Compile-time multitasking support for the Scheme programming language
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README.md

multischeme

This repo provides a sample implementation of an approach to add preemptive multitasking support to the Scheme programming language. This is meant as an alternative and/or complement to SRFI 18.

Language Changes

Multitasking is provided via a new type called a "task". Tasks are preemptive (they automatically yield control over to other tasks), but allow for arbitrarily large, user defined atomic blocks. The support for atomic blocks removes the need for additional types to synchronize multiple tasks, such as semaphores and mutexes.

The following new primitive methods are provided:

* (make-task <thunk>) - Creates a new, running task that invokes
  the given thunk
* (task? <expr>) - Returns #t if the passed expression is a task,
  otherwise #f
* (task-live? <expr>) - Returns #t if the passed expression is a
  live (running) task, otherwise #f
* (task-killed? <expr>) - Returns #t if the passed expression is a
  task that has been killed, otherwise #f
* (task-done? <expr>) - Returns #t if the passed expression is a
  task that exited normally, otherwise #f
* (task-kill <task>) - Kills the specified task immediately. Any
  child tasks are also killed. It is an error to call this with
  a non-task value. It is a no-op to call this with an already
  killed task. The return value is the passed in task.

This also introduces the concept of an atomic block; which is evaluated completely without any context switches occuring during its evaluation. The general principle of atomic blocks is that they are expressions which are known at compile time to only take a constant amount of time to evaluate, thus they can be run to completion without the risk of starving other tasks.

The set of atomic blocks is defined recursively; in a similar manner to how the language standard defines which calls are tail calls. The following expressions form atomic blocks:

* Constant expressions (numbers, characters, quoted expressions, etc.)
* Begin statements, when all of the nested statements are atomic blocks.
* Let statements, when the variable values and the body are all atomic
  blocks.
* If statements, when the test, consequent, and (optional) alternate are
  all atomic blocks.
* Lambda expressions (but not the application of a lambda).
* Set expressions, when the value being set is an atomic block.
* The application of certain primitive procedures, when the arguments
  are also atomic blocks. (The actual list of which procedures to
  consider to be atomic should include all procedures from the standard
  whose running time is not based on the size of its arguments).

Prerequisites

This implementation assumes the backing Scheme implementation supports SRFI 9, which adds support for record types, and SRFI 34, which adds support for exception handling.

Those assumptions are baked into this specific implementation, but the same multitasking primitives could be implemented without requiring those extensions.

Status

Build Status

This implementation supports all of R5RS except for macros and eval (and the interaction environment procedures related to eval). It also supports the extensions from SRFI 9 and SRFI 34. This code is also complete enough to bootstrap itself, and the new primitive procedures all work.

Implementation

Multitasking is implemented here in terms of a compile time source transformation.

Ideally, this should be performed some where in the middle of a Scheme compiler; after macro expansion and the removal of syntactic sugar, but before code generation. The file src/multitask.scm contains the code that you would drop into an existing Scheme compiler in order to add this feature in that way.

This repo also provides a simple compiler frontend to use with the multitasking transformation, to act as both a proof of concept and for end users who want to use this with an existing Scheme implementation out of the box. The frontend is defined in the file src/main.scm, and an example script showing how to use it with the Chicken Scheme compiler is provided in the file multischemec.sh.

Gory Details

The multitasking transformation is a variant on a continuation passing style (CPS) transformation.

In a traditional CPS transformation, the control flow in a program is made explicit by adding an additional continuation argument to every procedure. Instead of returning directly, procedures pass their return value to the continuation.

The important aspect of this is that the control flow in a CPS program is explicit; every transfer of control within a program has a corresponding procedure call. Since the transformation makes control flow explicit, we can modify it slightly in order to allow us to inject arbitrary control flow in a program.

This is done by making the procedures in the CPS program take two additional arguments instead of just one; a continuation, and a scheduler. Instead of passing its result to the continuation, a procedure passes its result and continuation to the scheduler. The scheduler can then decide what to do (e.g. continue processing the current task, or switch to another task).