How to control go routine numbers in function call

[Joseph Wang]

Hi everyone 

I have a simple question about go routines.

So I have a function like this

Func1() {

  for n := range array {

        go fun2()

  }

}

So as I can not make sure array has a fix size, then I cannot control go routines number. Does any one have some elegant ways that I can limit this function1 create fix number go routines?

Best

Joseph

[Joseph Wang]

I mean at same time only 4 go routines run and then creat another 4 etc.

[Michael Jones]

Here’s a helper...

https://github.com/korovkin/limiter/blob/master/README.md

You can do it using several other mechanisms as well.  

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Michael T. Jones
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[Krzysztof Kowalczyk]

See https://www.programming-books.io/essential/go/limiting-concurrency-with-a-semaphore-26ac6084d7404f7385e4eccaa3fd20de for one way that uses a channel as a semaphore to limit concurrency.

[Jesper Louis Andersen]

And if you prefer a talk, there is one by Bryan C. Mills,

https://www.youtube.com/watch?v=5zXAHh5tJqQ

which describes common concurrency patterns and how to apply them. In your case, a buffered channel can work as a (counting) semaphore. The invariant is: the number of tokens in the channel is the number of currently active workers. If the channel is full and you block, all worker slots are occupied. As workers complete, they remove a token from the channel, making room for another worker.

Finalization is important. You end by _filling_ the semaphore with tokens. These tokens have no workers associated with them, but are "virtual dummy sentinels" in a sense. Once the channel is full of these sentinel tokens, all the real workers must have exited, so you can go on[0]. You can also see this as vacant seats in a restaurant, say. As the restaurant wants to close, it starts filling up the seats with bulbasaurs. Once all seats have bulbasaurs in them, the restaurant can close.

[0] The section is at https://youtu.be/5zXAHh5tJqQ?t=1902 but the whole talk is definitely worth it.

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J.