Module Stdlib.Gc

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
*)
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: 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 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 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.

  • raises Invalid_argument

    if n is negative, return 0 if n is larger than the smoothing window.

  • since 4.03.0
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 Stdlib.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 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
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. 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.