Re: [PATCH RFC v2 3/3] io_uring: batch get(ctx->ref) across submits

From: Jens Axboe <axboe@kernel.dk>
To: Pavel Begunkov <asml.silence@gmail.com>,
	io-uring@vger.kernel.org, linux-kernel@vger.kernel.org
Cc: Tejun Heo <tj@kernel.org>, Dennis Zhou <dennis@kernel.org>,
	Christoph Lameter <cl@linux.com>
Subject: Re: [PATCH RFC v2 3/3] io_uring: batch get(ctx->ref) across submits
Date: Sat, 21 Dec 2019 10:01:07 -0700	[thread overview]
Message-ID: <fef3a245-d2a2-23b3-ff03-3e05af19b752@kernel.dk> (raw)
In-Reply-To: <fef4b765-338b-d3b0-7fd5-5672b92fd3e8@gmail.com>

On 12/21/19 9:48 AM, Pavel Begunkov wrote:
> On 21/12/2019 19:38, Jens Axboe wrote:
>> On 12/21/19 9:20 AM, Pavel Begunkov wrote:
>>> On 21/12/2019 19:15, Pavel Begunkov wrote:
>>>> Double account ctx->refs keeping number of taken refs in ctx. As
>>>> io_uring gets per-request ctx->refs during submission, while holding
>>>> ctx->uring_lock, this allows in most of the time to bypass
>>>> percpu_ref_get*() and its overhead.
>>>
>>> Jens, could you please benchmark with this one? Especially for offloaded QD1
>>> case. I haven't got any difference for nops test and don't have a decent SSD
>>> at hands to test it myself. We could drop it, if there is no benefit.
>>>
>>> This rewrites that @extra_refs from the second one, so I left it for now.
>>
>> Sure, let me run a peak test, qd1 test, qd1+sqpoll test on
>> for-5.6/io_uring, same branch with 1-2, and same branch with 1-3. That
>> should give us a good comparison. One core used for all, and we're going
>> to be core speed bound for the performance in all cases on this setup.
>> So it'll be a good comparison.
>>
> Great, thanks!

For some reason, not seeing much of a change between for-5.6/io_uring
and 1+2 and 1+2+3, it's about the same and results seem very stable.
For reference, top of profile with 1-3 applied looks like this:

+    3.92%  io_uring  [kernel.vmlinux]  [k] blkdev_direct_IO
+    3.87%  io_uring  [kernel.vmlinux]  [k] blk_mq_get_request
+    3.43%  io_uring  [kernel.vmlinux]  [k] io_iopoll_getevents
+    3.03%  io_uring  [kernel.vmlinux]  [k] __slab_free
+    2.87%  io_uring  io_uring          [.] submitter_fn
+    2.79%  io_uring  [kernel.vmlinux]  [k] io_submit_sqes
+    2.75%  io_uring  [kernel.vmlinux]  [k] bio_alloc_bioset
+    2.70%  io_uring  [nvme_core]       [k] nvme_setup_cmd
+    2.59%  io_uring  [kernel.vmlinux]  [k] blk_mq_make_request
+    2.46%  io_uring  [kernel.vmlinux]  [k] io_prep_rw
+    2.32%  io_uring  [kernel.vmlinux]  [k] io_read
+    2.25%  io_uring  [kernel.vmlinux]  [k] blk_mq_free_request
+    2.19%  io_uring  [kernel.vmlinux]  [k] io_put_req
+    2.06%  io_uring  [kernel.vmlinux]  [k] kmem_cache_alloc
+    2.01%  io_uring  [kernel.vmlinux]  [k] generic_make_request_checks
+    1.90%  io_uring  [kernel.vmlinux]  [k] __sbitmap_get_word
+    1.86%  io_uring  [kernel.vmlinux]  [k] sbitmap_queue_clear
+    1.85%  io_uring  [kernel.vmlinux]  [k] io_issue_sqe

-- 
Jens Axboe