1Gc(3) OCaml library Gc(3)
2
6 Gc - Memory management control and statistics; finalised values.
7
9 Module Gc
10
12 Module Gc
13 : sig end
14 Memory management control and statistics; finalised values.
17 type stat = {
24 minor_words : float ; (* Number of words allocated in the minor heap
25 since the program was started. This number is accurate in byte-code
26 programs, but only an approximation in programs compiled to native
27 code. *)
28 promoted_words : float ; (* Number of words allocated in the minor
29 heap that survived a minor collection and were moved to the major heap
30 since the program was started. *)
31 major_words : float ; (* Number of words allocated in the major heap,
32 including the promoted words, since the program was started. *)
33 minor_collections : int ; (* Number of minor collections since the
34 program was started. *)
35 major_collections : int ; (* Number of major collection cycles com‐
36 pleted since the program was started. *)
37 heap_words : int ; (* Total size of the major heap, in words. *)
38 heap_chunks : int ; (* Number of contiguous pieces of memory that
39 make up the major heap. *)
40 live_words : int ; (* Number of words of live data in the major heap,
41 including the header words. *)
42 live_blocks : int ; (* Number of live blocks in the major heap. *)
43 free_words : int ; (* Number of words in the free list. *)
44 free_blocks : int ; (* Number of blocks in the free list. *)
45 largest_free : int ; (* Size (in words) of the largest block in the
46 free list. *)
47 fragments : int ; (* Number of wasted words due to fragmentation.
48 These are 1-words free blocks placed between two live blocks. They are
49 not available for allocation. *)
50 compactions : int ; (* Number of heap compactions since the program
51 was started. *)
52 top_heap_words : int ; (* Maximum size reached by the major heap, in
53 words. *)
54 }
55 The memory management counters are returned in a stat record.
58 The total amount of memory allocated by the program since it was
60 started is (in words) minor_words + major_words - promoted_words .
61 Multiply by the word size (4 on a 32-bit machine, 8 on a 64-bit
62 machine) to get the number of bytes.
63 type control = {
67 mutable minor_heap_size : int ; (* The size (in words) of the minor
69 heap. Changing this parameter will trigger a minor collection.
70 Default: 32k. *)
71 mutable major_heap_increment : int ; (* The minimum number of words to
73 add to the major heap when increasing it. Default: 124k. *)
74 mutable space_overhead : int ; (* The major GC speed is computed from
76 this parameter. This is the memory that will be "wasted" because the
77 GC does not immediatly collect unreachable blocks. It is expressed as
78 a percentage of the memory used for live data. The GC will work more
79 (use more CPU time and collect blocks more eagerly) if space_overhead
80 is smaller. Default: 80. *)
81 mutable verbose : int ; (* This value controls the GC messages on
83 standard error output. It is a sum of some of the following flags, to
84 print messages on the corresponding events:
85 - 0x001 Start of major GC cycle.
87 - 0x002 Minor collection and major GC slice.
89 - 0x004 Growing and shrinking of the heap.
91 - 0x008 Resizing of stacks and memory manager tables.
93 - 0x010 Heap compaction.
95 - 0x020 Change of GC parameters.
97 - 0x040 Computation of major GC slice size.
99 - 0x080 Calling of finalisation functions.
101 - 0x100 Bytecode executable search at start-up.
103 - 0x200 Computation of compaction triggering condition. Default: 0.
105 *)
106 mutable max_overhead : int ; (* Heap compaction is triggered when the
108 estimated amount of "wasted" memory is more than max_overhead percent
109 of the amount of live data. If max_overhead is set to 0, heap com‐
110 paction is triggered at the end of each major GC cycle (this setting is
111 intended for testing purposes only). If max_overhead >= 1000000 , com‐
112 paction is never triggered. Default: 500. *)
113 mutable stack_limit : int ; (* The maximum size of the stack (in
115 words). This is only relevant to the byte-code runtime, as the native
116 code runtime uses the operating system's stack. Default: 256k. *)
117 mutable allocation_policy : int ; (* The policy used for allocating in
119 the heap. Possible values are 0 and 1. 0 is the next-fit policy,
120 which is quite fast but can result in fragmentation. 1 is the
121 first-fit policy, which can be slower in some cases but can be better
122 for programs with fragmentation problems. Default: 0. *)
123 }
124 The GC parameters are given as a control record. Note that these
127 parameters can also be initialised by setting the OCAMLRUNPARAM envi‐
128 ronment variable. See the documentation of ocamlrun.
129 val stat : unit -> stat
134 Return the current values of the memory management counters in a stat
136 record. This function examines every heap block to get the statistics.
