# 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. | `*)`
`` | `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. | `*)`
`` | `stack_size : `int`;` | `(*` | Current size of the stack, in words. Since 3.12.0 | `*)`

`}`

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: 256k. | `*)`
`` | `mutable 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. | `*)`
`` | `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 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: 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 and shared library search at start-up. `0x200` Computation of compaction-triggering condition. `0x400` Output GC statistics at program exit. 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. If compaction is permanently disabled, it is strongly suggested to set `allocation_policy` to 2. 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: 1024k. | `*)`
`` | `mutable allocation_policy : `int`;` | `(*` | The policy used for allocating in the major heap. Possible values are 0, 1 and 2. 0 is the next-fit policy, which is usually fast but can result in fragmentation, increasing memory consumption. 1 is the first-fit policy, which avoids fragmentation but has corner cases (in certain realistic workloads) where it is sensibly slower. 2 is the best-fit policy, which is fast and avoids fragmentation. In our experiments it is faster and uses less memory than both next-fit and first-fit. (since OCaml 4.10) The current default is next-fit, as the best-fit policy is new and not yet widely tested. We expect best-fit to become the default in the future. On one example that was known to be bad for next-fit and first-fit, next-fit takes 28s using 855Mio of memory, first-fit takes 47s using 566Mio of memory, best-fit takes 27s using 545Mio of memory. Note: When changing to a low-fragmentation policy, you may need to augment the `space_overhead` setting, for example using `100` instead of the default `80` which is tuned for next-fit. Indeed, the difference in fragmentation behavior means that different policies will have different proportion of "wasted space" for a given program. Less fragmentation means a smaller heap so, for the same amount of wasted space, a higher proportion of wasted space. This makes the GC work harder, unless you relax it by increasing `space_overhead`. Note: changing the allocation policy at run-time forces a heap compaction, which is a lengthy operation unless the heap is small (e.g. at the start of the program). Default: 0. Since 3.11.0 | `*)`
`` | `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 4.03.0 | `*)`
`` | `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 4.08.0 | `*)`
`` | `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 4.08.0 | `*)`
`` | `custom_minor_max_size : `int`;` | `(*` | Maximum amount of out-of-heap memory for each custom value allocated in the minor heap. When a custom value is allocated on the minor heap and holds more than this many bytes, only this value is counted against `custom_minor_ratio` and the rest is directly counted against `custom_major_ratio`. Note: this only applies to values allocated with `caml_alloc_custom_mem` (e.g. bigarrays). Default: 8192 bytes. Since 4.08.0 | `*)`

`}`

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`.

    val stat : unit -> stat

Return the current values of the memory management counters in a `stat` record. This function examines every heap block to get the statistics.

    val 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. This function is much faster than `stat` because it does not need to go through the heap.

    val counters : unit -> float * float * float

Return `(minor_words, promoted_words, major_words)`. This function is as fast as `quick_stat`.

    val minor_words : unit -> 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.

In native code this function does not allocate.

- Since 4.04

    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

`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).

    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 : 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 get_minor_free : unit -> int

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

- Since 4.03.0

    val get_bucket : int -> int

`get_bucket n` returns the current size of the `n`-th future bucket of the GC smoothing system. The unit is one millionth of a full GC.

- Since 4.03.0
- Raises `Invalid_argument` if `n` is negative, return 0 if n is larger than the smoothing window.

    val get_credit : unit -> int

`get_credit ()` returns the current size of the "work done in advance" counter of the GC smoothing system. The unit is one millionth of a full GC.

- Since 4.03.0

    val huge_fallback_count : unit -> int

Return the number of times we tried to map huge pages and had to fall back to small pages. This is always 0 if `OCAMLRUNPARAM` contains `H=1`.

- Since 4.03.0

    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 (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 `ref` are guaranteed to be heap-allocated and non-constant except when the length argument is `0`.

    val finalise_last : (unit -> unit) -> 'a -> unit

same as `Gc.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 `Gc.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 `Gc.finalise` are always called before the finalisation functions attached with `Gc.finalise_last`.

- Since 4.04

    val 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 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.

    val eventlog_pause : unit -> unit

`eventlog_pause ()` will pause the collection of traces in the runtime. Traces are collected if the program is linked to the instrumented runtime and started with the environment variable OCAML_EVENTLOG_ENABLED. Events are flushed to disk after pausing, and no new events will be recorded until `eventlog_resume` is called.

    val eventlog_resume : unit -> unit

`eventlog_resume ()` will resume the collection of traces in the runtime. Traces are collected if the program is linked to the instrumented runtime and started with the environment variable OCAML_EVENTLOG_ENABLED. This call can be used after calling `eventlog_pause`, or if the program was started with OCAML_EVENTLOG_ENABLED=p. (which pauses the collection of traces before the first event.)

    module Memprof: sig .. end

`Memprof` is a sampling engine for allocated memory words.