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Can anyone knowledgeable about compilers/GCs say why GO went with a conservative garbage collector? From any GO source code its trivial to pick out pointers from values. Is this information hard to preserve at runtime?



Probably ease of implementation. You can just grab the Boehm conservative GC, slap it in, and bang- you've got GC. You can even add GC this way to C/C++ programs.

Unfortunately, Boehm has draw backs- because it's getting no help from the compiler, it can't tell integers from pointers. So it has to treat everything that looks like it might be a pointer as a pointer, even if it's an integer (or floating point number). Which means that it's possible for garbage to not be collected, because there is an integer that happens to have the same value as the address of the garbage object. And, of course, once you can't collect that object, you can't collect all the objects it refers to (including false pointers), and so on.

The odds of this happening are a function of what percentage of the virtual address space is in use- once some critical threshold is reached, the amount of garbage that can't be collected due to false pointers just explodes. On 32-bit platforms, I've seen this happen with heap sizes of only a few hundred megabytes. And the advice to work around this is exactly what the responder said- use less memory, don't use large ints (which are more likely to be mistaken for pointers), etc. Also, the problem goes away (for the time being) on 64 bits, because the percentage of memory used drops. A terabyte of memory on a 64-bit system is the same fraction of the total address space as a kilobyte of memory is on a 32-bit system.

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A conservative GC is easier to implement. No other reason.

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I recall reading something along the lines of it being hard to use a precise GC due to the the "unsafe" package, but I am not knowledgeable at all.

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Here's an early LtU thread where someone predicted it'd probably have to use a conservative GC due to some of the addressing features: http://lambda-the-ultimate.org/node/3676#comment-52560

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