CODEWITHSUNDEEP
memorygarbage-collectionocaml
chrismamo1
chrismamo1

Reputation: 977

Why does OCaml need to create its own heaps for garbage collection?

According to this OCaml's memory is contained in two contiguous chunks of virtual memory managed by the OCaml runtime. I would like to know why this is necessary. Couldn't OCaml simply use the system malloc to allocate memory, and only use these heaps to store the block headers and pointers to the object's actual home in memory? This seems like a reinvention of the wheel when the operating system can do so much of the work instead.

I would also like to know why OCaml must allocate an entirely new major heap during the compact phase of garbage collection (source here), e.g.:

In the following rough sketch, let the letters A-D represent equally-sized pieces of OCaml blocks, and let . represent a freed space of the same size. From what I am given to understand, the OCaml garbage collector would "compact" this major heap:

[AAABB..CCCCCC.....DDD....]

By allocating a new major heap:

[.........................]

And copying the still-live blocks into it, before freeing the original heap:

[AAABBCCCCCCDDD...........]

How is this more efficient than simply rearranging those blocks inside the original heap? In the example above, a contiguity check could have avoided having to move blocks A and B at all, and in any case, how is it more efficient to ask the operating system to always allocate an entire new major heap?

Upvotes: 4

Views: 520

Answers (1)

Pierre Chambart
Pierre Chambart

Reputation: 1489

Every garbage collector handle their own heap for performance reasons. malloc is a quite slow, and compared to OCaml minor heap allocation it is multiple orders of magnitude slower (an allocation in the minor heap is done in 2 assembly instructions).

For the compaction, no that doesn't reallocate.

Upvotes: 2

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