Gc(3) OCaml library Gc(3)

# [NAME](#NAME)

Gc - Memory management control and statistics; finalised values.

# [Module](#Module)

Module Gc

# [Documentation](#Documentation)

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.

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.  
\*)  
live\_blocks : **int** ; (* Number of live blocks in the major heap.

See **live\_words** for a caveat about what "live" means.  
\*)  
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  
\*)  
forced\_major\_collections : **int** ; (* Number of forced full major collections completed since the program was started.

**Since** 4.12.0  
\*)  
}

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: 120.  
\*)

**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 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.

\*)

**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 default is best-fit.

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: If you change to next-fit, you may need to reduce the **space\_overhead** setting, for example using **80** instead of the default **120** which is tuned for best-fit. Otherwise, your program will need more memory.

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: 2.

**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.

**Since** 4.11

*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.)

**Since** 4.11

*module Memprof :* **sig end**

**Memprof** is a sampling engine for 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 callback 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.

2026-09-15 OCamldoc

---

Document: Gc(3o) from ocaml-4 4.14.4-r3 (Chainguard OS)
Source: https://github.com/ocaml/ocaml