The big news in J904 is multithreading. J verbs can now be run in
different cores, sharing the J global namespaces.
[A version of J with limited support for multithreading was revealed
last year by John Ference of Monument AI. John's work was an important
proof of concept, but the J904 version is an independent development
using pthreads instead of OpenMP. John has agreed with Jsoftware to
give Jsoftware access to the code and testcases used in his work. We
have not used the code.]
Two primitives support multithreading:
(0 T. '') creates a thread on a new core. A thread running a verb is
called a /task/. Even if you don't plan to create tasks, you should
create threads to allow JE to use them for its own purposes.
([x] u t. '' y) executes ([x] <@:u y) as a task to produce a boxed
result. The thread that executed t. continues to run. But the result
is a special kind of box called a /pyx/.
['pyx' is Greek for box, and has been brought into English on occasion
when a word has been needed for a special box. The most recent such
borrowing until now is for the box used by the Royal Mint to hold coins
for assay.]
A pyx looks on the outside like any other box. You can pass it as an
argument. But as soon as you look inside it, your thread is blocked
until the task producing the pyx's value has completed. Then you see the
value, and your thread continues.
Tasks share the global namespaces and thus can freely read, write, stomp
on, and delete names visible to other tasks.
We encourage you to experiment with threads and tasks. We are sure that
a layer of synchronizing primitives - semaphores, locks, and mutexes -
will be needed, but we would like to get practical experience with tasks
before we implement them. Suggestions welcome.
Henry Rich
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