Re: [PATCH] blk-mq: Use request queue-wide tags for tagset-wide sbitmap

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

On Mon, May 03, 2021 at 06:22:13PM +0800, John Garry wrote:
> The tags used for an IO scheduler are currently per hctx.
> 
> As such, when q->nr_hw_queues grows, so does the request queue total IO
> scheduler tag depth.
> 
> This may cause problems for SCSI MQ HBAs whose total driver depth is
> fixed.
> 
> Ming and Yanhui report higher CPU usage and lower throughput in scenarios
> where the fixed total driver tag depth is appreciably lower than the total
> scheduler tag depth:
> https://lore.kernel.org/linux-block/440dfcfc-1a2c-bd98-1161-cec4d78c6dfc@huawei.com/T/#mc0d6d4f95275a2743d1c8c3e4dc9ff6c9aa3a76b
> 

No difference any more wrt. fio running on scsi_debug with this patch in
Yanhui's test machine:

	modprobe scsi_debug host_max_queue=128 submit_queues=32 virtual_gb=256 delay=1
vs.
	modprobe scsi_debug max_queue=128 submit_queues=1 virtual_gb=256 delay=1

Without this patch, the latter's result is 30% higher than the former's.

note: scsi_debug's queue depth needs to be updated to 128 for avoiding io hang,
which is another scsi issue.


Thanks, 
Ming