Module Gc

The memory management counters are returned in a stat record. 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.

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.

Return 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: their returned field values are therefore 0. This function causes a full major collection.

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, 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.

Return (minor_words, promoted_words, major_words) for the current domain or potentially previous domains. This function is as fast as quick_stat.

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.

In native code this function does not allocate.

Return 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: their returned field values are therefore 0.

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: setting them therefore has no effect.

Trigger a minor collection.

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).

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

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

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

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

Return 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.

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

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.

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.

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.

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.

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 ())
   

delete_alarm a will stop the calls to the function associated to a. Calling delete_alarm a again has no effect.

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.

Notify the GC about some amount of collection work that was suspended during a ramp-up phase, to be resumed now.