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    <p>The short answer is that they're quite heavy (you can't launch 100+ of them cheaply), and they exhibit unexpected behaviours (somewhat mitigated by recent CPAN modules).</p> <p><b>You <i>can</i> safely use Perl ithreads by treating them as independent Actors</b>.</p> <ol> <li>Create a Thread::Queue::Any for "work".</li> <li>Launch multiple ithreads and "result" Queues passing them the ("work" + own "result") Queues by closure.</li> <li>Load (require) all the remaining code your application requires (not before threads!)</li> <li>Add work for the threads into the Queue as required.</li> </ol> <p>In "worker" ithreads:</p> <ol> <li>Bring in any common code (for any kind of job)</li> <li>Blocking-dequeue a piece of work from the Queue</li> <li>Demand-load any other dependencies required for this piece of work.</li> <li>Do the work.</li> <li>Pass the result back to the main thread via the "result" queue.</li> <li>Back to 2.</li> </ol> <p>If some "worker" threads start to get a little beefy, and you need to limit "worker" threads to some number then launch new ones in their place, then create a "launcher" thread first, whose job it is to launch "worker" threads and hook them up to the main thread.</p> <p><b>What are the main problems with Perl ithreads?</b></p> <p>They're a little inconvenient with for "shared" data as you need to explicity do the sharing (not a big issue).</p> <p>You need to look out for the behaviour of objects with DESTROY methods as they go out of scope in some thread (if they're still required in another!)</p> <p><b>The big one</b>: Data/variables that aren't explicitly shared are <b>CLONED</b> into new threads. This is a performance hit and probably not at all what you intended. The work around is to launch ithreads from a pretty much "pristine" condition (not many modules loaded).</p> <p>IIRC, there are modules in the Threads:: namespace that help with making dependencies explicit and/or cleaning up cloned data for new threads.</p> <p>Also, IIRC, there's a slightly different model using ithreads called "Apartment" threads, implemented by Thread::Appartment which has a different usage pattern and another set of trade-offs. </p> <p><b>The upshot:</b></p> <p>Don't use them unless you know what you're doing :-)</p> <p>Fork <i>may</i> be more efficient on Unix, but the IPC story is <i>much</i> simpler for ithreads. (This may have been mitigated by CPAN modules since I last looked :-)</p> <p>They're <i>still</i> better than Python's threads.</p> <p>There might, one day, be something much better in Perl 6.</p>
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    1. POUse cases for ithreads (interpreter threads) in Perl and rationale for using or not using them?
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    1. COThanks! This is the most detailed usage instruction for Perl's threads that I have seen to date. - In my question I stated that the *how* of using ithreads is covered while the *why* and *what for* are lacking. Thinking more about this, the *how*, the *why*, and the *what for* are closely related, and actually the *how* hasn't been accurate enough (for common Perl users like me) to answer the *why* and *what for*. - This answer is a step forward. Thanks again.
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    2. CO"They're still better than Python's threads." – can you explain this? Please, it's very intresting
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    3. COMostly it's about the Python GIL in most implementations (predominantly cpython but others to lesser extents). Basically, it means that OS threads on Python cannot actually execute CPU work concurrently -- which completely defeats the point of them. You can use them (Python threads) to make I/O a little more concurrent, but it simpler to use event-oriented APIs to do that.
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