Prolog threads can exchange data using dynamic predicates, database records, and other globally shared data. These provide no suitable means to wait for data or a condition as they can only be checked in an expensive polling loop. Message queues provide a means for threads to wait for data or conditions without using the CPU.
Each thread has a message queue attached to it that is identified by the thread. Additional queues are created using message_queue_create/1.
If more than one thread is waiting for messages on the given queue and at least one of these is waiting with a partially instantiated Term, the waiting threads are all sent a wake-up signal, starting a rush for the available messages in the queue. This behaviour can seriously harm performance with many threads waiting on the same queue as all-but-the-winner perform a useless scan of the queue. If there is only one waiting thread or all waiting threads wait with an unbound variable, an arbitrary thread is restarted to scan the queue.101See the documentation for the POSIX thread functions pthread_cond_signal() v.s. pthread_cond_broadcast() for background information.
Please note that non-unifying messages remain in the queue. After the
following has been executed, thread 1 has the term b(gnu)
in its queue and continues execution using A = gnat.
<thread 1> thread_get_message(a(A)), <thread 2> thread_send_message(Thread_1, b(gnu)), thread_send_message(Thread_1, a(gnat)),
See also thread_peek_message/1.
message_queue_create(Alias), but according to the
ISO draft on Prolog threads.
deadline
option. If both options are provided, the earliest time is effective.
It Time is 0 or 0.0, thread_get_message/3 examines the queue but does not suspend if no matching term is available. Note that unlike thread_peek_message/2, a matching term is removed from the queue.
It Time < 0, thread_get_message/3 fails immediately.
The size(Size) property is always present and may be
used to enumerate the created message queues. Note that this predicate
does
not enumerate threads, but can be used to query the properties
of the default queue of a thread.
Explicit message queues are designed with the worker-pool model in mind, where multiple threads wait on a single queue and pick up the first goal to execute. Below is a simple implementation where the workers execute arbitrary Prolog goals. Note that this example provides no means to tell when all work is done. This must be realised using additional synchronisation.
%% create_workers(?Id, +N)
%
% Create a pool with Id and number of workers.
% After the pool is created, post_job/1 can be used to
% send jobs to the pool.
create_workers(Id, N) :-
message_queue_create(Id),
forall(between(1, N, _),
thread_create(do_work(Id), _, [])).
do_work(Id) :-
repeat,
thread_get_message(Id, Goal),
( catch(Goal, E, print_message(error, E))
-> true
; print_message(error, goal_failed(Goal, worker(Id)))
),
fail.
%% post_job(+Id, +Goal)
%
% Post a job to be executed by one of the pool's workers.
post_job(Id, Goal) :-
thread_send_message(Id, Goal).
These predicates provide a mechanism to make another thread execute some goal as an interrupt. Signalling threads is safe as these interrupts are only checked at safe points in the virtual machine. Nevertheless, signalling in multithreaded environments should be handled with care as the receiving thread may hold a mutex (see with_mutex/2). Signalling probably only makes sense to start debugging threads and to cancel no-longer-needed threads with throw/1, where the receiving thread should be designed carefully to handle exceptions at any point.
Signals (interrupts) do not cooperate well with the world of multithreading, mainly because the status of mutexes cannot be guaranteed easily. At the call-port, the Prolog virtual machine holds no locks and therefore the asynchronous execution is safe.
Goal can be any valid Prolog goal, including throw/1 to make the receiving thread generate an exception, and trace/0 to start tracing the receiving thread.
In the Windows version, the receiving thread immediately executes the signal if it reaches a Windows GetMessage() call, which generally happens if the thread is waiting for (user-)input.
Besides queues (section 8.3.1) threads can share and exchange data using dynamic predicates. The multithreaded version knows about two types of dynamic predicates. By default, a predicate declared dynamic (see dynamic/1) is shared by all threads. Each thread may assert, retract and run the dynamic predicate. Synchronisation inside Prolog guarantees the consistency of the predicate. Updates are logical: visible clauses are not affected by assert/retract after a query started on the predicate. In many cases primitives from section 8.4 should be used to ensure that application invariants on the predicate are maintained.
Besides shared predicates, dynamic predicates can be declared with the thread_local/1 directive. Such predicates share their attributes, but the clause list is different in each thread.
Thread-local dynamic predicates are intended for maintaining thread-specific state or intermediate results of a computation.
It is not recommended to put clauses for a thread-local predicate into a file, as in the example below, because the clause is only visible from the thread that loaded the source file. All other threads start with an empty clause list.
:- thread_local
foo/1.
foo(gnat).
DISCLAIMER Whether or not this declaration is appropriate in the sense of the proper mechanism to reach the goal is still debated. If you have strong feelings in favour or against, please share them in the SWI-Prolog mailing list.