RE: [PATCH] hpsa: scsi-mq support

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

> -----Original Message-----
> From: linux-scsi-owner@vger.kernel.org [mailto:linux-scsi-
> owner@vger.kernel.org] On Behalf Of Christoph Hellwig
> Sent: Sunday, November 13, 2016 5:29 PM
> To: Hannes Reinecke
> Cc: Christoph Hellwig; Hannes Reinecke; Christoph Hellwig; Martin K.
Petersen;
> James Bottomley; Don Brace; linux-scsi@vger.kernel.org; Jens Axboe
> Subject: Re: [PATCH] hpsa: scsi-mq support
>
> On Sun, Nov 13, 2016 at 10:44:47AM +0100, Hannes Reinecke wrote:
> > One day to mark with bright red in the calendar.
> >
> > Christoph Hellwig is telling me _NOT_ to use scsi-mq.
>
> That's not what I'm doing.
>
> > This patch was done so see what would needed to be done to convert a
> > legacy driver.
> > As I was under the impression that scsi-mq is the way forward, seeing
> > that it should be enabled per default.
> > But I must have been mistaken. Apparently.
>
> What I am doing is to tell you you should not expose multiple queues
unless the
> hardware actually has multiple submissions queues.  The blk-mq and
scsi-mq
> code works just fine with a single submission queue, and the hpsa driver
in
> particular works really well with scsi-mq and a single submission queue.
E.g. the
> RAID HBA on slide 19 of this presentations is an hpsa one:

I have similar results for MegaRaid where seen MR driver gives significant
improvement for Single Submission Queue and multiple Completion Queue.
Having said that scsi-mq is enabled but with single Queue is more than
enough to maximize improvement of SCSI-MQ.

Major advantage was seen while IO load is cross the boundary of Physical
CPU socket.

>From this discussion I understood that - Similar logical changes proposed
for megaraid_sas  and we are not really going to gain with fake multiple
submission exposed to SML.

Kashyap

>
> http://events.linuxfoundation.org/sites/events/files/slides/scsi.pdf
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