What's wrong with threading on IO? For example if you have a webserver which has to wait on replies from various other resources while serving enough clients to

What's wrong with threading on IO? For example if you have a webserver which has to wait on replies from various other resources while serving enough clients to saturate itself. Sure, performance wise it is probably better to have a fixed number of threads which do their own scheduling in userspace but this requires much more work then a simple if(!finished()){yield();} and making a thread per client.