Perl 6 and Parrot Essentials, Second Edition

Threads

Threads allow multiple pieces of code to run in parallel. This is useful when you have multiple physical CPUs to share the load of running individual threads. With a single processor, threads still provide the feeling of parallelism, but without any improvement in execution time. Even worse, sometimes using threads on a single processor will actually slow down your program.

Still, many algorithms can be expressed more easily in terms of parallel running pieces of code and many applications profit from taking advantage of multiple CPUs. Threads can vastly simplify asynchronous programs like internet servers: a thread splits off, waits for some I/O to happen, handles it, and relinquishes the processor again when it’s done.

Parrot compiles in thread support by default (at least, if the platform provides some kind of support for it). Unlike Perl 5, compiling with threading support doesn’t impose any execution time penalty for a non-threaded program. Like exceptions and events, threads are still under development, so you can expect significant changes in the near future.

As outlined in the previous chapter, Parrot implements three different threading models. The following example uses the third model, which takes advantage of shared data. It uses a TQueue (thread-safe queue) object to synchronize the two parallel running threads. This is only a simple example to illustrate threads, not a typical usage of threads (no one really wants to spawn two threads just to print out a simple ...