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.