Stdlib.Gc(3)

📦
version 5.5.0 · 19 versions
, in arch rolling

🌸 Stdlib.Gc — bestie edition 💅

okay so this is Stdlib.Gc, and it lives in the OCaml library (that's the manual name, now you know 🫶). The official NAME line is just Stdlib.Gc and honestly the original description is <italic>no description</italic> — iconic of them to say nothing 😭. Anyway. Sit down, grab a strawberry drink 🍓, we're talking about the garbage collector and it's actually going to be fine.

✨ Module ✨

Module Stdlib.Gc

📖 Documentation 📖

Module Gc : (module Stdlib__Gc)

🎀 type stat — the receipts 🧾

type stat = {
  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 ;
  top_heap_words : int ;
  stack_size : int ;
  forced_major_collections : int ;
  live_stacks_words : int ;
}

and here's what every single field is giving, no skips 💖:

  • 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 got moved to the major heap, since the program started.
  • major_words : float — number of words allocated in the major heap, including the promoted words, since the program started.
  • minor_collections : int — how many minor collections have happened since the program started.
  • major_collections : int — how many major collection cycles have been completed since the program 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. ⚠️ this metric is currently not available in OCaml 5: the field value is always 0.
  • live_words : int — number of words of live data in the major heap, including the header words.
okay so about "live" 👀 — "live" words means every word in the major heap that isn't currently known to be collectable, which includes words that became 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 compute gc stats, because then "live" words has the simple meaning of "reachable by the program". One caveat: 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. 💀
  • live_blocks : int — number of live blocks in the major heap. (See live_words for the caveat about what "live" means — same energy ✨.)
  • free_words : int — number of words in the free list.
  • free_blocks : int — number of blocks in the free list. ⚠️ this metric is currently not available in OCaml 5: the field value is always 0.
  • largest_free : int — size (in words) of the largest block in the free list. ⚠️ this metric is currently not available in OCaml 5: the field value is always 0.
  • fragments : int — number of wasted words due to fragmentation. These are 1-word 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 current OCaml stack, in words.
🦋 Since 5.5 restored for the current stack
  • forced_major_collections : int — number of forced full major collections completed since the program was started.
🦋 Since 4.12
  • live_stacks_words : int — total space allocated outside of the OCaml heap for stack fragments. This includes stack metadata and stack fragments stored inside the stack fragment cache.
🦋 Since 5.5

so the memory management counters get returned in a stat record, and 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. math slay 💅

🎀 type control — the knobs ✨

type control = {
  minor_heap_size : int ;
  major_heap_increment : int ;
  space_overhead : int ;
  verbose : int ;
  max_overhead : int ;
  stack_limit : int ;
  allocation_policy : int ;
  window_size : int ;
  custom_major_ratio : int ;
  custom_minor_ratio : int ;
  custom_minor_max_size : int ;
}

field by field, because we don't gatekeep 🫶:

  • 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.
  • 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.
⚠️ this field is currently not available in OCaml 5: the field value is always 0.
  • space_overhead : int — the major GC speed is computed from this parameter, along with small_heap_limit. 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. The amount of overhead space used by the GC is approximately:
- (live data size) * space_overhead when live data is greater than small_heap_limit
-less than small_heap_limit + (live data size) * space overhead when live data is smaller than small_heap_limit

Default: 120.

  • 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:
    • 0x0001 — start and end of major GC cycle.
    • 0x0002 — minor collection and major GC slice.
    • 0x0004 — growing and shrinking of the heap.
    • 0x0008 — resizing of stacks and memory manager tables.
    • 0x0010 — heap compaction.
    • 0x0020 — change of GC parameters.
    • 0x0040 — computation of major GC slice size.
    • 0x0080 — calling of finalisation functions.
    • 0x0100 — bytecode executable and shared library search at start-up.
    • 0x0200 — computation of compaction-triggering condition.
    • 0x0400 — output GC statistics at program exit.
    • 0x0800 — GC debugging messages.
    • 0x1000 — address space reservation changes.

Default: 0.

  • 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.
⚠️ this field is currently not available in OCaml 5: the field value is always 0.
  • stack_limit : int — the maximum size of the fiber stacks (in words). Default: 128M.
  • allocation_policy : int — the policy used for allocating in the major heap.
⚠️ this field is currently not available in OCaml 5: the field value is always 0.

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

  • 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.
🦋 Since 4.03 ⚠️ this field is currently not available in OCaml 5: the field value is always [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
  • 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
  • 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, bestie 📝. Note that these parameters can also be initialised by setting the OCAMLRUNPARAM environment variable. See the documentation of ocamlrun. iykyk ✨

🔥 val stat : unit -> stat

Returns the current values of the memory management counters in a stat record that represents the program's total memory stats. The heap_chunks, free_blocks, largest_free, and stack_size metrics are currently not available in OCaml 5, so their returned field values are therefore 0. Heads up: this function causes a full major collection (so like, not free, she's doing work 💅).

