Questions on parallel programming terminology

[tkf]

@juliohm

refers to real threads as 1:Threads.nthreads() , correct?

Yes, if what you mean by “real thread” is OS thread.

@juliohm

From what I grasped, “green threads” aren’t real threads, what are they then? Should new users learn about them at all?

I think it depends. If you want to, say, implement something like pmap, you probably should. But if you just want to use pmap then it’s probably OK to forget about it.

@juliohm

First, I understand that these three concepts are restricted to process pools, so nothing to do with threads.

Channels and futures in general are communication mechanisms in concurrent programming. The Julia type Channel can be used for communication within a process and RemoteChannel and Future are for communication across processes. So, Channel cannot be used with a process pool (but it can be used with thread pool; e.g., with @spawn and @threads).

(Though looking at the second question, maybe you already get this idea?)

Note my emphasis on concurrency and not parallelism. The difference is a bit subtle and I’m not sure everybody agrees with a single definition. But I like this explanation by Rob Pike:

Concurrency Is Not Parallelism

I’m bringing it up because you may be confused that Channel is used even without threading, i.e., parallelism. Concurrency is about dealing with multiple things at once so Channel is useful even though you use only one CPU (e.g., using @async).

@juliohm

Third, I understand that a Future is just a channel that is returned by a remote function call?

Future can be used for passing at most one value (or an exception) to the peer while RemoteChannel can be used for passing arbitrary number of values.

@juliohm

Can we submit work to remote processes running on remote machines, and use multi-threading there with a specific number of real Threads threads instead of “green threads” as well?

I don’t think you can do it just with Distributed. One option is to use Transducers.jl: Thread- and process-based parallelisms in Transducers.jl (+ some news)

@juliohm

Also, this functionality needs to necessarily live in pmap ? How to deal with this hybrid type of parallelism more explicitly?

I think we need more high-level abstractions and easy-to-use APIs, developed outside stdlib.

@juliohm

I couldn’t get however the remote_do documentation. What is it?

The documentation says

Unlike remotecall, it does not store the result of computation, nor is there a way to wait for its completion.

So, it’s just an API that does not send back the result by default. You need to use RemoteChannel etc. for communication.