Heavy weight and light weight thread

Question

What are the Light weight and heavy weight threads in terms of Java?

Answer 1

Java standard threads are reasonably heavy in comparison to Erlang threads which are very light spawnable processes. Erlang demonstrates a distributed finite state machine. However as an example, http://kilim.malhar.net/ , a Java extension library based on the Actor model of concurrency, proposes a construct for light weight threads in java. Instead of Thread implementing run(), a Kilim thread implements from the Kilim library using an execute() method. Apparently it shows Java's runtime outperforms Erlang's (atleast in a local environment AFAIK). Java did actually have such things in the original language spec called 'green threads' but subsequent Java versions dropped them in favor of native threads

Answer 2

In most systems Light weight threads are the normal threads you create with the help of library, like p_threads in linux.

While Heavy weight, in some systems, refer to a system process, with its own virtual memory and a more complex structure, like information about the process performance/statistics.

For more information:

http://www.computerworld.com/s/article/66405/Processes_and_Threads

http://msdn.microsoft.com/en-us/library/ms684841(VS.85).aspx

Answer 3

It's related to the amount of "context" associated with a thread, and consequently the amount of time it takes to perform a "context switch".

Heavyweight threads, (usually kernel/os level threads) have a lot of context (hardware registers, kernel stacks, etc). So it takes a lot of time to switch between threads. Heavyweight threads may also have restrictions on them, for example, on some OSes, kernel threads cannot be pre-empted, which means they can't forcibly be switched out until they give up control.

Lightweight threads on the other hand (usually, user space threads) have much less context. (They essentially share the same hardware context), they only need to store the context of the user stack, hence the time taking to switch lightweight threads is much shorter.

On most OSes, any threads you create as a programmer in user space will be lightweight in comparison to the kernel space threads. There is no formal definition of heavyweight and lightweight, it's just more of a comparison between threads with more context and threads with less context. Don't forget that every OS has its own different implementation of threads, and the lines between heavy and light threads are not necessarily clearly defined. In some programming languages and frameworks, when you create a "Thread" you might not even be getting a full thread, you might just be getting some abstraction that hides the real number of threads underneath.

[Some OSes allow threads to share address space, so threads that would usually be quite heavy, are slightly lighter]