Hacker News Reader

Comment by sovande | Hacker News Reader

I don't understand why co-routines, green threads or any other n:1 user-level threading model are of much interest in today's multi-core processor world? To utilise the computing power in front of you, you really need to use 1:1 kernel level threads in your program.

This is also the biggest problem I have with node.js which is inherently single-threaded (libuv's thread-pool aside) and only uses a fraction of the CPU power available on your computer.

[Edit]: A trivial, but practical example which demonstrate the power of using your machine's capabilities compared to just a fraction. If you use Make to build your project. Try 'make -j n' where n is 2 x cores on your machine. And observed the speed of building in parallel compared to serialised.

View on HN

The main use I've found for them is converting state machines to coroutines. Logic that is tedious, cryptic or unmaintainable with state machines becomes trivial with coroutines:

http://eli.thegreenplace.net/2009/08/29/co-routines-as-an-al...

I agree though that someone needs to adapt coroutines to run concurrently. As long as there is no shared memory and you only use pipes to transmit messages (optimized with something like copy-on-write), there should be a way to partition the coroutines by the number of processors. I think it would work if you could switch the stack atomically.

One thing to also consider is that if you use nonblocking system calls, you get very high CPU utilization you're just not going to get with threads and blocking. I imagine users goof this with node.js though. It would be nice to have pure nonblocking middleware..

It just depends what you're doing / what you want.

If you're doing N different things that are all actively using the CPU, then yeah, you want real threads. But if you maybe have a bunch of different things "in flight" that are mostly just sitting around waiting for something to else to happen to make progress, then something like coroutines or green threads could make sense 'cuz of lower startup and memory overhead.

It can also be handy just for code structure. You want two things making forward progress at once, and instead of using callbacks and a bunch of variables to try to maintain state, you just use coroutines and the implicit state therein, but since you didn't go to real, executing-in-parallel threads, you don't have to worry about all the synchronization/race avoidance junk that comes from that (but of course, you also don't get the multi-core speedup).

Modern runtimes use M:N, not N:1.

Reply on news.ycombinator.com