Module Stdlib.Gc

Memory management control and statistics; finalised values.

type stat = {
  1. minor_words: float,
    /*

    Number of words allocated in the minor heap since the program was started.

    */
  2. 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.

    */
  3. major_words: float,
    /*

    Number of words allocated in the major heap, including the promoted words, since the program was started.

    */
  4. minor_collections: int,
    /*

    Number of minor collections since the program was started.

    */
  5. major_collections: int,
    /*

    Number of major collection cycles completed since the program was started.

    */
  6. heap_words: int,
    /*

    Total size of the major heap, in words.

    */
  7. heap_chunks: int,
    /*

    Number of contiguous pieces of memory that make up the major heap. This metrics is currently not available in OCaml 5: the field value is always 0.

    */
  8. live_words: int,
    /*

    Number of words of live data in the major heap, including the header words.

    Note that "live" words refers to every word in the major heap that isn't currently known to be collectable, which includes words that have become unreachable by the program after the start of the previous gc cycle. It is typically much simpler and more predictable to call Gc.full_major (or Gc.compact) then computing gc stats, as then "live" words has the simple meaning of "reachable by the program". One caveat is that a single call to Gc.full_major will not reclaim values that have a finaliser from Gc.finalise (this does not apply to Gc.finalise_last). If this caveat matters, simply call Gc.full_major twice instead of once.

    */
  9. live_blocks: int,
    /*

    Number of live blocks in the major heap.

    See live_words for a caveat about what "live" means.

    */
  10. free_words: int,
    /*

    Number of words in the free list.

    */
  11. free_blocks: int,
    /*

    Number of blocks in the free list. This metrics is currently not available in OCaml 5: the field value is always 0.

    */
  12. largest_free: int,
    /*

    Size (in words) of the largest block in the free list. This metrics is currently not available in OCaml 5: the field value is always 0.

    */
  13. 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.

    */
  14. compactions: int,
    /*

    Number of heap compactions since the program was started.

    */
  15. top_heap_words: int,
    /*

    Maximum size reached by the major heap, in words.

    */
  16. stack_size: int,
    /*

    Current size of the stack, in words. This metrics is currently not available in OCaml 5: the field value is always 0.

    • since 3.12
    */
  17. forced_major_collections: int,
    /*

    Number of forced full major collections completed since the program was started.

    • since 4.12
    */
};

The memory management counters are returned in a stat record. These counters give values for the whole program.

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 = {
  1. minor_heap_size: int,
    /*

    The size (in words) of the minor heap. Changing this parameter will trigger a minor collection. The total size of the minor heap used by this program is the sum of the heap sizes of the active domains. Default: 256k.

    */
  2. major_heap_increment: int,
    /*

    How much to add to the major heap when increasing it. If this number is less than or equal to 1000, it is a percentage of the current heap size (i.e. setting it to 100 will double the heap size at each increase). If it is more than 1000, it is a fixed number of words that will be added to the heap. Default: 15.

    */
  3. 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 immediately 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: 120.

    */
  4. 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 and end 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 and shared library search at start-up.
    • 0x200 Computation of compaction-triggering condition.
    • 0x400 Output GC statistics at program exit. Default: 0.
    */
  5. 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.

    */
  6. stack_limit: int,
    /*

    The maximum size of the fiber stacks (in words). Default: 1024k.

    */
  7. allocation_policy: int,
    /*

    The policy used for allocating in the major heap.

    This option is ignored in OCaml 5.x.

    Prior to OCaml 5.0, possible values were 0, 1 and 2.

    • 0 was the next-fit policy
    • 1 was the first-fit policy (since OCaml 3.11)
    • 2 was the best-fit policy (since OCaml 4.10)
    • since 3.11
    */
  8. window_size: int,
    /*

    The size of the window used by the major GC for smoothing out variations in its workload. This is an integer between 1 and 50. Default: 1.

    • since 4.03
    */
  9. custom_major_ratio: int,
    /*

    Target ratio of floating garbage to major heap size for out-of-heap memory held by custom values located in the major heap. The GC speed is adjusted to try to use this much memory for dead values that are not yet collected. Expressed as a percentage of major heap size. The default value keeps the out-of-heap floating garbage about the same size as the in-heap overhead. Note: this only applies to values allocated with caml_alloc_custom_mem (e.g. bigarrays). Default: 44.

    • since 4.08
    */
  10. custom_minor_ratio: int,
    /*

    Bound on floating garbage for out-of-heap memory held by custom values in the minor heap. A minor GC is triggered when this much memory is held by custom values located in the minor heap. Expressed as a percentage of minor heap size. Note: this only applies to values allocated with caml_alloc_custom_mem (e.g. bigarrays). Default: 100.

    • since 4.08
    */
  11. custom_minor_max_size: int,
    /*

    Maximum amount of out-of-heap memory for each custom value allocated in the minor heap. Custom values that hold more than this many bytes are allocated on the major heap. Note: this only applies to values allocated with caml_alloc_custom_mem (e.g. bigarrays). Default: 70000 bytes.

