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Loom

 Fibers, Continuations and Tail-Calls

Table of Contents

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  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 DetectLocksInCompiledFrames to the java command  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 UseNewCode to the java command  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