Baby Steps

A blog about programming, and tiny ways to improve it.

Proposed JS Parallelism vs Actors

In one of the comments on yesterday’s post, Tushar Pokle asked why I would champion my model over an Erlang model of strict data separation. There are several answers to this question. The simplest answer is that Web Workers already provide an actors model, though they do not make tasks particularly cheap (it’s possible to work around this by creating a fixed number of workers and sending tasks for them to execute).

The better answer is that I don’t think that Erlang’s actor model and the model I propose are that far apart. I see this model as a kind of “delimited actor”. Why would I say that, since it does not seem to resemble actors at a superficial level? The reason is that, in my model, each child is quite isolated from one another. In a typical “shared memory” model, processes communicate by modifying common data structures. This turns out to be highly unreliable.

In the model I propose (which needs a name), processes may share data structures, but they cannot communicate this way, as those structures are immutable. In fact, the only way that sibling processes can communicate with one another is by joining each other. This allows them to recieve the other processes result. This is effectively a one-shot message from one task to another.

So, in a way my model is a simplification of actors: it allows you to spawn a set of actors. The parent data which they share is effectively an initial message from the parent to each child. The child’s result is then a one time message from each child to the parent (or to other siblings in a DAG-like fashion).

I don’t actually think my model is a good choice as the only model for parallelism in your language, but I think it complements actors quite well. Consider what you would do in Erlang if you want to process the members of a list in parallel: you would create a task for each member of the list and send it whatever context is requires. You would then receive back the new values and construct the new list. In other words, you would implement precisely the messaging pattern that this model defines.

Of course, as often happens, supporting only a limited model for messaging lets you optimize things in the implementation. Because we know that the child processes are only of limited duration, we don’t have to copy the parent’s data but can instead allow them to reference it (readonly) directly. Similarly, because we know that each child is dead when its result is received, we don’t have to copy the result into the parent’s address space, but can again reuse the values directly. Finally, the garbage collector does not have to consider the case of cross-process garbage collection: the parent’s data is immutable, so whatever is live will remain live. The data accessible to each child is disjoint, so we can collect data owned by each child independently without looking at the others.

I am tempted to call my model “delimited actors”, but I think it looks sufficiently different from actors that this name might be misleading. But, as I just argued, I think it is closer to actors than to the “shared memory” model that has caused so many problems and difficulties.

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