Custom memory allocator and memory region

[mratsim]

I would like to use `cudaHostAlloc` as a custom allocator.

It's a replacement to C `malloc` that

  * allocates page-locked memory
  * greatly improve Host<->GPU memory transfer via Direct Memory Access
  * enable non-blocking host<->GPU memcpy w.r.t. both host and GPU computations



Here is [Nim 
implementation](https://github.com/unicredit/nimcuda/blob/82c0bf43ff94d4839e233d7d2a0735345b66c8b4/nimcuda/cuda_runtime_api.nim#L3228)
    
    
    proc cudaHostAlloc*(pHost: ptr pointer;
                        size: csize;
                        flags: cuint): cudaError_t
    {.cdecl, importc: "cudaHostAlloc", dynlib: libcudart.so.}
    

**Questions**

1\. Is it possible to replace the default memory allocator for say `seq`?

2\. If not is it possible to use a custom memory allocator for a custom ref 
object?

I had a look at [Nim's memory 
regions](https://nim-lang.org/docs/manual.html#types-memory-regions) I have a 
feel that it may help but I don't understand how to use it in practice.
    
    
    type
      UncheckedArray {.unchecked.}[T] = array[0..100_000_000, T]
      
      PinnedArray[T] = ref object
        len: int
        data: UncheckedArray[T]
    
    
    # Allocate 50 kB
    # this will be replaced by cudaHostAlloc code
    
    var memRegion = alloc(50_000)
    
    # Doesn't compile
    var foo: PinnedArray[int] ptr memRegion
    
    # I get "Error: type expected"
    # Nimsuggest also says "region needs to be an object type"