Loom prototype publicly available

[Ron Pressler]

Hi.

You can now build and try our initial Loom prototype.

	$ hg clone http://hg.openjdk.java.net/loom/loom  
	$ cd loom  
	$ hg update -r fibers 
	$ sh configure   
	$ make images

(Note that you must switch to the fibers branch)

- Supported platforms: Mac and Linux on x86-64  
- Tested GCs: Parallel and G1
- Supported compilers: C1, C2, Graal
- Missing features: JVM TI (fiber debugging), fiber/continuation serialization

Work on tail calls has not yet begun.

Current status will be tracked in a wiki page we will set up soon.

The fibers API is in the java.lang.Fiber class, and is minimal at this stage.
Thread locals should work as expected (and be associated with the fiber), and
calls to Thread.currentThread() would return a consistent result, so a lot of
existing code should work. Most relevant IO operations, as well as
java.util.concurrent constructs, Thread.sleep etc. will now result in the fiber
— rather than the underlying kernel thread — being blocked.

Object.wait() would still block the underlying kernel thread, as would trying to
park a fiber while a native monitor is held or a native method is on the stack.

The current implementation performs a full stack copy by default. To experiment
with the lazy-copying solution I described in a previous email, add
`-XX:+UnlockExperimentalVMOptions -XX:+UseNewCode`, but note that in that mode
exceptions thrown inside continuations may not work (probably crash the VM) and
stack traces would be missing frames.

This is an initial prototype made to be simple to implement so we could
experiment with different APIs, compatibility etc.. Performance, therefore, is
far from stellar. Designing a good API and writing an implementation with good
performance are our top priorities.

You’re welcome to try the prototype and share your impressions here. On Monday,
we’ll present Loom at JVMLS, and more updates will follow.


Ron