Clozure CL provides facilities which enable multiple threads of execution (threads, sometimes called lightweight processes or just processes, though the latter term shouldn't be confused with the OS's notion of a process) within a lisp session. This document describes those facilities and issues related to multithreaded programming in Clozure CL.

Wherever possible, I'll try to use the term "thread" to denote a lisp thread, even though many of the functions in the API have the word "process" in their name. A lisp-process is a lisp object (of type CCL:PROCESS) which is used to control and communicate with an underlying native thread. Sometimes, the distinction between these two (quite different) objects can be blurred; other times, it's important to maintain.

Lisp threads share the same address space, but maintain their own execution context (stacks and registers) and their own dynamic binding context.

Traditionally, Clozure CL's threads have been cooperatively scheduled: through a combination of compiler and runtime support, the currently executing lisp thread arranged to be interrupted at certain discrete points in its execution (typically on entry to a function and at the beginning of any looping construct). This interrupt occurred several dozen times per second; in response, a handler function might observe that the current thread had used up its time slice and another function (the lisp scheduler) would be called to find some other thread that was in a runnable state, suspend execution of the current thread, and resume execution of the newly executed thread. The process of switching contexts between the outgoing and incoming threads happened in some mixture of Lisp and assembly language code; as far as the OS was concerned, there was one native thread running in the Lisp image and its stack pointer and other registers just happened to change from time to time.

Under Clozure CL's cooperative scheduling model, it was possible (via the use of the CCL:WITHOUT-INTERRUPTS construct) to defer handling of the periodic interrupt that invoked the lisp scheduler; it was not uncommon to use WITHOUT-INTERRUPTS to gain safe, exclusive access to global data structures. In some code (including much of Clozure CL itself) this idiom was very common: it was (justifiably) believed to be an efficient way of inhibiting the execution of other threads for a short period of time.

The timer interrupt that drove the cooperative scheduler was only able to (pseudo-)preempt lisp code: if any thread called a blocking OS I/O function, no other thread could be scheduled until that thread resumed execution of lisp code. Lisp library functions were generally attuned to this constraint, and did a complicated mixture of polling and "timed blocking" in an attempt to work around it. Needless to say, this code is complicated and less efficient than it might be; it meant that the lisp was a little busier than it should have been when it was "doing nothing" (waiting for I/O to be possible.)

For a variety of reasons - better utilization of CPU resources on single and multiprocessor systems and better integration with the OS in general - threads in Clozure CL 0.14 and later are preemptively scheduled. In this model, lisp threads are native threads and all scheduling decisions involving them are made by the OS kernel. (Those decisions might involve scheduling multiple lisp threads simultaneously on multiple processors on SMP systems.) This change has a number of subtle effects:

As a broad generalization: code that's been aggressively tuned to the constraints of the cooperative scheduler may need to be redesigned to work well with the preemptive scheduler (and code written to run under Clozure CL's interface to the native scheduler may be less portable to other CL implementations, many of which offer a cooperative scheduler and an API similar to Clozure CL (< 0.14) 's.) At the same time, there's a large overlap in functionality in the two scheduling models, and it'll hopefully be possible to write interesting and useful MP code that's largely independent of the underlying scheduling details.

The keyword :OPENMCL-NATIVE-THREADS is on *FEATURES* in 0.14 and later and can be used for conditionalization where required.