Programming Perl, 3rd Edition

The Thread Model

The thread model of multiprocessing was first introduced to Perl as an experimental feature in version 5.005. (By "thread model", we mean threads that share data resources by default, not the new ithreads of version 5.6.) In some senses, this thread model is still an experimental feature even in 5.6, because Perl is a rich language and multithreading can make a muddle of even the simplest language. There are still various nooks and crannies of Perl semantics that don't interact very well with the notion of everything being shared. The new ithreads model is an attempt to bypass these problems, and at some future point, the current thread model may be subsumed under the ithread model (when we get an interface to ithreads that says "share everything you can by default"). But despite its warts, the current "experimental" thread model continues to be useful in many real-world situations where the only alternative to being a guinea pig is even less desirable. Reasonably robust applications can be written in threaded Perl, but you have to be very careful. You should at least consider using fork instead, if you can think of a way to solve your problem with pipes instead of shared data structures.

But some algorithms are easier to express if multiple tasks have easy and efficient access to the same pool of data.^[2] This makes for code that can be smaller and simpler. And because the kernel does not have to copy page tables for data (even if doing copy-on-write) at thread ...