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

Pavel Begunkov
2022-07-20T13:32:58Z

On 7/18/22 03:19, David Ahern wrote:
> On 7/14/22 12:55 PM, Pavel Begunkov wrote:
>>>>>> 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.
>>
>> It was a quick test with my laptop, not a super fast CPU, preemptible
>> kernel, etc., and considering that the fact that it processes receives
>> from in the same send syscall roughly doubles the overhead, 87Gb/s
>> looks ok. It's not like MSG_ZEROCOPY would look much different, even
>> more to that all sends here will be executed sequentially in io_uring,
>> so no extra parallelism or so. As for 1200, I think 4GB/s is reasonable,
>> it's just the kernel overhead per byte is too high, should be same with
>> just send(2).
>
> ?
> It's a stream socket so those sends are coalesced into MTU sized packets.

That leaves syscall and io_uring overhead, locking the socket, etc.,
which still requires more cycles than just copying 1200 bytes. And
the used CPU is not blazingly fast, could be that a better CPU/setup
will saturate 100G

>>>> 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).
>>
>> Haven't seen it back then. In general io_uring doesn't stop submitting
>> requests if one request fails, at least because we're trying to execute
>> requests asynchronously. And in general, requests can get executed
>> out of order, so most probably submitting a bunch of requests to a single
>> TCP sock without any ordering on io_uring side is likely a bug.
>
> TCP socket buffer fills resulting in a partial send (i.e, for a given
> sqe submission only part of the write/send succeeded). io_uring was not
> handling that case.

Shouldn't have been different from send(2) with MSG_NOWAIT, can be short
and the user should handle it. Also I believe Jens pushed just recently
in-kernel retries on the io_uring side for TCP in such cases.

> I'll try to find some time to resurrect the iperf3 patch and try top of
> tree kernel.

Awesome

>> You can link io_uring requests, i.e. IOSQE_IO_LINK, guaranteeing
>> execution ordering. And if you meant links in the message, I agree
>> that it was not the best decision to consider len < sqe->len not
>> an error and not breaking links, but it was later added that
>> MSG_WAITALL would also change the success condition to
>> len==sqe->len. But all that is relevant if you was using linking.

--
Pavel Begunkov