Re: [PATCH] hpsa: scsi-mq support

[Christoph Hellwig <hch@infradead.org>]

On Fri, Nov 11, 2016 at 07:22:05PM +0100, Hannes Reinecke wrote:
> Well, it's the same as with megasas and mpt3sas. Each of those have
> a single MMIO register where the driver writes the address of the
> command into. What exactly the hardware does in the back doesn't
> really matter here; the command is in memory and the hardware can
> access it as it sees fit. So from that point of view we can assume
> having a submission queue to match the completion queue;
> With that setup we do have a contention point on the single command
> register, but that's about it.
> We still should benefit from scsi-mq, though.

How do we benefit from scsi-mq in this case?  We still hit global
cachelines like commands_outstanding in the driver, and we lost the
batching done by the ctx -> hw_ctx layering for the single queue
blk-mq case.  We also get much less efficient merging and will not
have the chance of having and I/O schedule in the near future.

But back to my question from the last mail:  What workload is improved
by using this patch?