17.9   Module 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_free : int;
  fragments : int;
  compactions : int
}

The memory management counters are returned in a stat record. The fields of this record are:
minor_words Number of words allocated in the minor heap since the program was started.
promoted_words Number of words allocated in the minor heap that survived a minor collection and were moved to the major heap since the program was started.
major_words Number of words allocated in the major heap, including the promoted words, since the program was started.
minor_collections Number of minor collections since the program was started.
major_collections Number of major collection cycles, not counting the current cycle, since the program was started.
heap_words Total number of words in the major heap.
heap_chunks Number of times the major heap size was increased since the program was started.
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_free 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.
compactions Number of heap compactions since the program was started.

The total amount of memory allocated by the program since it was started 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 : int;
  mutable max_overhead : int;
  mutable stack_limit : int
}

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. Default: 32k.
major_heap_increment The minimum number of words to add to the major heap when increasing it. Default: 62k.
space_overhead The major GC speed is computed from this parameter. This is the memory that will be "wasted" because the GC does not immediatly collect unreachable objects. It is expressed as a percentage of the memory used for live data. 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. Default: 42.
max_overhead Heap compaction is triggered when the estimated amount of free memory is more than max_overhead percent of the amount of live data. If max_overhead is set to 0, heap compaction is triggered at the end of each major GC cycle (this setting is intended for testing purposes only). If max_overhead >= 1000000, compaction is never triggered. Default: 1000000.
verbose This value controls the GC messages on standard error output. It is a sum of some of the following flags, to print messages on the corresponding events:
1 Start of major GC cycle.
2 Minor collection and major GC slice.
4 Growing and shrinking of the heap.
8 Resizing of stacks and memory manager tables.
16 Heap compaction.
32 Change of GC parameters.
64 Computation of major GC slice size. Default: 0.
stack_limit The maximum size of the stack (in words). This is only relevant to the byte-code runtime, as the native code runtime uses the operating system's stack. Default: 256k.

val stat : unit -> stat

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

val print_stat : out_channel -> unit

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

val get : unit -> control

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

val set : control -> unit

set r changes the GC parameters according to the control record r. The normal usage is: Gc.set { (Gc.get()) with Gc.verbose = 13 } .

val minor : unit -> unit

Trigger a minor collection.

val major : unit -> unit

Finish the current major collection cycle.

val full_major : unit -> unit

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

val compact : unit -> unit = "gc_compaction";;

Perform a full major collection and compact the heap. Note that heap compaction is a lengthy operation.