Async's analog of `Core_kernel.Gc`.

We remove the `Expert` module, which has functions that are superseded by Async-friendly functions below.

`include module type of Core_kernel.Gc with module Gc.Expert := Core_kernel.Gc.Expert`

This is a wrapper around INRIA's standard `Gc` module. Provides memory management control and statistics, and finalized values.

`module Stat = Core_kernel.Gc.Stat`
`type stat``= Stat.t```

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.

`module Control = Core_kernel.Gc.Control`
`type control``= Control.t```

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.

`external stat : Core_kernel__.Import.unit ‑> stat = "caml_gc_stat"`

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

`external quick_stat : Core_kernel__.Import.unit ‑> stat = "caml_gc_quick_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.

`external counters : Core_kernel__.Import.unit ‑> Core_kernel__.Import.float * Core_kernel__.Import.float * Core_kernel__.Import.float = "caml_gc_counters"`

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

`external minor_words : Core_kernel__.Import.unit ‑> Core_kernel__.Import.int = "core_kernel_gc_minor_words"`

The following functions return the same as `(Gc.quick_stat ()).Stat.f`, avoiding any allocation (of the `stat` record or a float). On 32-bit machines the `int` may overflow.

Note that `minor_words` does not allocate, but we do not annotate it as `noalloc` because we want the compiler to save the value of the allocation pointer register (%r15 on x86-64) to the global variable `caml_young_ptr` before the C stub tries to read its value.

`external major_words : Core_kernel__.Import.unit ‑> Core_kernel__.Import.int = "core_kernel_gc_major_words"`
`external promoted_words : Core_kernel__.Import.unit ‑> Core_kernel__.Import.int = "core_kernel_gc_promoted_words"`
`external minor_collections : Core_kernel__.Import.unit ‑> Core_kernel__.Import.int = "core_kernel_gc_minor_collections"`
`external major_collections : Core_kernel__.Import.unit ‑> Core_kernel__.Import.int = "core_kernel_gc_major_collections"`
`external heap_words : Core_kernel__.Import.unit ‑> Core_kernel__.Import.int = "core_kernel_gc_heap_words"`
`external heap_chunks : Core_kernel__.Import.unit ‑> Core_kernel__.Import.int = "core_kernel_gc_heap_chunks"`
`external compactions : Core_kernel__.Import.unit ‑> Core_kernel__.Import.int = "core_kernel_gc_compactions"`
`external top_heap_words : Core_kernel__.Import.unit ‑> Core_kernel__.Import.int = "core_kernel_gc_top_heap_words"`
`external major_plus_minor_words : Core_kernel__.Import.unit ‑> Core_kernel__.Import.int = "core_kernel_gc_major_plus_minor_words"`

This function returns `major_words () + minor_words ()`. It exists purely for speed (one call into C rather than two). Like `major_words` and `minor_words`, `major_plus_minor_words` avoids allocating a `stat` record or a float, and may overflow on 32-bit machines.

This function is not marked `[@@noalloc]` to ensure that the allocation pointer is up-to-date when the minor-heap measurement is made.

`external get : Core_kernel__.Import.unit ‑> control = "caml_gc_get"`

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

`external set : control ‑> Core_kernel__.Import.unit = "caml_gc_set"`

`set r` changes the GC parameters according to the `control` record `r`. The normal usage is: `Gc.set { (Gc.get()) with Gc.Control.verbose = 0x00d }`

`external minor : Core_kernel__.Import.unit ‑> Core_kernel__.Import.unit = "caml_gc_minor"`

Trigger a minor collection.

`external major_slice : Core_kernel__.Import.int ‑> Core_kernel__.Import.int = "caml_gc_major_slice"`

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.

`external major : Core_kernel__.Import.unit ‑> Core_kernel__.Import.unit = "caml_gc_major"`

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

`external full_major : Core_kernel__.Import.unit ‑> Core_kernel__.Import.unit = "caml_gc_full_major"`

Do a minor collection, finish the current major collection cycle, and perform a complete new cycle. This will collect all currently unreachable blocks.

