Green threads

In computer programming, green threads are threads that are scheduled by a runtime library or virtual machine (VM) instead of natively by the underlying operating system. Green threads emulate multithreaded environments without relying on any native OS capabilities, and they are managed in user space instead of kernel space, enabling them to work in environments that do not have native thread support.

Green threads refers to the name of the Java thread library. The Green Team is the name of the team at Sun Microsystems that designed the Java thread library.

On a multi-core processor, native thread implementations can automatically assign work to multiple processors, whereas green thread implementations normally cannot. Green threads can be started much faster on some VMs. On uniprocessor computers, however, the most efficient model has not yet been clearly determined.

Benchmarks on computers running the (long outdated) Linux kernel version 2.2 have shown that:

When a green thread executes a blocking system call, not only is that thread blocked, but all of the threads within the process are blocked. To avoid that problem, green threads must use asynchronous I/O operations, although the increased complexity on the user side can be reduced if the virtual machine implementing the green threads spawn specific I/O processes (hidden to the user) for each I/O operation.

There are also mechanisms which allow use of native threads and reduce the overhead of thread activation and synchronization:

In Java 1.1, green threads were the only threading model used by the JVM, at least on Solaris. As green threads have some limitations compared to native threads, subsequent Java versions dropped them in favor of native threads.

An exception to this is the Squawk virtual machine, which is a mixture between an operating system for low-power devices and a Java virtual machine. It uses green threads in order to keep the native code to an absolute minimum and to support the migration of its isolates.

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