[PATCHSET v4 0/9] Support for provided buffers for send

[Jens Axboe <axboe@kernel.dk>]

Hi,

We never supported provided buffers for sends, because it didn't seem
to make a lot of sense. But it actually does make a lot of sense! If
an app is receiving data, doing something with it, and then sending
either the same or another buffer out based on that, then if we use
provided buffers for sends we can guarantee that the sends are
serialized. This is because provided buffer rings are FIFO ordered,
as it's a ring buffer, and hence it doesn't really matter if you
have more than one send inflight.

This provides a nice efficiency win, but more importantly, it reduces
the complexity in the application as it no longer needs to track a
potential backlog of sends. The app just sets up a send based buffer
ring, exactly like it does for incoming data. And that's it, no more
dealing with serialized sends.

In some testing with proxy [1], in basic shuffling of packets I see a
68% improvement with this over manually dealing with serializing sends.
That's a pretty big win on top of making the app simpler. Using
multishot further brings a nice improvement on top, about 10% extra on
top.

You can also find the patches here:

https://git.kernel.dk/cgit/linux/log/?h=io_uring-send-queue

[1] https://git.kernel.dk/cgit/liburing/tree/examples/proxy.c

Changes since v3:

- Drop MSG_MORE patch, separate thing anyway. Moved the flags hunk into
  the respective send/sendmsg patches, where they actually belonged.
- Rename IORING_FEAT_SEND_BUFS to IORING_FEAT_SEND_BUF_SELECT
- Enable MSG_WAITALL for send multishot. If set, then we retry via
  poll, if not set, we terminate the multishot sequence. This also
  fixes send multishot with short send in general.
- Add other networking related patch for recv/recvmsg multishot,
  managing IORING_CQE_F_MORE better rather than always needing to hit
  -ENOBUFS to terminate.

-- 
Jens Axboe