Re: [PATCH 09/11] megaraid_sas: switch fusion adapters to MQ

[Ming Lei <ming.lei@redhat.com>]

On Thu, Jan 09, 2020 at 11:55:12AM +0000, John Garry wrote:
> On 09/12/2019 10:10, Sumit Saxena wrote:
> > On Mon, Dec 2, 2019 at 9:09 PM Hannes Reinecke <hare@suse.de> wrote:
> > > 
> > > Fusion adapters can steer completions to individual queues, and
> > > we now have support for shared host-wide tags.
> > > So we can enable multiqueue support for fusion adapters and
> > > drop the hand-crafted interrupt affinity settings.
> > 
> > Hi Hannes,
> > 
> > Ming Lei also proposed similar changes in megaraid_sas driver some
> > time back and it had resulted in performance drop-
> > https://patchwork.kernel.org/patch/10969511/
> > 
> > So, we will do some performance tests with this patch and update you.
> > 
> 
> Hi Sumit,
> 
> I was wondering if you had a chance to do this test yet?
> 
> It would be good to know, so we can try to progress this work.

Looks most of the comment in the following link isn't addressed:

https://lore.kernel.org/linux-block/20191129002540.GA1829@ming.t460p/

> Firstly too much((nr_hw_queues - 1) times) memory is wasted. Secondly IO
> latency could be increased by too deep scheduler queue depth. Finally CPU
> could be wasted in the retrying of running busy hw queue.
> 
> Wrt. driver tags, this patch may be worse, given the average limit for
> each LUN is reduced by (nr_hw_queues) times, see hctx_may_queue().
> 
> Another change is bt_wait_ptr(). Before your patches, there is single
> .wait_index, now the number of .wait_index is changed to nr_hw_queues.
> 
> Also the run queue number is increased a lot in SCSI's IO completion, see
> scsi_end_request().

I guess memory waste won't be a blocker.

But it may not be one accepted behavior to reduce average active queue
depth for each LUN by nr_hw_queues times, meantime scheduler queue depth
is increased by nr_hw_queues times, compared with single queue.

thanks,
Ming