RE: About scsi device queue depth

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

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>
> On 12/01/2021 11:44, Kashyap Desai wrote:
> >>>> loops = 10000
> >>> Is there any effect on random read IOPS when you decrease sdev queue
> >>> depth? For sequential IO, IO merge can be enhanced by that way.
> >>>
> >> Let me check...
> > John - Can you check your test with rq_affinity = 2  and nomerges=2 ?
> >
> > I have noticed similar drops (whatever you have reported) -  "once we
> > reach near pick of sdev or host queue depth, sometimes performance
> > drop due to contention."
> > But this behavior keep changing since kernel changes in this area is
> > very actively happened in past. So I don't know the exact details
> > about kernel version etc.
> > I have similar setup (16 SSDs) and I will try similar test on latest
> > kernel.
> >
> > BTW - I remember that rq_affinity=2 play major role in such issue.  I
> > usually do testing with rq_affinity = 2.
> >
>
> Hi Kashyap,
>
> As requested:
>
> rq_affinity=1, nomerges=0 (default)
>
> sdev queue depth	num jobs=1
> 8			1650
> 16			1638
> 32			1612
> 64			1573
> 254			1435 (default for LLDD)
>
> rq_affinity=2, nomerges=2
>
> sdev queue depth	num jobs=1
> 8			1236
> 16			1423
> 32			1438
> 64			1438
> 254			1438 (default for LLDD)
>
> Setup as original: fio read, 12x SAS SSDs

What is an issue with rq_affinity=2 and nomerges=2 ?
It looks like - "Dropping IOPs from peak (1.6M) is not an issue here but we
are not able to reach peak IOPs. IOPS increase gradually and it saturate."

>
> So, again, we see that performance changes from changing sdev queue depth
> depends on workload and then also other queue config.

Can you send me "dmesg" logs of your setup. What is host_busy counter while
you run your test ? I am not able to duplicate the issue on my setup.

To avoid any io merge issue, can we use rand_read data ?

Kashyap

>
> Thanks,
> John

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