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: 62k. *)
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 }
118 The GC parameters are given as a control record. Note that these
121 parameters can also be initialised by setting the OCAMLRUNPARAM envi‐
122 ronment variable. See the documentation of ocamlrun.
123 val stat : unit -> stat
128 Return the current values of the memory management counters in a stat
130 record. This function examines every heap block to get the statistics.
131 val quick_stat : unit -> stat
136 Same as stat except that live_words , live_blocks , free_words ,
138 free_blocks , largest_free , and fragments are set to 0. This function
139 is much faster than stat because it does not need to go through the
140 heap.
141 val counters : unit -> float * float * float
146 Return (minor_words, promoted_words, major_words) . This function is
148 as fast at quick_stat .
149 val get : unit -> control
154 Return the current values of the GC parameters in a control record.
156 val set : control -> unit
161 set r changes the GC parameters according to the control record r .
164 The normal usage is: Gc.set { (Gc.get()) with Gc.verbose = 0x00d }
165 val minor : unit -> unit
171 Trigger a minor collection.
173 val major_slice : int -> int
178 Do a minor collection and a slice of major collection. The argument is
180 the size of the slice, 0 to use the automatically-computed slice size.
181 In all cases, the result is the computed slice size.
182 val major : unit -> unit
187 Do a minor collection and finish the current major collection cycle.
189 val full_major : unit -> unit
194 Do a minor collection, finish the current major collection cycle, and
196 perform a complete new cycle. This will collect all currently unreach‐
197 able blocks.
198 val compact : unit -> unit
203 Perform a full major collection and compact the heap. Note that heap
205 compaction is a lengthy operation.
206 val print_stat : Pervasives.out_channel -> unit
211 Print the current values of the memory management counters (in human-
213 readable form) into the channel argument.
214 val allocated_bytes : unit -> float
219 Return the total number of bytes allocated since the program was
221 started. It is returned as a float to avoid overflow problems with int
222 on 32-bit machines.
223 val finalise : ('a -> unit) -> 'a -> unit
228 finalise f v registers f as a finalisation function for v . v must be
231 heap-allocated. f will be called with v as argument at some point
232 between the first time v becomes unreachable and the time v is col‐
233 lected by the GC. Several functions can be registered for the same
234 value, or even several instances of the same function. Each instance
235 will be called once (or never, if the program terminates before v
236 becomes unreachable).
237 The GC will call the finalisation functions in the order of dealloca‐
239 tion. When several values become unreachable at the same time (i.e.
240 during the same GC cycle), the finalisation functions will be called in
241 the reverse order of the corresponding calls to finalise . If finalise
242 is called in the same order as the values are allocated, that means
243 each value is finalised before the values it depends upon. Of course,
244 this becomes false if additional dependencies are introduced by assign‐
245 ments.
246 Anything reachable from the closure of finalisation functions is con‐
248 sidered reachable, so the following code will not work as expected:
249 - let v = ... in Gc.finalise (fun x -> ...) v
251 Instead you should write:
253 - let f = fun x -> ... ;; let v = ... in Gc.finalise f v
255 The f function can use all features of O'Caml, including assignments
257 that make the value reachable again. It can also loop forever (in this
258 case, the other finalisation functions will be called during the execu‐
259 tion of f). It can call finalise on v or other values to register
260 other functions or even itself. It can raise an exception; in this
261 case the exception will interrupt whatever the program was doing when
262 the function was called.
263 finalise will raise Invalid_argument if v is not heap-allocated. Some
266 examples of values that are not heap-allocated are integers, constant
267 constructors, booleans, the empty array, the empty list, the unit
268 value. The exact list of what is heap-allocated or not is implementa‐
269 tion-dependent. Some constant values can be heap-allocated but never
270 deallocated during the lifetime of the program, for example a list of
271 integer constants; this is also implementation-dependent. You should
272 also be aware that compiler optimisations may duplicate some immutable
273 values, for example floating-point numbers when stored into arrays, so
274 they can be finalised and collected while another copy is still in use
275 by the program.
276 The results of calling String.make , String.create , Array.make , and
278 Pervasives.ref are guaranteed to be heap-allocated and non-constant
279 except when the length argument is 0 .
280 val finalise_release : unit -> unit
285 A finalisation function may call finalise_release to tell the GC that
287 it can launch the next finalisation function without waiting for the
288 current one to return.
289 type alarm
293 An alarm is a piece of data that calls a user function at the end of
296 each major GC cycle. The following functions are provided to create
297 and delete alarms.
298 val create_alarm : (unit -> unit) -> alarm
303 create_alarm f will arrange for f to be called at the end of each major
306 GC cycle, starting with the current cycle or the next one. A value of
307 type alarm is returned that you can use to call delete_alarm .
308 val delete_alarm : alarm -> unit
313 delete_alarm a will stop the calls to the function associated to a .
316 Calling delete_alarm a again has no effect.
317OCamldoc 2007-05-24 Gc(3)