Quasar uses byte code instrumentation to do things you can't do with "regular" java. How exactly they can do things whne the JVM is the same JVM? good question,

Quasar uses byte code instrumentation to do things you can't do with "regular" java. How exactly they can do things whne the JVM is the same JVM? good question, I don't know, I know implementing user threads simply means keeping stack information, keeping a scheduler, handling signals, without asking the kernel / OS for a native thread (had to implement user threads in C in the past, not fun) Maybe the JVM allows for more fine grained control on PC location, and allows to "go to" different places in the code without invoking a native thread. But I know that when you create a Quasar co-routine / green thread / user thread / continuation or whatever we want to call it, then it's not the same as Akka. In Akka, if you have a "conversation" between a million actors in a circle, you can still get 1 thread to be reused, if there is no need to use more than one. But if you try to do 3 things in 100% parallel, you might find yourself with 3 threads (from the little that I know). In quasar, you can do all that, without invoking an OS thread, I think this is the main difference. http://docs.paralleluniverse.co/quasar/ http://docs.paralleluniverse.co/quasar/