyap
> 
> > -----Original Message-----
> > From: Kashyap Desai [mailto:kashyap.desai@xxxxxxxxxxxx]
> > Sent: Thursday, October 20, 2016 3:39 PM
> > To: linux-scsi@xxxxxxxxxxxxxxx
> > Subject: Device or HBA level QD throttling creates randomness in
> sequetial
> > workload
> >
> > [ Apologize, if you find more than one instance of my email.
> > Web based email client has some issue, so now trying git send mail.]
> >
> > 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 and 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
> >
> > <KD> After LBA "00 03 c0 3c" next command is with LBA "00 03 c0 5b".
> > Two 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.
> >
> > Thanks, Kashyap

-- 
Omar