[PATCHSET,v2,0/5] Send and receive bundles

Message

Jens Axboe April 20, 2024, 1:29 p.m. UTC

Hi,

I went back to the drawing board a bit on the send multishot, and this
is what came out.

First support was added for provided buffers for send. This works like
provided buffers for recv/recvmsg, and the intent here to use the buffer
ring queue as an outgoing sequence for sending.

But the real meat is adding support for picking multiple buffers at the
time, what I dubbed "bundles" here. Rather than just pick a single buffer
for send, it can pick a bunch of them and send them in one go. The idea
here is that the expensive part of a request is not the sqe issue, it's
the fact that we have to do each buffer separately. That entails calling
all the way down into the networking stack, locking the socket, checking
what needs doing afterwards (like flushing the backlog), unlocking the
socket, etc. If we have an outgoing send queue, then pick what buffers
we have (up to a certain cap), and pass them to the networking stack in
one go.

Bundles must be used with provided buffers, obviously. At completion
time, they pass the starting buffer ID in cqe->flags, like any other
provided buffer completion. cqe->res is the TOTAL number of bytes sent,
so it's up to the application to iterate buffers to figure out how many
completed. This part is trivial. I'll push the proxy changes out soon,
just need to cleanup them up as I did the sendmsg bundling too and would
love to compare.

With that in place, I added support for recv for bundles as well. Exactly
the same as the send side - if we have a known amount of data pending,
pick enough buffers to satisfy the receive and post a single completion
for that round. Buffer ID in cqe->flags, cqe->res is the total number of
buffers sent. Receive can be used with multishot as well - fire off one
multishot recv, and keep getting big completions. Unfortunately, recvmsg
multishot is just not as efficient as recv, as it carries additional
data that needs copying. recv multishot with bundles provide a good
alternative to recvmsg, if all you need is more than one range of data.
I'll compare these too soon as well.

This is obviously a bigger win for smaller packets than for large ones,
as the overall cost of entering sys_sendmsg/sys_recvmsg() in terms of
throughput decreases as the packet size increases. For the extreme end,
using 32b packets, performance increases substantially. Runtime for
proxying 32b packets between three machines on a 10G link for the test:

Send ring:		3462 msec		1183Mbit
Send ring + bundles	 844 msec		4853Mbit

and bundles reach 100% bandwidth at 80b of packet size, compared to send
ring alone needing 320b to reach 95% of bandwidth (I didn't redo that
test so don't have the 100% number).

Patches are on top of my for-6.9/io_uring branch and can also be found
here:

https://git.kernel.dk/cgit/linux/log/?h=io_uring-recvsend-bundle

Changes since v1:
- Shuffle some hunks around
- Fix various bugs
- Rebase on current 6.10 branch

 include/linux/io_uring_types.h |   3 +
 include/uapi/linux/io_uring.h  |  10 ++
 io_uring/io_uring.c            |   3 +-
 io_uring/kbuf.c                | 157 +++++++++++++++++-
 io_uring/kbuf.h                |  53 ++++--
 io_uring/net.c                 | 284 ++++++++++++++++++++++++++++-----
 io_uring/opdef.c               |   1 +
 7 files changed, 456 insertions(+), 55 deletions(-)