Each call to zmq::poll is painfully slow when we have many open zmq
sockets, such as when we have 1800 outbound connections (i.e. connected
to every other service node, as services nodes might have sometimes and
the Session push notification server *always* has).
In testing on my local Ryzen 5950 system each time we go back to
zmq::poll incurs about 1.5ms of (mostly system) CPU time with 2000 open
outbound sockets, and so if we're being pelted with a nearly constant
stream of requests (such as happens with the Session push notification
server) we incur massive CPU costs every time we finish processing
messages and go back to wait (via zmq::poll) for more.
In testing a simple ZMQ (no OxenMQ) client/server that establishes 2000
connections to a server, and then has the server send a message back on
a random connection every 1ms, we get atrocious CPU usage: the proxy
thread spends a constant 100% CPU time. Virtually all of this is in the
poll call itself, though, so we aren't really bottlenecked by how much
can go through the proxy thread: in such a scenario the poll call uses
its CPU then returns right away, we process the queue of messages, and
return to another poll call. If we have lots of messages received in
that time, though (because messages are coming fast and the poll was
slow) then we process a lot all at once before going back to the poll,
so the main consequences here are that:
1) We use a huge amount of CPU
2) We introduce latency in a busy situation because the CPU has to make
the poll call (e.g. 1.5ms) before the next message can be processed.
3) If traffic is very bursty then the latency can manifest another
problem: in the time it takes to poll we could accumulate enough
incoming messages to overfill our internal per-category job queue,
which was happening in the SPNS.
(I also tested with 20k connections, and the poll time scaling was
linear: we still processed everything, but in larger chunks because
every poll call took about 15ms, and so we'd have about 15 messages at a
time to process with added latency of up to 15ms).
Switching to epoll *drastically* reduces the CPU usage in two ways:
1) It's massively faster by design: there's a single setup and
communication of all the polling details to the kernel which we only
have to do when our set of zmq sockets changes (which is relatively
rare).
2) We can further reduce CPU time because epoll tells us *which* sockets
need attention, and so if only 1 connection out of the 2000 sent us
something we can only bother checking that single socket for
messages. (In theory we can do the same with zmq::poll by querying
for events available on the socket, but in practice it doesn't
improve anything over just trying to read from them all).
In my straight zmq test script, using epoll instead reduced CPU usage in
the sends-every-1ms scenario from a constant pegged 100% of a core to an
average of 2-3% of a single core. (Moreover this CPU usage level didn't
noticeably change when using 20k connections instead of 2k).
Internal messages (control messages, worker messages) are always 3 parts
or less, so we can optimize by using a stack allocated std::array for
those cases rather than needing to continually clear and expand a heap
allocated vector.
bt_*, hex, base32z, base64 all moved to oxen-encoding a while ago; this
finishes the move by removing them from oxenmq and instead making oxenmq
depend on oxen-encoding.
Jason Rhinelander
Sean Darcy