Re: [PATCH net-next v4 00/27] io_uring zerocopy send

David Ahern
2022-07-13T23:45:55Z

On 7/11/22 5:56 AM, Pavel Begunkov wrote:
> On 7/8/22 15:26, Pavel Begunkov wrote:
>> On 7/8/22 05:10, David Ahern wrote:
>>> On 7/7/22 5:49 AM, Pavel Begunkov wrote:
>>>> NOTE: Not be picked directly. After getting necessary acks, I'll be
>>>> working
>>>>        out merging with Jakub and Jens.
>>>>
>>>> The patchset implements io_uring zerocopy send. It works with both
>>>> registered
>>>> and normal buffers, mixing is allowed but not recommended. Apart
>>>> from usual
>>>> request completions, just as with MSG_ZEROCOPY, io_uring separately
>>>> notifies
>>>> the userspace when buffers are freed and can be reused (see API
>>>> design below),
>>>> which is delivered into io_uring's Completion Queue. Those
>>>> "buffer-free"
>>>> notifications are not necessarily per request, but the userspace has
>>>> control
>>>> over it and should explicitly attaching a number of requests to a
>>>> single
>>>> notification. The series also adds some internal optimisations when
>>>> used with
>>>> registered buffers like removing page referencing.
>>>>
>>>>  From the kernel networking perspective there are two main changes.
>>>> The first
>>>> one is passing ubuf_info into the network layer from io_uring
>>>> (inside of an
>>>> in kernel struct msghdr). This allows extra optimisations, e.g.
>>>> ubuf_info
>>>> caching on the io_uring side, but also helps to avoid cross-referencing
>>>> and synchronisation problems. The second part is an optional
>>>> optimisation
>>>> removing page referencing for requests with registered buffers.
>>>>
>>>> Benchmarking with an optimised version of the selftest (see [1]),
>>>> which sends
>>>> a bunch of requests, waits for completions and repeats. "+ flush"
>>>> column posts
>>>> one additional "buffer-free" notification per request, and just "zc"
>>>> doesn't
>>>> post buffer notifications at all.
>>>>
>>>> NIC (requests / second):
>>>> IO size | non-zc    | zc             | zc + flush
>>>> 4000    | 495134    | 606420 (+22%)  | 558971 (+12%)
>>>> 1500    | 551808    | 577116 (+4.5%) | 565803 (+2.5%)
>>>> 1000    | 584677    | 592088 (+1.2%) | 560885 (-4%)
>>>> 600     | 596292    | 598550 (+0.4%) | 555366 (-6.7%)
>>>>
>>>> dummy (requests / second):
>>>> IO size | non-zc    | zc             | zc + flush
>>>> 8000    | 1299916   | 2396600 (+84%) | 2224219 (+71%)
>>>> 4000    | 1869230   | 2344146 (+25%) | 2170069 (+16%)
>>>> 1200    | 2071617   | 2361960 (+14%) | 2203052 (+6%)
>>>> 600     | 2106794   | 2381527 (+13%) | 2195295 (+4%)
>>>>
>>>> Previously it also brought a massive performance speedup compared to
>>>> the
>>>> msg_zerocopy tool (see [3]), which is probably not super interesting.
>>>>
>>>
>>> can you add a comment that the above results are for UDP.
>>
>> Oh, right, forgot to add it
>>
>>
>>> You dropped comments about TCP testing; any progress there? If not, can
>>> you relay any issues you are hitting?
>>
>> Not really a problem, but for me it's bottle necked at NIC bandwidth
>> (~3GB/s) for both zc and non-zc and doesn't even nearly saturate a CPU.
>> Was actually benchmarked by my colleague quite a while ago, but can't
>> find numbers. Probably need to at least add localhost numbers or grab
>> a better server.
>
> Testing localhost TCP with a hack (see below), it doesn't include
> refcounting optimisations I was testing UDP with and that will be
> sent afterwards. Numbers are in MB/s
>
> IO size | non-zc    | zc
> 1200    | 4174      | 4148
> 4096    | 7597      | 11228

I am surprised by the low numbers; you should be able to saturate a 100G
link with TCP and ZC TX API.

>
> Because it's localhost, we also spend cycles here for the recv side.
> Using a real NIC 1200 bytes, zc is worse than non-zc ~5-10%, maybe the
> omitted optimisations will somewhat help. I don't consider it to be a
> blocker. but would be interesting to poke into later. One thing helping
> non-zc is that it squeezes a number of requests into a single page
> whenever zerocopy adds a new frag for every request.
>
> Can't say anything new for larger payloads, I'm still NIC-bound but
> looking at CPU utilisation zc doesn't drain as much cycles as non-zc.
> Also, I don't remember if mentioned before, but another catch is that
> with TCP it expects users to not be flushing notifications too much,
> because it forces it to allocate a new skb and lose a good chunk of
> benefits from using TCP.

I had issues with TCP sockets and io_uring at the end of 2020:
https://www.spinics.net/lists/io-uring/msg05125.html

have not tried anything recent (from 2022).