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0 pointshsn9153y ago0 comments

    func Allocate[T](arena *Arena) *T {
       var bytes = arena.Bump(sizeof(T), alignof(T))
       return (*T) bytes
    }

Any reason why this would not work?

Maybe you need to cast to unsafe.Pointer or something before returning, but in theory this _should_ work.

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3 comments · 1 top-level

morelisp3y ago· 2 in thread

It could probably be made to work, but would it reduce any GC work compared to any other kind of object pool? The returned value will still be accessible from some stack root and need to be scanned, I think regardless of whether the arena was already scanned - the arena would mark the span of the T itself, but any subfields with pointers would need to be scanned from the T as it would need the type information. And if T has no pointer subfields it would not be scanned anyway. So maybe best case you save a mark per T?

The advantage of a bump allocator is being able to throw it all out at once at the end and never have to check it while "in use", and I don't think Go's GC would let you do that.

hsn915OP3y ago

Right, the pointers would still be scanned by the GC, so we can't elliminate the cost of the GC, unless we completely turn it off (I think that is possible).

But the point of the arena allocator is to make allocations and deallocatins very fast. Say you want to allocate many small objects for a very short amount of time and then get rid of all of them. Doing it on an Arena would save a lot of computation that would be required in a typical heap allocator.

morelisp3y ago

> But the point of the arena allocator is to make allocations and deallocatins very fast... Doing it on an Arena would save a lot of computation that would be required in a typical heap allocator.

Relative to a completely naive allocator yes, but relative to any other kind of pooling (e.g. Go's internal small object pools, or a `sync.Pool`, or just a `make([]T, 1000)`) the advantage of a bump allocator is marginal without the ability to avoid the mark overhead and actually throw it all out at once.

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