Module Gc

module Gc: sig  end
Memory management control and statistics; finalised values.

type stat = {
   minor_words : float; (*Number of words allocated in the minor heap since the program was started. This number is accurate in byte-code programs, but only an approximation in programs compiled to native code.*)
   promoted_words : float; (*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 : float; (*Number of words allocated in the major heap, including the promoted words, since the program was started.*)
   minor_collections : int; (*Number of minor collections since the program was started.*)
   major_collections : int; (*Number of major collection cycles completed since the program was started.*)
   heap_words : int; (*Total size of the major heap, in words.*)
   heap_chunks : int; (*Number of contiguous pieces of memory that make up the major heap.*)
   live_words : int; (*Number of words of live data in the major heap, including the header words.*)
   live_blocks : int; (*Number of live blocks in the major heap.*)
   free_words : int; (*Number of words in the free list.*)
   free_blocks : int; (*Number of blocks in the free list.*)
   largest_free : int; (*Size (in words) of the largest block in the free list.*)
   fragments : int; (*Number of wasted words due to fragmentation. These are 1-words free blocks placed between two live blocks. They are not available for allocation.*)
   compactions : int; (*Number of heap compactions since the program was started.*)
   top_heap_words : int; (*Maximum size reached by the major heap, in words.*)
The memory management counters are returned in a stat record.

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; (*The size (in words) of the minor heap. Changing this parameter will trigger a minor collection. Default: 32k.*)
   mutable major_heap_increment : int; (*The minimum number of words to add to the major heap when increasing it. Default: 62k.*)
   mutable space_overhead : int; (*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 blocks. It is expressed as a percentage of the memory used for live data. The GC will work more (use more CPU time and collect blocks more eagerly) if space_overhead is smaller. Default: 80.*)
   mutable verbose : int; (*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:
  • 0x001 Start of major GC cycle.
  • 0x002 Minor collection and major GC slice.
  • 0x004 Growing and shrinking of the heap.
  • 0x008 Resizing of stacks and memory manager tables.
  • 0x010 Heap compaction.
  • 0x020 Change of GC parameters.
  • 0x040 Computation of major GC slice size.
  • 0x080 Calling of finalisation functions.
  • 0x100 Bytecode executable search at start-up.
  • 0x200 Computation of compaction triggering condition. Default: 0.
   mutable max_overhead : int; (*Heap compaction is triggered when the estimated amount of "wasted" 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: 500.*)
   mutable stack_limit : int; (*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.*)
The GC parameters are given as a control record.

val stat : unit -> stat
Return the current values of the memory management counters in a stat record.
val counters : unit -> float * float * float
Return (minor_words, promoted_words, major_words). Much faster than stat.
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 = 0x00d }
val minor : unit -> unit
Trigger a minor collection.
val major_slice : int -> int
Do a minor collection and a slice of major collection. The argument is the size of the slice, 0 to use the automatically-computed slice size. In all cases, the result is the computed slice size.
val major : unit -> unit
Do a minor collection and finish the current major collection cycle.
val full_major : unit -> unit
Do a minor collection, finish the current major collection cycle, and perform a complete new cycle. This will collect all currently unreachable blocks.
val compact : unit -> unit
Perform a full major collection and compact the heap. Note that heap compaction is a lengthy operation.
val print_stat : Pervasives.out_channel -> unit
Print the current values of the memory management counters (in human-readable form) into the channel argument.
val allocated_bytes : unit -> float
Return the total number of bytes allocated since the program was started. It is returned as a float to avoid overflow problems with int on 32-bit machines.
val finalise : ('a -> unit) -> 'a -> unit
finalise f v registers f as a finalisation function for v. v must be heap-allocated. f will be called with v as argument at some point between the first time v becomes unreachable and the time v is collected by the GC. Several functions can be registered for the same value, or even several instances of the same function. Each instance will be called once (or never, if the program terminates before v becomes unreachable).

A number of pitfalls are associated with finalised values: finalisation functions are called asynchronously, sometimes even during the execution of other finalisation functions. In a multithreaded program, finalisation functions are called from any thread, thus they must not acquire any mutex.

Anything reachable from the closure of finalisation functions is considered reachable, so the following code will not work as expected:

  • let v = ... in Gc.finalise (fun x -> ...) v
Instead you should write:
  • let f = fun x -> ... ;; let v = ... in Gc.finalise f v
The f function can use all features of O'Caml, including assignments that make the value reachable again. It can also loop forever (in this case, the other finalisation functions will be called during the execution of f). It can call finalise on v or other values to register other functions or even itself. It can raise an exception; in this case the exception will interrupt whatever the program was doing when the function was called.

finalise will raise Invalid_argument if v is not heap-allocated. Some examples of values that are not heap-allocated are integers, constant constructors, booleans, the empty array, the empty list, the unit value. The exact list of what is heap-allocated or not is implementation-dependent. Some constant values can be heap-allocated but never deallocated during the lifetime of the program, for example a list of integer constants; this is also implementation-dependent. You should also be aware that compiler optimisations may duplicate some immutable values, for example floating-point numbers when stored into arrays, so they can be finalised and collected while another copy is still in use by the program.

The results of calling String.make, String.create, Array.make, and Pervasives.ref are guaranteed to be heap-allocated and non-constant except when the length argument is 0.

type alarm
An alarm is a piece of data that calls a user function at the end of each major GC cycle. The following functions are provided to create and delete alarms.

val create_alarm : (unit -> unit) -> alarm
create_alarm f will arrange for f to be called at the end of each major GC cycle, starting with the current cycle or the next one. A value of type alarm is returned that you can use to call delete_alarm.
val delete_alarm : alarm -> unit
delete_alarm a will stop the calls to the function associated to a. Calling delete_alarm a again has no effect.