[BenoitP]

The quasar library comes with bytecode manipulation to implement lightweight threads. It saves the program counter and local stack, and pushes it back when resuming. The space cost of context switching is reduced to its minimum.

Now this is still on the regular JVM, which will probably have difficulties JITting code that resides in those green-threads (but still better performance than erlang). Also, the entire application is subject to the regular hiccups: gc requires global safepoints, which are reached by making each thread wait when it reaches a local safepoint; on top of the stop-the-world gc operations themselves.

But things are improving: RedHat is developing Shenandoah, a GC that has STW pauses under 10ms. The only thing left is making concurrent the scanning of garbage roots(those reside in the registers and stacks). Their goal for version 2 is to get completely rid of pauses.