Module Picos

The architecture of Picos

The core concepts of Picos are

• Trigger — ability to await for a signal,
• Computation — a cancelable computation, and
• Fiber — an independent thread of execution,

that are implemented in terms of the effects

• Trigger.Await — to suspend and resume a fiber,
• Computation.Cancel_after — to cancel a computation after given period of time,
• Fiber.Current — to obtain the unique handle of the current fiber,
• Fiber.Yield — to cooperatively request rescheduling the current fiber, and
• Fiber.Spawn — to start new fibers,

that can be used to implement many kinds of higher level concurrent programming facilities.

Understanding cancelation

A central idea of Picos is to provide a collection of building blocks for parallelism safe cancelation. Consider the following characteristic example:

Mutex.protect mutex begin fun () ->
  while true do
    Condition.wait condition mutex
  done
end

Assume that the fiber executing the above computation might be canceled, at any point, by another fiber running in parallel. How could that be done both effectively and safely?

• To be effective, cancelation should take effect as soon as possible. In this case, cancelation should take effect even during the Mutex.lock inside Mutex.protect and the Condition.wait operations when the fiber might be in a suspended state awaiting for a signal to continue.
• To be safe, cancelation should not leave the program in an invalid state or cause the program to leak memory. In this case, the ownership of the mutex must be transferred to the next fiber or be left unlocked and no references to unused objects must be left in the mutex or the condition variable.

Picos allows Mutex and Condition to be implemented such that cancelation may safely take effect at or during calls to Mutex.lock and Condition.wait.

Cancelation in Picos

The Fiber concept in Picos corresponds to an independent thread of execution. A fiber may explicitly forbid or permit the scheduler from propagating cancelation to it. This is important for the implementation of some key concurrent abstractions such as condition variables, where it is necessary to forbid cancelation when the associated mutex is reacquired.

Each fiber has an associated Computation. A computation is something that needs to be completed either by returning a value through it or by canceling it with an exception. To cancel a fiber one cancels the computation associated with the fiber.

Before a computation has been completed, it is also possible to attach a Trigger to the computation and also to later detach the trigger from the computation. A trigger attached to a computation is signaled as the computation is completed.

The Trigger concept in Picos is what allows a fiber to be suspended and later resumed. A fiber can create a trigger, add it to any shared data structure(s), and await for the trigger to be signaled. The await operation, which is implemented by the scheduler, also, in case the fiber permits cancelation, attaches the trigger to the computation of the fiber when it suspends the fiber. This is what allows a fiber to be resumed via cancelation of the computation.

The return value of await tells whether the fiber was resumed normally or due to being canceled and the caller then needs to properly handle either case. After being canceled, depending on the concurrent abstraction being implemented, the caller might need to e.g. remove references to the trigger from the shared data structures, cancel asynchronous IO operations, or transfer ownership of a mutex to the next fiber in the queue of the mutex.

Modules reference

We first open the Picos module

open Picos

and define a simple scheduler for running the examples in this document on OCaml 4

let run main =
  Picos_threaded.run ~forbid:false main

using the basic thread based scheduler and on OCaml 5

let run main =
  Picos_fifos.run ~forbid:false main

using the basic effects based scheduler that come with Picos as samples.

Auxiliary modules

Core modules

Please note that the example code snippets in this documentation may e.g. use the Domain and Unix modules in order to be able to describe Picos concepts in isolation in the absence of a Picos compatible scheduler.