Re: [PATCH] block: Improve IOPS by removing the fairness code

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

On Thu, Mar 21, 2024 at 03:46:05PM -0700, Bart Van Assche wrote:
> There is an algorithm in the block layer for maintaining fairness
> across queues that share a tag set. The sbitmap implementation has
> improved so much that we don't need the block layer fairness algorithm
> anymore and that we can rely on the sbitmap implementation to guarantee
> fairness.
> 
> This patch removes the following code and structure members:
> - The function hctx_may_queue().
> - blk_mq_hw_ctx.nr_active and request_queue.nr_active_requests_shared_tags
>   and also all the code that modifies these two member variables.
> 
> On my test setup (x86 VM with 72 CPU cores) this patch results in 2.9% more
> IOPS. IOPS have been measured as follows:
> 
> $ modprobe null_blk nr_devices=1 completion_nsec=0
> $ fio --bs=4096 --disable_clat=1 --disable_slat=1 --group_reporting=1 \
>       --gtod_reduce=1 --invalidate=1 --ioengine=psync --ioscheduler=none \
>       --norandommap --runtime=60 --rw=randread --thread --time_based=1 \
>       --buffered=0 --numjobs=64 --name=/dev/nullb0 --filename=/dev/nullb0

The above test just covers single LUN test.

But the code you removed is actually for providing fairness over multi-LUN,
do you have multi-LUN test result for supporting the change?

Thanks,
Ming