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

Pavel Begunkov
2019-12-21T17:27:13Z

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On 21/12/2019 20:01, Jens Axboe wrote:
> 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:

I see. I'll probably drop the last one, as it only complicates things.

My apologies for misleading terminology. Read-only QD1 (submit and
wait until the userspace completes it) obviously won't saturate a CPU.
Writes probably wouldn't as well (though, depends on HW). And it would be
better to say -- submit by one, complete in a bunch.
Just curious, what you used for testing? Is it fio?

>
> + 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
>
>

--
Pavel Begunkov

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