Main

Loom

 Fibers, Continuations and Tail-Calls

Supported Platforms

Mac and Linux on x86-64

Download and Build from Source

$ 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 before building)

Missing Features

• JVM TI support (fiber debugging)

• Forced preemption

• Cloning continuations

• Serialization of fiber/continuation

   

Continuations

Design

The primitive continuation construct is that of a scoped (AKA multiple-named-prompt), stackful, one-shot (non-reentrant) delimited continuation. The continuation can be cloned, and thus used to implement reentrant delimited continuations. The construct is exposed via the java.lang.Continuation class. Continuations are intended as a low-level API, that application authors are not intended to use directly. They will use higher-level constructs built on top of continuations, such as fibers or generators.

A continuation object is constructed by passing two arguments to the constructor: a Runnable target that serves as the body of the continuation, and a java.lang.ContinuationScope. The scope is the delimited continuation's prompt, that allows continuations to be nested. One could think of such "scoped continuations" as nested try/catch blocks, where the scope is the type of the exception thrown, which determines the handler called.

A continuation is started by calling Continuation.run, which would start executing the body in the continuation's target, and returns either when the continuation terminates (the body runs to completion, and terminates either normally or abnormally), or when it yields on the continuation's scope. To query the reason for run returning, use Continuation.isDone, which returns true if the body has terminated, or false if it has yielded.

A call to the static Continuation.yield suspends the current continuation and all enclosing continuations up until the innermost one with the scope passd to yield, causing the run method of that continuation to return.

The Continuation class does not provide a mechanism of communication between run and yield (i.e., neither takes or returns a value that is passed to/received from the other), unlike most implementations of delimited continuations. However, implementing a class that does allow this kind of communication on top of the Continuation class is straightforward, and will likely be included in the JDK.

Implementation

Performance

Current yield/continue performance is far from stellar. The reason is that we focused on getting a working prototype using existing Hotspot mechanisms, some of which have not been designed to be used so frequently. We are now working on improving performance both by optimizing the actual freeze/thaw logic, as well as optimizing those existing VM mechanisms. 

One mechanism that is particularly slow is the one used to detect whether a frame is holding a native monitor (synchronized block). Because this mechanism is so slow, the monitor detection can be turned off (which will break fiber code that runs through synchronized blocks) by adding -XX:-DetectLocksInCompiledFrames to the java command line.

An important performance feature is lazy-copying of frames. This feature is currently turned off by default because it is missing some important functionality, in particular it does not support the throwing of exceptions, and stack traces will miss some frames (those that have not been thawed). To turn lazy copying on, add -XX:+UnlockExperimentalVMOptions -XX:+UseNewCode to the java command line.

Fibers

Design

Implementation

Fibers are implemented in the core libraries. A fiber is implemented as a continuation (of fiber scope) that is wrapped as a task and scheduled by a j.u.c.Executor.

Tail Calls

Design