Original bug ID: 7495
Reporter: @mmottl
Assigned to: @damiendoligez
Status: acknowledged (set by @mshinwell on 2017-03-07T12:33:34Z)
Resolution: open
Priority: normal
Severity: minor
Version: 4.04.0
Category: runtime system and C interface
Monitored by: @mshinwell @mmottl

Bug description

Consider the following code:

let () =
  let open Bigarray in
  for _i = 1 to 100_000 do
    let vec = Array1.create float64 c_layout 1_000_000 in
    (* Makes sure that physical memory is actually allocated *)
    Array1.fill vec 0.
  done

This code will let memory consumption quickly go into the GB range. A comparable implementation using float arrays will stay stable in the low MB range:

let () =
  for _i = 1 to 100_000 do
    ignore (Array.make 1_000_000 0.)
  done

I remember that this is a long-standing problem, probably the same as #4616, and likely affects custom block allocation in general. Not sure whether it's already tracked in another still active issue. Since the GC will likely undergo significant changes in the near future due to attempts to parallelize it, I think this issue should remain on people's mind. Some practical numerical code can run into problems with this limitation.