🦋 val quick_stat : unit -> stat

Returns a record with the current values of the memory management counters like stat. Unlike stat, quick_stat does not perform a full major collection, and hence, is much faster ⚡. However — and this is the catch — quick_stat reports the counters sampled at the last minor collection or at the end of the last major collection cycle (whichever is the latest). Hence, the memory stats returned by quick_stat are not instantaneously accurate. it's giving "close enough" energy 🙈

💜 val counters : unit -> float * float * float

Returns (minor_words, promoted_words, major_words) for the current domain or potentially previous domains. This function is as fast as quick_stat. literally same speed, no cap 🏎️

🌸 val 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. (native code bestie, we love you, but it's an approximation 😭)

In native code this function does not allocate.

🦋 Since 4.04

🎀 val get : unit -> control

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

The major_heap_increment, max_overhead, allocation_policy, and window_size fields are currently not available in OCaml 5, so their returned field values are therefore 0.

🚨 Alert unsynchronized_access. GC parameters are a mutable global state. (yeah. global state. we know. 💀)

💅 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 }

The major_heap_increment, max_overhead, allocation_policy, and window_size fields are currently not available in OCaml 5, so setting them therefore has no effect.

🚨 Alert unsynchronized_access. GC parameters are a mutable global state.

⭐ val minor : unit -> unit

Triggers a minor collection. that's it, that's the whole vibe ✨

🌈 val major_slice : int -> int

okay bestie we're gonna stop right here because there's more of this page coming in part 2 👀 — don't go anywhere, the major_slice lore continues 🫶

  • major_slice n — okay so bestie, this does 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 — does a minor collection and finishes the current major collection cycle. Slay. ✨
  • val full_major : unit -> unit — does a minor collection, finishes the current major collection cycle, and performs a complete new cycle. This will collect all currently unreachable blocks. Iconic cleanup energy. 🧹
  • val compact : unit -> unit — performs a full major collection and compacts the heap. Note that heap compaction is a lengthy operation. Lowkey a big chore, but sometimes you gotta. 🫶
  • val print_stat : out_channel -> unit — prints the current values of the memory management counters (in human-readable form) of the total program into the channel argument. Cute lil memory receipt. 🧾
  • val allocated_bytes : unit -> float — returns 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. No cap, that float is doing real work. 💖
  • val get_minor_free : unit -> int — returns the current size of the free space inside the minor heap of this domain.

Since 4.03

  • 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. Teamwork, but make it GC. 💫
  • 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.
  • val 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 ())
  • 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. Polite, no drama. 🎀
  • val eventlog_pause : unit -> unit — Deprecated. Use Runtime_events.pause instead.
  • val eventlog_resume : unit -> unit — Deprecated. Use Runtime_events.resume instead.

🧚 module Memprof : sig end 🧚

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.

Since blocks are composed of several words, a block can potentially be sampled several times. If a block is sampled several times, then each of the callbacks is called once for each event of this block: the multiplicity is given in the n_samples field of the allocation structure.

This engine makes it possible to implement a low-overhead memory profiler as an OCaml library.

Note: this API is EXPERIMENTAL. It may change without prior notice. 👀
  • type suspended_collection_work
  • val ramp_up : (unit -> 'a) -> 'a * suspended_collection_work — In general, the OCaml GC assumes that the program runs in a "steady state" where peak memory usage remains constant: for each newly allocated work, it assumes that one work has become unreachable and will try to collect it during the next GC slice.

This assumption is incorrect at the points during program execution where the live memory increases instead of remaining stable: the steady-state assumption will make the GC work harder at no benefit as it will not find more memory to collect.

ramp_up f puts the current domain in a "ramp-up" phase for the duration of the evaluation of f (), letting the GC know that the steady-state assumption does not hold; it should be used when you know that the live memory of the program will increase significantly.

During a ramp-up phase, the GC will not try to work harder for new allocations: the corresponding collection work is "suspended". The total amount of suspended collection work is returned by ramp_up along with the result of the function.

If the user discards this suspended work (by doing nothing with it), the GC will never accelerate to recover the corresponding amount of memory. This is appropriate if the ramp-up work allocates long-lived memory that remains live until the end of the program execution.

If the user knows that at a certain point in the program the live memory consumption has been reduced by the corresponding amount -- typically, because the memory allocated during ramp_up has become unused -- then they should call Gc.ramp_down below to have the GC "resume" this collection work.

If f () raises an exception, the ramp-up phase terminates, the collection work that was suspended is resumed, and the exception is re-raised.

If f () performs an effect, the effect is not handled and an Effect.Unhandled exception is thrown instead. Bestie, read that twice. 🫶

  • val ramp_down : suspended_collection_work -> unit — notify the GC about some amount of collection work that was suspended during a ramp-up phase, to be resumed now. Resume the vibe. 🌈

🌸 module Tweak : sig end 🌸

GC Tweaks are unstable and undocumented configurable GC parameters, primarily intended for use by GC developers.

As well as using Gc.Tweak.set "foo" 42, they can also be configured in OCAMLRUNPARAM, using the following syntax:

OCAMLRUNPARAM='Xfoo=42'

Additionally, OCAMLRUNPARAM=Xhelp will show the available GC tweaks.

Since 5.5

That's the whole vibe from Stdlib.Gc(3) 💖 — source/version: OCamldoc, date: 2026-06-22. Stay iconic, bestie! 👑✨

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