[MCRed]

The problem is the JVM. There's a reason erlang has its own virtual machine. Without virtual machine support, doing concurrency is really just faking it, and will result in significant bugs later on that are essentially impossible to debug.

[15155]

I take a neutral stance on this: the JVM isn't as amenable to the exact kind of concurrency that Erlang provides (green-threads with local GC backed by immutable data), but that isn't a fair assessment of concurrency.

Erlang (et. al. green thread models) has a method of concurrency that works well for situations you're using Erlang for. It's not the most efficient or most technically advanced mechanism for handling concurrency.

That being said-

> Without virtual machine support, doing concurrency is really just faking it

This is patently false, because it implies that: - A VM is necessary for "real" concurrency - (by deduction) Anything not utilizing an erlang-identical model is "faking it."

> will result in significant bugs later on that are essentially impossible to debug

I'm going to need to see an anecdote or citation for this.

Personally-speaking, I have a much easier time (quickly) understanding the execution of event-loop-backed cooperative multitasking than high-level, arbitrarily-preempted threads.

Also - I cannot see any reason why Quasar for the JVM would be any easier or harder to debug (from a theoretical, not tooling or practical standpoint) than Erlang actors.

The nuances differentiating the two models aren't great enough to merit bugs that you might see in one place or the other. Unbound, CPU-heavy tasks will cause problems for both systems.

[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.