trio (original) (raw)

https://bugs.python.org/issue27906 suggests that it's important to be able to accept multiple connections in a single event loop tick.

It's trivial to implement, too -- just

def accept_nowait(self): try: return self._sock.accept() except BlockingIOError: raise _core.WouldBlock from None

But...

I am... not currently able to understand how/why this can help. Consider a simple model where after accepting each connection, we do a fixed bolus of CPU-bound work taking T seconds and then immediately send the response, so each connection is handled in a single loop step. If we accept only 1 connection per loop step, then each loop step takes 1 * T seconds and we handle 1 connection/T seconds on average. If we accept 100 connections per loop step, then each loop step takes 100 * T seconds and we handle 1 connection/T seconds on average.

Did the folks in the bug report above really just need an increased backlog parameter to absorb bursts? Batching accept() certainly increases the effective listen queue depth (basically making it unlimited), but "make your queues unbounded" is not a generically helpful strategy.

The above analysis is simplified in that (a) it ignores other work going on in the system and (b) it assumes each connection triggers a fixed amount of synchronous work to do. If it's wrong it's probably because one of these factors matters somehow. The "other work" part obviously could matter, if the other work is throttlable at the event loop level, in the sense that if loop steps take longer then they actually do less work. Which is not clear here (the whole idea of batching accept is make it not have this property, so if this is how we're designing all our components then it doesn't work...).

I guess one obvious piece of "other work" that scales with the number of passes through the loop is just, loop overhead. One would hope this is not too high, but it is not nothing.

If we do want this, then #13 will want to use it.

The same question probably applies to recvfrom / sendto -- right now a task can only send or receive 1 UDP packet per event loop tick.