[PATCHSET v3 0/8] 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 36% improvement with this over manually dealing with sends. That's
a pretty big win on top of making the app simpler. Using multishot
further brings a nice improvement on top. Need to rebench on the 10G
setup, in a vm it's looking pretty tasty.

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 v2:

- Add prep patches unifying send/recv header handling
- Add multishot mode as well
- Fix issue with REQ_F_BL_EMPTY on ring provided buffers

-- 
Jens Axboe