RE: [PATCH v2 11/15] megaraid_sas: Set device queue_depth same as HBA can_queue value in scsi-mq mode

[Shivasharan Srikanteshwara <shivasharan.srikanteshwara@broadcom.com>]

> -----Original Message-----
> From: Kashyap Desai [mailto:kashyap.desai@broadcom.com]
> Sent: Tuesday, July 11, 2017 9:18 PM
> To: Christoph Hellwig; Shivasharan Srikanteshwara
> Cc: linux-scsi@vger.kernel.org; martin.petersen@oracle.com;
> thenzl@redhat.com; jejb@linux.vnet.ibm.com; Sumit Saxena; hare@suse.com
> Subject: RE: [PATCH v2 11/15] megaraid_sas: Set device queue_depth same as
> HBA can_queue value in scsi-mq mode
>
> > -----Original Message-----
> > From: Christoph Hellwig [mailto:hch@lst.de]
> > Sent: Tuesday, July 11, 2017 7:28 PM
> > To: Shivasharan S
> > Cc: linux-scsi@vger.kernel.org; martin.petersen@oracle.com;
> > thenzl@redhat.com; jejb@linux.vnet.ibm.com;
> > kashyap.desai@broadcom.com; sumit.saxena@broadcom.com;
> hare@suse.com;
> > hch@lst.de
> > Subject: Re: [PATCH v2 11/15] megaraid_sas: Set device queue_depth
> > same
> as
> > HBA can_queue value in scsi-mq mode
> >
> > On Wed, Jul 05, 2017 at 05:00:25AM -0700, Shivasharan S wrote:
> > > Currently driver sets default queue_depth for VDs at 256 and JBODs
> > > based on interface type, ie., for SAS JBOD QD will be 64, for SATA
> JBOD QD
> > will be 32.
> > > During performance runs with scsi-mq enabled, we are seeing better
> > > results by setting QD same as HBA queue_depth.
> >
> > Please no scsi-mq specifics.  just do this unconditionally.
>
> Chris -  Intent for mq specific check is mainly because of sequential work
> load
> for HDD is having penalty due to mq scheduler issue.
> We did this exercise prior to mq-deadline support.
>
> Making generic change for non-mq and mq was good, but we may see some
> user may not like to see regression.
> E.a In case of, QD = 32 for SATA PD file system creation may be faster
> compare
> to large QD. There may be a soft merger at block layer due to queue depth
> throttling. Eventually, FS creation goes fast due to IO merges, but same
> will not
> be true if we change queue depth logic (means, increase device queue depth
> to
> HBA QD.)
>
> We have choice to completely remove this patch and ask users to do sysfs
> settings in case of scsi-mq performance issue for HDD sequential work
> load.
> Having this patch, we want to provide better QD settings as default from
> driver.
>
>
> Thanks, Kashyap

Hi Christoph,
As Kashyap mentioned, the performance issues seen were specific to scsi-mq
enabled case when running sequential workloads with HDDs.
Making this generic might result in regressions in some scenarios for
non-mq.
That was the idea behind making the change specific to scsi-mq only.

Let us know if you are ok with having this as is or we could remove
this patch completely and have users manually tune queue depth settings if
they
are seeing performance issues with scsi-mq enabled.

Thanks,
Shivasharan