On Mon, Oct 21, 2019 at 12:49:53PM +0100, John Garry wrote:

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Yes, we share tags among all queues, but we generate the tag - known as IPTT
- in the LLDD now, as we can no longer use the request tag (as it is not
unique per all queues):

https://github.com/hisilicon/kernel-dev/commit/087b95af374be6965583c1673032fb33bc8127e8#diff-f5d8fff19bc539a7387af5230d4e5771R188

As I said, the branch is messy and I did have to fix 087b95af374.

Firstly this way may waste lots of memory, especially the queue depth is
big, such as, hisilicon V3's queue depth is 4096.

Secondly, you have to deal with queue busy efficiently and correctly,
for example, your real hw tags(IPTT) can be used up easily, and how
will you handle these dispatched request?

I have not seen scenario of exhausted IPTT. And IPTT count is same as SCSI
host.can_queue, so SCSI midlayer should ensure that this does not occur.

That check isn't correct, and each hw queue should have allowed
.can_queue in-flight requests.

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Finally, you have to evaluate the performance effect, this is highly
related with how to deal with out-of-IPTT.

Some figures from our previous testing:

Managed interrupt without exposing multiple queues: 3M IOPs
Managed interrupt with exposing multiple queues: 2.6M IOPs

Then you see the performance regression.


Thanks,
Ming