## `gc`: memory management control and statistics

    type stat = {
      minor_words : int;
      promoted_words : int;
      major_words : int;
      minor_collections : int;
      major_collections : int;
      heap_words : int;
      heap_chunks : int;
      live_words : int;
      live_blocks : int;
      free_words : int;
      free_blocks : int;
      largest_words : int;
      fragments : int
    }

  The memory management counters are returned in a `stat` record. All the numbers are computed since the start of the program. The fields of this record are:
  `minor_words` Number of words allocated in the minor heap.
  `promoted_words` Number of words allocated in the minor heap that survived a minor collection and were moved to the major heap.
  `major_words` Number of words allocated in the major heap, including the promoted words.
  `minor_collections` Number of minor collections.
  `major_collections` Number of major collection cycles, not counting the current cycle.
  `heap_words` Total size of the major heap, in words.
  `heap_chunks` Number of times the major heap size was increased.
  `live_words` Number of words of live data in the major heap, including the header words.
  `live_blocks` Number of live objects in the major heap.
  `free_words` Number of words in the free list.
  `free_blocks` Number of objects in the free list.
  `largest_words` Size (in words) of the largest object in the free list.
  `fragments` Number of wasted words due to fragmentation. These are 1-words free blocks placed between two live objects. They cannot be inserted in the free list, thus they are not available for allocation.
  The total amount of memory allocated by the program is (in words) `minor_words + major_words - promoted_words`. Multiply by the word size (4 on a 32-bit machine, 8 on a 64-bit machine) to get the number of bytes.

    type control = {
      mutable minor_heap_size : int;
      mutable major_heap_increment : int;
      mutable space_overhead : int;
      mutable verbose : bool
    }

  The GC parameters are given as a `control` record. The fields are:
  `minor_heap_size` The size (in words) of the minor heap. Changing this parameter will trigger a minor collection.
  `major_heap_increment` The minimum number of words to add to the major heap when increasing it.
  `space_overhead` The major GC speed is computed from this parameter. This is the percentage of heap space that will be `wasted` because the GC does not immediatly collect unreachable objects. The GC will work more (use more CPU time and collect objects more eagerly) if `space_overhead` is smaller. The computation of the GC speed assumes that the amount of live data is constant.
  `verbose` This flag controls the GC messages on standard error output.

    value stat : unit -> stat

  Return the current values of the memory management counters in a `stat` record.

    value print_stat : io__out_channel -> unit

  Print the current values of the memory management counters (in human-readable form) into the channel argument.

    value get : unit -> control

  Return the current values of the GC parameters in a `control` record.

    value set : control -> unit

  `set r` changes the GC parameters according to the `control` record `r`. The normal usage is:

             let r = gc__get () in    (* Get the current parameters. *)
               r.verbose <- true;     (* Change some of them. *)
               gc__set r              (* Set the new values. *)

    value minor : unit -> unit

  Trigger a minor collection.

    value major : unit -> unit

  Finish the current major collection cycle.

    value full_major : unit -> unit

  Finish the current major collection cycle and perform a complete new cycle. This will collect all currently unreachable objects.