How does tricolor garbage collection work?

Golang’s garbage collector uses a “tricolor” algorithm. This means it divides the heap objects into three sets: black, white, and grey. Initially, all objects are white, and as the algorithm proceeds, objects are moved into the grey and then black sets, in such a way that eventually the orphaned (collectible) objects are left in the white set, which is then cleared. An important property of this algorithm is that it can run concurrently with the “mutator” (program).

The meaning of the three sets is this:

The important invariant is the “tricolor” invariant: no pointers go directly from the black set to the white set. It is this invariant which lets us eventually clear the white set.

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