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

David Ahern
2022-07-18T02:19:27Z

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.

>
>>> 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.

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

>
> 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.
>