Fibers, Continuations and Tail-Calls
Loom is under active development, which means that information and advice given here might change in the future.
Mac and Linux on x86-64
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(Note that you must switch to the
fibers branch before building)
JVM TI support (fiber debugging)
- Cloning continuations
Serialization of fiber/continuation
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.
Continuation class does not provide a mechanism of communication between
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.
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 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