Note: None of this is implemented.
It can be a huge performance benefit that threads share memory and other resources, particularly when the threads need to exchange or share large, complex data structures. However, thread-shared memory also opens up safety risks when communications are not properly synchronized.
To mediate these risks, Cone offers several mechanisms that facilitate fast, convenient, safe communications between threads.
Race condition safety
Any data passed or shared between threads must be done so safely. Cone ensures this by only allowing "sendable" data to be sent to another thread. The only type of data that is not sendable is a reference that has the "mut" or "const" permission, or any aggregate type containing such a reference. Data of any other type is sendable, including shared, immutable references, unique references (which are moved from one thread to another), and any other move or copy type.
These restrictions apply to all mechanisms that support communications between threads, such as thread creation, concurrent data structures, and more.
An efficient model for thread-based concurrency is independent actors (threads) that throw work to each other in the form of messages placed on an actor's queue. Each actor pulls off messages from its queue in order, and then performs the work requested by the message. For now, let's assume that actors do not need shared, mutable access to objects nor do they perform any resource-blocking operations.
A straightforward way to implement this actor model involves using library-packaged actors that own and use concurrent message queues (or channels). Such multiple-producer-single-consumer (MPSC) channels are themselves library packaged-delivered concurrent FIFO data structures. Any actor that wants some actor to perform work adds that work to the actor's queue in the form of a message. As part of a continuous loop, the actor owning the queue pops a message off the queue and completely performs the non-blocking work it requests.
Any particular channel's messages are all the same type, but might very well be some sort of variant type, allowing different kinds of messages to be sent. Cone makes it easier to support this kind of message-passing by supporting the definition of queued-methods for the message type. For example:
struct CommandMessage counter u32 qfn increment(): ++counter qfn decrement(): --counter
A message can be sent to the actor in what looks like a normal method call:
However, instead of executing the function right away, the message is appended to the actor's queue. Later, the actor can pop this message off and dispatch it, which automatically then dispatches to the appropriate method's logic, updating the counter that is part of the actor's state.
A "qfn" will never specify a return type, because it is always being performed asynchronously with no guarantee of any value ever being calculated or returned. If a return value is desired, then one can pass the along a reference to the actor to which a message back can be sent.
Shared, Mutable Objects
In most cases, the above approach is the most efficient when threads do not require shared access to mutable objects. This is because using thread-shared, mutable objects carries at least these performance penalties: the run-time cost of the atomic synchronization primitives (especially locks), and the way mutation invalidates the memory cache for other threads.
For this reason (and to minimize the risk of deadlocks), it is desirable to send only small-sized messages between threads (typically references) and only use the static permissions on references. Static permissions offer the best performance, because they guarantee at compile-time that shared, mutable access to the same memory location cannot happen between threads. No run-time mechanisms are needed to enforce these guarantees. Although mutable objects cannot be shared between threads, immutable can be. Even better, it is possible to move a uniquely-owned object from one thread to another.
That said, there are definitely situations where we might want threads to share access to mutable objects. This can be supported using lock permissions, shared data structures or atomic primitives.
Cone's locked permissions have already been introduced. Lock permissions use runtime locks to synchronize when multiple threads need (mutable) access to the same object.
The locks provide an atomic envelope around a set of operations, so that no other thread can access the object while another has it locked. If threads request multiple locks at the same time, careful design is needed to avoid the risk of deadlocks.
Concurrent data structures
The message queue used by actor is one of several kinds of concurrent data structures. Other examples include concurrent arrays and dictionaries.
Concurrent data structures are implemented in a way that makes it always possible to access and mutate these structures across multiple threads. Every operation (method) on these types is essentially atomic. Importantly, these operations never use locks and never block.
Atomic primitives are a small collection of atomic (mutable) operations that are safe to perform on objects shared between threads. For example, it is safe to increment or decrement some shared integer.
These atomic operations are the building blocks that make possible many of the synchronized communication mechanisms already described.