Re: [RFC 00/12] io_uring zerocopy send

[Willem de Bruijn]

> >>> 1) we pass a bvec, so no page table walks.
> >>> 2) zerocopy_sg_from_iter() is just slow, adding a bvec optimised version
> >>>     still doing page get/put (see 4/12) slashed 4-5%.
> >>> 3) avoiding get_page/put_page in 5/12
> >>> 4) completion events are posted into io_uring's CQ, so no
> >>>     extra recvmsg for getting events
> >>> 5) no poll(2) in the code because of io_uring
> >>> 6) lot of time is spent in sock_omalloc()/free allocating ubuf_info.
> >>>     io_uring caches the structures reducing it to nearly zero-overhead.
> >>
> >> Nice set of complementary optimizations.
> >>
> >> We have looked at adding some of those as independent additions to
> >> msg_zerocopy before, such as long-term pinned regions. One issue with
> >> that is that the pages must remain until the request completes,
> >> regardless of whether the calling process is alive. So it cannot rely
> >> on a pinned range held by a process only.
> >>
> >> If feasible, it would be preferable if the optimizations can be added
> >> to msg_zerocopy directly, rather than adding a dependency on io_uring
> >> to make use of them. But not sure how feasible that is. For some, like
> >> 4 and 5, the answer is clearly it isn't.  6, it probably is?
>
> Forgot about 6), io_uring uses the fact that submissions are
> done under an per ring mutex, and completions are under a per
> ring spinlock, so there are two lists for them and no extra
> locking. Lists are spliced in a batched manner, so it's
> 1 spinlock per N (e.g. 32) cached ubuf_info's allocations.
>
> Any similar guarantees for sockets?

For datagrams it might matter, not sure if it would show up in a
profile. The current notification mechanism is quite a bit more
heavyweight than any form of fixed ubuf pool.

For TCP this matters less, as multiple sends are not needed and
completions are coalesced, because in order.