Gc(3) OCaml library Gc(3)NAMEGc - Memory management control and statistics; finalised values.
Module
Module GcDocumentation
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. This number is accurate in byte-code
programs, but only an approximation in programs compiled to native
code. *)
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 com‐
pleted 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. *)
}
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: 32k. *)
mutable major_heap_increment : int ; (* The minimum number of words to
add to the major heap when increasing it. Default: 124k. *)
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 immediatly 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 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 search at start-up.
- 0x200 Computation of compaction triggering condition. 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 com‐
paction is triggered at the end of each major GC cycle (this setting is
intended for testing purposes only). If max_overhead >= 1000000 , com‐
paction is never triggered. If compaction is permanently disabled, it
is strongly suggested to set allocation_policy to 1. 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: 256k. *)
mutable allocation_policy : int ; (* The policy used for allocating in
the heap. Possible values are 0 and 1. 0 is the next-fit policy,
which is quite fast but can result in fragmentation. 1 is the
first-fit policy, which can be slower in some cases but can be better
for programs with fragmentation problems. Default: 0. *)
}
The GC parameters are given as a control record. Note that these
parameters can also be initialised by setting the OCAMLRUNPARAM envi‐
ronment 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 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
Do a minor collection and a slice of major collection. The argument is
the size of the slice, 0 to use the automatically-computed slice size.
In all cases, the result is the computed slice size.
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 unreach‐
able 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 : Pervasives.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 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 and the time v is col‐
lected 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 dealloca‐
tion. 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 assign‐
ments.
Anything reachable from the closure of finalisation functions is con‐
sidered reachable, so the following code will not work as expected:
- let v = ... in Gc.finalise (fun x -> ...) v
Instead you should write:
- 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 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 implementa‐
tion-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. You should
also be aware that compiler optimisations may duplicate some immutable
values, for example floating-point numbers when stored into arrays, so
they can be finalised and collected while another copy is still in use
by the program.
The results of calling String.make , String.create , Array.make , and
Pervasives.ref are guaranteed to be heap-allocated and non-constant
except when the length argument is 0 .
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.
OCamldoc 2013-09-28 Gc(3)
