NAME¶
Gc - Memory management control and statistics; finalised values.
Module¶
Module Gc
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. 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 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. *)
}
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 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. 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 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 at
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 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 :
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 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.
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 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 O'Caml, 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
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. 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.