CODEWITHSUNDEEP
memorygogarbage-collection
thwd
thwd

Reputation: 24868

How does Go's precise GC work?

Go 1.3 implements a precise garbage collector.

How does it precisely identify pointers?

Upvotes: 3

Views: 1099

Answers (1)

VonC
VonC

Reputation: 1328912

Looking at "Changes to the garbage collector", the mechanasim seems simple:

Starting with Go 1.3, the runtime assumes that values with pointer type contain pointers and other values do not.

This assumption is fundamental to the precise behavior of both stack expansion and garbage collection.

Programs that use package unsafe to store integers in pointer-typed values are illegal and will crash if the runtime detects the behavior.
Programs that use package unsafe to store pointers in integer-typed values are also illegal but more difficult to diagnose during execution.

This reddit thread add:

Basically a GC has to find out which objects are reachable, to do this it has to follow pointers lying on the stack to every object they point to and then follow the pointers in the objects to every object they point to until it no longer encounters new objects.
Every object not encountered by the GC is then garbage.

The problem with that is that it requires the GC to know what a pointer is:

  • a precise GC has that information,
  • a conservative GC has to assume that every value on the stack may be a pointer if it is identical with the address of an allocated object.

As a result conservate GCs tend to keep a lot of unreachable objects around and have to do more work (traverse dead object graphs).

Upvotes: 5

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