Thread Reuse: How it works, why it works

[ffrostfall]

updated 5/15/2022 to fix code example (thanks @jackdotink)

local freeThread

local function functionPasser(fn, ...)
	local aquiredThread = freeThread
	freeThread = nil
	fn(...)
	freeThread = aquiredThread
end

local function yielder()
	while true do
		functionPasser(coroutine.yield())
	end
end

local function SpawnWithReuse(fn, ...)
	if not freeThread then
		freeThread = coroutine.create(yielder)
		coroutine.resume(freeThread)
	end

	task.spawn(freeThread, fn, ...)
end

(putting credit where it’s due, I learned this pattern from @Stravant’s GoodSignal)

---

What does this do?

This snippet of code allows us to re-use threads instead of constantly creating new ones. This helps with performance. While it may look intimidating, it’s not that complex.

---

How does this work?

These lines of code may seem complex and complicated, but I can assure you this process is very simple.

All we really need to know are a few things:

• Threads can only contain one function
  - task.spawn can resume a thread, aka make it stop yielding
• We can pass arguments into coroutine.yield() through task.spawn(function, argsPassedIntoYield)

So let’s go over this process real quickly:

1. When we want to spawn a thread, detect if there’s already a freeThread
2. If not, create a thread with the function yielder.
3. Resume the thread we just created- this runs the yielder function.
4. After that, grab the freeThread reference, and set the freeThread variable to nil. We keep the value this way, but any reference to it is cleared, so nothing else can detect it or interfere.
5. Resume the thread again- except this time, it’s yielding. Remember how I said we can pass arguments into coroutine.yield? We pass the function through. That runs a function which… literally just runs the function you put into it. This means that yielder:
   1. Waits until a function gets passed through
   2. When a function does pass through, run it.
   3. Continues waiting
6. After we run yielder, put freeThread back, because it’s done- it’s not running any code.

---

Why does this work?

Because in this scenario, we are only creating a thread if we cannot access the existing thread. This means we don’t need to consistently spawn threads- which increases the speed ever so slightly. However, this is important at an incredibly large scale when you’re spawning threads up to hundreds of times a frame, resuming a thread instead of creating a thread can save a lot of time. This optimization method particularly helped my project BridgeNet- thread reuse is not implemented yet, but it helped speed it up by over double.

Hope this explanation helped!

[BostonWhaIer]

This reminds me a little bit of a programming paradigm called object pooling which is where you have a collection of instances already created, and than you take them in and out of the game tree as necessary to reduce the time it takes to create a new instance.

With that being said I never considered that you could re-use an existing coroutine for another task which is pretty interesting. But I wouldn’t necessarily call this thread re-use since your not necessarily reusing the threads for the same purpose here, but your instead recycling them to carry out another job.

[ffrostfall]

Yeah, that’s pretty much what this is- the only main difference being we’re only using a single “pool” to save performance.

As per what it’s called- you’re reusing the same thread to do multiple tasks. I do not like arguing semantics though, so I’m just going to shut up and leave it at that.

[BostonWhaIer]

Yeah I didn’t come here to have a dictionary argument either, just was providing some food for thought

[jackdotink (Jack)]

Apologies for bumping, but to future readers:

The code example given for thread reuse does not work! The corrected code is below:

local freeThread

local function functionPasser(fn, ...)
	local aquiredThread = freeThread
	freeThread = nil
	fn(...)
	freeThread = aquiredThread
end

local function yielder()
	while true do
		functionPasser(coroutine.yield())
	end
end

local function SpawnWithReuse(fn, ...)
	if not freeThread then
		freeThread = coroutine.create(yielder)
		coroutine.resume(freeThread)
	end

	task.spawn(freeThread, fn, ...)
end

[cmdr33]

-------------------


Apologies for bumping, but to future readers:

The code example given for thread reuse does not work! The corrected code is below:

local freeThread

local function functionPasser(fn, ...)
	local aquiredThread = freeThread
	freeThread = nil
	fn(...)
	freeThread = aquiredThread
end

local function yielder()
	while true do
		functionPasser(coroutine.yield())
	end
end

local function SpawnWithReuse(fn, ...)
	if not freeThread then
		freeThread = coroutine.create(yielder)
		coroutine.resume(freeThread)
	end

	task.spawn(freeThread, fn, ...)
end

-------------------

````END QUOTE

thanks :D