Loom
Fibers, Continuations and Tail-Calls
Note
Loom is under active development, which means that information and advice given here might change in the future.
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.
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
Overview
Content Tools
ThemeBuilder