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

Jens Axboe
2019-12-21T17:01:10Z

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