On 10/01/2020 12:09, John Garry 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/
OK, but I was waiting for results first, which I hoped would not take
too long. They weren't forgotten, for sure. Let me check them now.
Hi Ming,
I think that your questions here were related to the shared scheduler
tags, which was Hannes' proposal (I initially had it in v2 series, but
dropped it for v3).
I was just content to maintain the concept of shared driver tags.
Thanks,
John
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.
Yeah, that's a trade-off. And, as I remember, memory waste does not seem
extreme.
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,
John
