A coroutine is a generalization of the subroutine. Forget for a
moment everything you've been taught about calling functions,
and stacks, etc... Think back to
BASIC, and the
GOTO statement. Imagine a more powerful
GOTO that could jump back and forth between
functions, passing arguments, and you begin to get the idea
Coroutines are a bit of ancient forgotten computer-science lore; stamped out of the collective memory by the hegemony of C. But they are useful in a wide variety of situations that can only be clumsily solved using 'standard' tools like threads and processes.
Coroutines can be used to simplify just about any difficult state-machine programming problem. Any time you find yourself tempted to build a complex state machine to solve what should be a simple problem, you've actually been pining for coroutines. With them, you can usually turn an 'inside-out' problem into a 'right-side-out' problem, and replace a few pages of hairy code with a single function.
A popular application of coroutines is in the construction of lightweight threading libraries (in fact, on Win32 coroutines are called 'fibers'). Coroutine threads are more scalable than OS threads - a desktop machine can easily juggle tens of thousands of them without gobbling up the entire virtual memory space.
Efficient coroutines hold out the possibility of building powerful, stateful internet servers that are massively scalable.
If you can convince your OS to deliver events, you can avoid the
In the latest versions of Linux (2.3.x) there are kernel tweaks that allow posix realtime signals to be sent to indicate I/O events. This is an improvement over SIGIO because realtime signals are queued and can be fetched in batches, so the whole mechanism should scale much better.
Anyway, here are the beginnings of my attempt to build an I/O scheduler around linux's realtime signals.
Soon I hope to plug this into our existing coroutine scheduler infrastructure.
FreeBSD-5.0 (-CURRENT) and possible 4.1-RELEASE include a new feature for kernel event delivery called kqueue() . Doug White is working on a Python interface, which I have hacked and plugged into egp-coro.
Note: The kqueue() interface just changed, so this code will require some rearrangement...
Win32 includes a robust event-delivery API, even supporting async file I/O. If you're interested in working on this, please contact me so we can coordinate effort and hopefully produce a portable high-level framework.