Module Base__Import0.Gc
Memory management control and statistics; finalised values.
type stat
=
{
}
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
=
{
}
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 ofocamlrun
.
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 thatlive_words
,live_blocks
,free_words
,free_blocks
,largest_free
, andfragments
are set to 0. This function is much faster thanstat
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 asquick_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 thecontrol
recordr
. The normal usage is:Gc.set { (Gc.get()) with Gc.verbose = 0x00d }
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. Ifn
= 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 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 : Stdlib.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 withint
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 then
-th future bucket of the GC smoothing system. The unit is one millionth of a full GC. RaiseInvalid_argument
ifn
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
containsH=1
.- since
- 4.03.0
val finalise : ('a -> unit) -> 'a -> unit
finalise f v
registersf
as a finalisation function forv
.v
must be heap-allocated.f
will be called withv
as argument at some point between the first timev
becomes unreachable (including through weak pointers) and the timev
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 beforev
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
. Iffinalise
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 callsfinalise_release
). It can callfinalise
onv
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 raiseInvalid_argument
ifv
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 typesfloat
are sometimes allocated and sometimes not, so finalising them is unsafe, andfinalise
will also raiseInvalid_argument
for them. Values of type'a Lazy.t
(for any'a
) are likefloat
in this respect, except that the compiler sometimes optimizes them in a way that preventsfinalise
from detecting them. In this case, it will not raiseInvalid_argument
, but you should still avoid callingfinalise
on lazy values.The results of calling
String
.make,Bytes
.make,Bytes
.create,Array
.make, andStdlib.ref
are guaranteed to be heap-allocated and non-constant except when the length argument is0
.
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 tofinalise
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 withfinalise
are always called before the finalisation functions attached withfinalise_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 forf
to be called at the end of each major GC cycle, starting with the current cycle or the next one. A value of typealarm
is returned that you can use to calldelete_alarm
.
val delete_alarm : alarm -> unit
delete_alarm a
will stop the calls to the function associated toa
. Callingdelete_alarm a
again has no effect.