Re: [RFC 00/12] io_uring zerocopy send

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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.