137 val quick_stat : unit -> stat
142 Same as stat except that live_words , live_blocks , free_words ,
144 free_blocks , largest_free , and fragments are set to 0. This function
145 is much faster than stat because it does not need to go through the
146 heap.
147 val counters : unit -> float * float * float
152 Return (minor_words, promoted_words, major_words) . This function is
154 as fast at quick_stat .
155 val get : unit -> control
160 Return the current values of the GC parameters in a control record.
162 val set : control -> unit
167 set r changes the GC parameters according to the control record r .
170 The normal usage is: Gc.set { (Gc.get()) with Gc.verbose = 0x00d }
171 val minor : unit -> unit
177 Trigger a minor collection.
179 val major_slice : int -> int
184 Do a minor collection and a slice of major collection. The argument is
186 the size of the slice, 0 to use the automatically-computed slice size.
187 In all cases, the result is the computed slice size.
188 val major : unit -> unit
193 Do a minor collection and finish the current major collection cycle.
195 val full_major : unit -> unit
200 Do a minor collection, finish the current major collection cycle, and
202 perform a complete new cycle. This will collect all currently unreach‐
203 able blocks.
204 val compact : unit -> unit
209 Perform a full major collection and compact the heap. Note that heap
211 compaction is a lengthy operation.
212 val print_stat : Pervasives.out_channel -> unit
217 Print the current values of the memory management counters (in
219 human-readable form) into the channel argument.
220 val allocated_bytes : unit -> float
225 Return the total number of bytes allocated since the program was
227 started. It is returned as a float to avoid overflow problems with int
228 on 32-bit machines.
229 val finalise : ('a -> unit) -> 'a -> unit
234 finalise f v registers f as a finalisation function for v . v must be
237 heap-allocated. f will be called with v as argument at some point
238 between the first time v becomes unreachable and the time v is col‐
239 lected by the GC. Several functions can be registered for the same
240 value, or even several instances of the same function. Each instance
241 will be called once (or never, if the program terminates before v
242 becomes unreachable).
243 The GC will call the finalisation functions in the order of dealloca‐
245 tion. When several values become unreachable at the same time (i.e.
246 during the same GC cycle), the finalisation functions will be called in
247 the reverse order of the corresponding calls to finalise . If finalise
248 is called in the same order as the values are allocated, that means
249 each value is finalised before the values it depends upon. Of course,
250 this becomes false if additional dependencies are introduced by assign‐
251 ments.
252 Anything reachable from the closure of finalisation functions is con‐
254 sidered reachable, so the following code will not work as expected:
255 - let v = ... in Gc.finalise (fun x -> ...) v
257 Instead you should write:
259 - let f = fun x -> ... ;; let v = ... in Gc.finalise f v
261 The f function can use all features of O'Caml, including assignments
263 that make the value reachable again. It can also loop forever (in this
264 case, the other finalisation functions will be called during the execu‐
265 tion of f). It can call finalise on v or other values to register
266 other functions or even itself. It can raise an exception; in this
267 case the exception will interrupt whatever the program was doing when
268 the function was called.
269 finalise will raise Invalid_argument if v is not heap-allocated. Some
272 examples of values that are not heap-allocated are integers, constant
273 constructors, booleans, the empty array, the empty list, the unit
274 value. The exact list of what is heap-allocated or not is implementa‐
275 tion-dependent. Some constant values can be heap-allocated but never
276 deallocated during the lifetime of the program, for example a list of
277 integer constants; this is also implementation-dependent. You should
278 also be aware that compiler optimisations may duplicate some immutable
279 values, for example floating-point numbers when stored into arrays, so
280 they can be finalised and collected while another copy is still in use
281 by the program.
282 The results of calling String.make , String.create , Array.make , and
284 Pervasives.ref are guaranteed to be heap-allocated and non-constant
285 except when the length argument is 0 .
286 val finalise_release : unit -> unit
291 A finalisation function may call finalise_release to tell the GC that
293 it can launch the next finalisation function without waiting for the
294 current one to return.
295 type alarm
299 An alarm is a piece of data that calls a user function at the end of
302 each major GC cycle. The following functions are provided to create
303 and delete alarms.
304 val create_alarm : (unit -> unit) -> alarm
309 create_alarm f will arrange for f to be called at the end of each major
312 GC cycle, starting with the current cycle or the next one. A value of
313 type alarm is returned that you can use to call delete_alarm .
314 val delete_alarm : alarm -> unit
319 delete_alarm a will stop the calls to the function associated to a .
322 Calling delete_alarm a again has no effect.
323OCamldoc 2017-03-22 Gc(3)