`external compact : Core_kernel__.Import.unit ‑> Core_kernel__.Import.unit = "caml_gc_compaction"`

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

`val print_stat : Pervasives.out_channel ‑> Core_kernel__.Import.unit`

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

`val allocated_bytes : Core_kernel__.Import.unit ‑> Core_kernel__.Import.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 keep_alive : _ ‑> Core_kernel__.Import.unit`

`keep_alive a` ensures that `a` is live at the point where `keep_alive a` is called. It is like `ignore a`, except that the compiler won't be able to simplify it and potentially collect `a` too soon.

`val tune : ?⁠logger:(Core_kernel__.Import.string ‑> Core_kernel__.Import.unit) ‑> ?⁠minor_heap_size:Core_kernel__.Import.int ‑> ?⁠major_heap_increment:Core_kernel__.Import.int ‑> ?⁠space_overhead:Core_kernel__.Import.int ‑> ?⁠verbose:Core_kernel__.Import.int ‑> ?⁠max_overhead:Core_kernel__.Import.int ‑> ?⁠stack_limit:Core_kernel__.Import.int ‑> ?⁠allocation_policy:Core_kernel__.Import.int ‑> ?⁠window_size:Core_kernel__.Import.int ‑> Core_kernel__.Import.unit ‑> Core_kernel__.Import.unit`

Adjust the specified GC parameters.

`module Allocation_policy = Core_kernel.Gc.Allocation_policy`

The policy used for allocating in the heap.

`val disable_compaction : ?⁠logger:(Core_kernel__.Import.string ‑> Core_kernel__.Import.unit) ‑> allocation_policy:[ `Don't_change | `Set_to of Allocation_policy.t ] ‑> Core_kernel__.Import.unit ‑> Core_kernel__.Import.unit`
`module Expert = Core_kernel.Gc.Expert`

The `Expert` module contains functions that novice users should not use, due to their complexity.

`val add_finalizer : 'a Core_kernel.Heap_block.t ‑> ('a Core_kernel.Heap_block.t ‑> unit) ‑> unit`

`add_finalizer b f` ensures that `f` runs after `b` becomes unreachable. `f b` will run in its own Async job. If `f` raises, the unhandled exception will be raised to the monitor that called `add_finalizer b f`.

The OCaml runtime only supports finalizers on heap blocks, hence `add_finalizer` requires `b : _ Heap_block.t`.

The runtime essentially maintains a set of finalizer pairs:

          'a Heap_block.t * ('a Heap_block.t -> unit)

Each call to `add_finalizer` adds a new pair to the set. It is allowed for many pairs to have the same heap block, the same function, or both. Each pair is a distinct element of the set.

After a garbage collection determines that a heap block `b` is unreachable, it removes from the set of finalizers all finalizer pairs `(b, f)` whose block is `b`, and then and runs `f b` for all such pairs. Thus, a finalizer registered with `add_finalizer` will run at most once.

In a finalizer pair `(b, f)`, it is a mistake for the closure of `f` to reference (directly or indirectly) `b` -- `f` should only access `b` via its argument. Referring to `b` in any other way will cause `b` to be kept alive forever, since `f` itself is a root of garbage collection, and can itself only be collected after the pair `(b, f)` is removed from the set of finalizers.

The `f` function can use all features of OCaml and Async, since it runs as an ordinary Async job. `f` can even make `b` reachable again. It can even call `add_finalizer` on `b` or other values to register other finalizer functions.

`val add_finalizer_exn : 'a ‑> ('a ‑> unit) ‑> unit`

`add_finalizer_exn b f` is like `add_finalizer`, but will raise if `b` is not a heap block.

`val add_finalizer_last : 'a Core_kernel.Heap_block.t ‑> (unit ‑> unit) ‑> unit`

Same as `add_finalizer` except that the function is not called until the value has become unreachable for the last time. This means that the finalization function does not receive the value as an argument. Every weak pointer and ephemeron that contained this value as key or data is unset before running the finalization function.

`val add_finalizer_last_exn : 'a ‑> (unit ‑> unit) ‑> unit`
`module Alarm : sig ... end`

A GC alarm calls a user function after the end of each major GC cycle.