    • since 4.08
    */
};

The GC parameters are given as a control record. Note that these parameters can also be initialised by setting the OCAMLRUNPARAM environment variable. See the documentation of ocamlrun.

let stat: unit => stat;

Return the current values of the memory management counters in a stat record that represent the program's total memory stats. This function causes a full major collection.

let quick_stat: unit => stat;

Same as stat except that live_words, live_blocks, free_words, free_blocks, largest_free, and fragments are set to 0. Due to per-domain buffers it may only represent the state of the program's total memory usage since the last minor collection or major cycle. This function is much faster than stat because it does not need to trigger a full major collection.

let counters: unit => (float, float, float);

Return (minor_words, promoted_words, major_words) for the current domain or potentially previous domains. This function is as fast as quick_stat.

let minor_words: unit => float;

Number of words allocated in the minor heap by this domain or potentially previous domains. This number is accurate in byte-code programs, but only an approximation in programs compiled to native code.

In native code this function does not allocate.

  • since 4.04
let get: unit => control;

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

  • alert unsynchronized_access GC parameters are a mutable global state.
let 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 }

  • alert unsynchronized_access GC parameters are a mutable global state.
let minor: unit => unit;

Trigger a minor collection.

let major_slice: int => int;

major_slice n Do a minor collection and a slice of major collection. n is the size of the slice: the GC will do enough work to free (on average) n words of memory. If n = 0, the GC will try to do enough work to ensure that the next automatic slice has no work to do. This function returns an unspecified integer (currently: 0).

let major: unit => unit;

Do a minor collection and finish the current major collection cycle.

let 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.

let compact: unit => unit;

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

let print_stat: out_channel => unit;

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

let allocated_bytes: unit => float;

Return the number of bytes allocated by this domain and potentially a previous domain. It is returned as a float to avoid overflow problems with int on 32-bit machines.

let get_minor_free: unit => int;

Return the current size of the free space inside the minor heap of this domain.

  • since 4.03
let 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 (including through weak pointers) 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).

The GC will call the finalisation functions in the order of deallocation. When several values become unreachable at the same time (i.e. during the same GC cycle), the finalisation functions will be called in the reverse order of the corresponding calls to finalise. If finalise is called in the same order as the values are allocated, that means each value is finalised before the values it depends upon. Of course, this becomes false if additional dependencies are introduced by assignments.

In the presence of multiple OCaml threads it should be assumed that any particular finaliser may be executed in any of the threads.

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 _ -> ...v...) v

Instead you should make sure that v is not in the closure of the finalisation function by writing:

  • let f = fun x -> ... let v = ... in Gc.finalise f v

The f function can use all features of OCaml, including assignments that make the value reachable again. It can also loop forever (in this case, the other finalisation functions will not be called during the execution of f, unless it calls finalise_release). 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 guaranteed to be 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. Note that values of types float are sometimes allocated and sometimes not, so finalising them is unsafe, and finalise will also raise Invalid_argument for them. Values of type 'a Lazy.t (for any 'a) are like float in this respect, except that the compiler sometimes optimizes them in a way that prevents finalise from detecting them. In this case, it will not raise Invalid_argument, but you should still avoid calling finalise on lazy values.

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

let finalise_last: (unit => unit) => 'a => unit;

same as finalise except the value is not given as argument. So you can't use the given value for the computation of the finalisation function. The benefit is that the function is called after the value is unreachable for the last time instead of the first time. So contrary to finalise the value will never be reachable again or used again. In particular every weak pointer and ephemeron that contained this value as key or data is unset before running the finalisation function. Moreover the finalisation functions attached with finalise are always called before the finalisation functions attached with finalise_last.

  • since 4.04
let finalise_release: unit => unit;

A finalisation function may call finalise_release to tell the GC that it can launch the next finalisation function without waiting for the current one to return.

type alarm;

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

let create_alarm: (unit => unit) => alarm;

create_alarm f will arrange for f to be called at the end of major GC cycles, not caused by f itself, starting with the current cycle or the next one. f will run on the same domain that created the alarm, until the domain exits or delete_alarm is called. A value of type alarm is returned that you can use to call delete_alarm.

It is not guaranteed that the Gc alarm runs at the end of every major GC cycle, but it is guaranteed that it will run eventually.

As an example, here is a crude way to interrupt a function if the memory consumption of the program exceeds a given limit in MB, suitable for use in the toplevel:

let run_with_memory_limit (limit : int) (f : unit -> 'a) : 'a =
  let limit_memory () =
    let mem = Gc.(quick_stat ()).heap_words in
    if mem / (1024 * 1024) > limit / (Sys.word_size / 8) then
      raise Out_of_memory
  in
  let alarm = Gc.create_alarm limit_memory in
  Fun.protect f ~finally:(fun () -> Gc.delete_alarm alarm ; Gc.compact ())
let 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.

let eventlog_pause: unit => unit;
  • deprecated Use Runtime_events.pause instead.
let eventlog_resume: unit => unit;
  • deprecated Use Runtime_events.resume instead.
module Memprof: { ... };

Memprof is a profiling engine which randomly samples allocated memory words. Every allocated word has a probability of being sampled equal to a configurable sampling rate. Once a block is sampled, it becomes tracked. A tracked block triggers a user-defined callback as soon as it is allocated, promoted or deallocated.