Module Gc

module Gc = struct ... end 
stat 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 size of the major heap, in words.
  • heap_chunks Number of times the major heap size was increased since the program was started (including the initial allocation of the heap).
  • live_words Number of words of live data in the major heap, including the header words.
  • live_blocks Number of live blocks in the major heap.
  • free_words Number of words in the free list.
  • free_blocks Number of blocks in the free list.
  • largest_free Size (in words) of the largest block in the free list.
  • fragments Number of wasted words due to fragmentation. These are 1-words free blocks placed between two live blocks. 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.
= {
minor_words :  float ;
promoted_words :  float ;
major_words :  float ;
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 ;
control = {
:  int ;
:  int ;
:  int ;
:  int ;
:  int ;
:  int ;
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.


stat : unit -> stat
Return the current values of the memory management counters in a stat record.

counters : unit -> float * float * float
Return (minor_words, promoted_words, major_words). Much faster than stat.

get : unit -> control
Return the current values of the GC parameters in a control record.

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

minor : unit -> unit
Trigger a minor collection.

major : unit -> unit
Finish the current major collection cycle.

full_major : unit -> unit
Finish the current major collection cycle and perform a complete new cycle. This will collect all currently unreachable blocks.

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

print_stat : Pervasives.out_channel -> unit
Print the current values of the memory management counters (in human-readable form) into the channel argument.

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.

finalise : ('b -> unit) -> 'b -> unit
Gc.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 the GC deallocates v).
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 cannot not acquire any mutex.
Anything reachable from the closure of finalisation functions is considered reachable, so the following code will not work: The f function can use all features of O'Caml, including assignments that make the value reachable again (indeed, the value is already reachable from the stack during the execution of the function). It can also loop forever (in this case, the other finalisation functions will be called during the execution of f). It can call Gc.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.
Gc.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. You should also be aware that some optimisations will duplicate some immutable values, especially floating-point numbers when stored into arrays, so they can be finalised and collected while another copy is still in use by the program.

create_alarm : (unit -> unit) -> alarm
create_alarm f will arrange for f to be called at the end of each major GC cycle. A value of type alarm is returned that you can use to call delete_alarm.

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.