linux-kernel - Re: [PATCH v6 7/8] selftests: ublk: kublk: decouple ublk

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Message-ID: <aB3AyJdO7-mFzjFf@fedora>
Date: Fri, 9 May 2025 16:46:00 +0800
From: Ming Lei <ming.lei@...hat.com>
To: Uday Shankar <ushankar@...estorage.com>
Cc: Jens Axboe <axboe@...nel.dk>,
	Caleb Sander Mateos <csander@...estorage.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Shuah Khan <shuah@...nel.org>, Jonathan Corbet <corbet@....net>,
	linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-kselftest@...r.kernel.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH v6 7/8] selftests: ublk: kublk: decouple ublk_queues from
 ublk server threads

On Wed, May 07, 2025 at 03:49:41PM -0600, Uday Shankar wrote:
> Add support in kublk for decoupled ublk_queues and ublk server threads.
> kublk now has two modes of operation:
> 
> - (preexisting mode) threads and queues are paired 1:1, and each thread
>   services all the I/Os of one queue
> - (new mode) thread and queue counts are independently configurable.
>   threads service I/Os in a way that balances load across threads even
>   if load is not balanced over queues. requires passing --round_robin
> 
> The new mode of operation is exercised by the new test_generic_08, which
> issues I/O against a single queue and verifies that each of the 8 ublk
> server threads handles exactly 1/8 of the total I/O count. Under the old
> mode of operation (i.e. without --round_robin), all I/O goes to one ublk
> server thread, and the test fails.

Hi Uday,

I just setup two ublk devices in the following way:

[root@...st-40 ublk]# ./kublk add -t null -q 2  --nthreads=4 --round_robin
dev id 0: nr_hw_queues 2 queue_depth 128 block size 512 dev_capacity 524288000
	max rq size 1048576 daemon pid 8516 flags 0x842 state LIVE
	queue 0: affinity(0 )
	queue 1: affinity(8 )

[root@...st-40 ublk]# ./kublk add -t null -q 2
dev id 1: nr_hw_queues 2 queue_depth 128 block size 512 dev_capacity 524288000
	max rq size 1048576 daemon pid 8539 flags 0x42 state LIVE
	queue 0: affinity(0 )
	queue 1: affinity(8 )

Then run 'fio/t/io_uring -p0 /dev/ublkb[0|1]', and IOPS of /dev/ublkb0 is
just 1/3 of /dev/ublkb1.

For ublk/loop, '-q 2  --nthreads=4 --round_robin' is still a bit slower
than '-q 2', but the gap isn't too big(<10%).


Thanks,
Ming