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 = {
23 minor_words : float ; (* Number of words allocated in the minor heap
24 since the program was started. This number is accurate in byte-code
25 programs, but only an approximation in programs compiled to native
26 code.
27 *)
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 *)
32 major_words : float ; (* Number of words allocated in the major heap,
33 including the promoted words, since the program was started.
34 *)
35 minor_collections : int ; (* Number of minor collections since the
36 program was started.
37 *)
38 major_collections : int ; (* Number of major collection cycles com‐
39 pleted since the program was started.
40 *)
41 heap_words : int ; (* Total size of the major heap, in words.
42 *)
43 heap_chunks : int ; (* Number of contiguous pieces of memory that
44 make up the major heap.
45 *)
46 live_words : int ; (* Number of words of live data in the major heap,
47 including the header words.
48 *)
49 live_blocks : int ; (* Number of live blocks in the major heap.
50 *)
51 free_words : int ; (* Number of words in the free list.
52 *)
53 free_blocks : int ; (* Number of blocks in the free list.
54 *)
55 largest_free : int ; (* Size (in words) of the largest block in the
56 free list.
57 *)
58 fragments : int ; (* Number of wasted words due to fragmentation.
59 These are 1-words free blocks placed between two live blocks. They are
60 not available for allocation.
61 *)
62 compactions : int ; (* Number of heap compactions since the program
63 was started.
64 *)
65 top_heap_words : int ; (* Maximum size reached by the major heap, in
66 words.
67 *)
68 stack_size : int ; (* Current size of the stack, in words.
69 Since 3.12.0
72 *)
73 }
74 The memory management counters are returned in a stat record.
77 The total amount of memory allocated by the program since it was
79 started is (in words) minor_words + major_words - promoted_words .
80 Multiply by the word size (4 on a 32-bit machine, 8 on a 64-bit
81 machine) to get the number of bytes.
82 type control = {
85 mutable minor_heap_size : int ; (* The size (in words) of the minor
87 heap. Changing this parameter will trigger a minor collection.
88 Default: 256k.
89 *)
90 mutable major_heap_increment : int ; (* How much to add to the major
92 heap when increasing it. If this number is less than or equal to 1000,
93 it is a percentage of the current heap size (i.e. setting it to 100
94 will double the heap size at each increase). If it is more than 1000,
95 it is a fixed number of words that will be added to the heap. Default:
96 15.
97 *)
98 mutable space_overhead : int ; (* The major GC speed is computed from
100 this parameter. This is the memory that will be "wasted" because the
101 GC does not immediately collect unreachable blocks. It is expressed as
102 a percentage of the memory used for live data. The GC will work more
103 (use more CPU time and collect blocks more eagerly) if space_overhead
104 is smaller. Default: 80.
105 *)
106 mutable verbose : int ; (* This value controls the GC messages on
108 standard error output. It is a sum of some of the following flags, to
109 print messages on the corresponding events:
110 - 0x001 Start of major GC cycle.
112 - 0x002 Minor collection and major GC slice.
114 - 0x004 Growing and shrinking of the heap.
116 - 0x008 Resizing of stacks and memory manager tables.
118 - 0x010 Heap compaction.
120 - 0x020 Change of GC parameters.
122 - 0x040 Computation of major GC slice size.
124 - 0x080 Calling of finalisation functions.
126 - 0x100 Bytecode executable and shared library search at start-up.
128 - 0x200 Computation of compaction-triggering condition.
130 - 0x400 Output GC statistics at program exit. Default: 0.
132 *)
134 mutable max_overhead : int ; (* Heap compaction is triggered when the
136 estimated amount of "wasted" memory is more than max_overhead percent
137 of the amount of live data. If max_overhead is set to 0, heap com‐
138 paction is triggered at the end of each major GC cycle (this setting is
139 intended for testing purposes only). If max_overhead >= 1000000 , com‐
140 paction is never triggered. If compaction is permanently disabled, it
141 is strongly suggested to set allocation_policy to 1. Default: 500.
142 *)
143 mutable stack_limit : int ; (* The maximum size of the stack (in
145 words). This is only relevant to the byte-code runtime, as the native
146 code runtime uses the operating system's stack. Default: 1024k.
147 *)
148 mutable allocation_policy : int ; (* The policy used for allocating in
150 the heap. Possible values are 0 and 1. 0 is the next-fit policy,
151 which is quite fast but can result in fragmentation. 1 is the
152 first-fit policy, which can be slower in some cases but can be better
153 for programs with fragmentation problems. Default: 0.
154 Since 3.11.0
157 *)
158 window_size : int ; (* The size of the window used by the major GC
159 for smoothing out variations in its workload. This is an integer
160 between 1 and 50. Default: 1.
161 Since 4.03.0
164 *)
165 }
166 The GC parameters are given as a control record. Note that these
169 parameters can also be initialised by setting the OCAMLRUNPARAM envi‐
170 ronment variable. See the documentation of ocamlrun .
171 val stat : unit -> stat
175 Return the current values of the memory management counters in a stat
177 record. This function examines every heap block to get the statistics.
178 val quick_stat : unit -> stat
182 Same as stat except that live_words , live_blocks , free_words ,
184 free_blocks , largest_free , and fragments are set to 0. This function
185 is much faster than stat because it does not need to go through the
186 heap.
187 val counters : unit -> float * float * float
191 Return (minor_words, promoted_words, major_words) . This function is
193 as fast as quick_stat .
194 val minor_words : unit -> float
198 Number of words allocated in the minor heap since the program was
200 started. This number is accurate in byte-code programs, but only an
201 approximation in programs compiled to native code.
202 In native code this function does not allocate.
204 Since 4.04
207 val get : unit -> control
211 Return the current values of the GC parameters in a control record.
213 val set : control -> unit
217 set r changes the GC parameters according to the control record r .
220 The normal usage is: Gc.set { (Gc.get()) with Gc.verbose = 0x00d }
221 val minor : unit -> unit
226 Trigger a minor collection.
228 val major_slice : int -> int
232 major_slice n Do a minor collection and a slice of major collection. n
235 is the size of the slice: the GC will do enough work to free (on aver‐
236 age) n words of memory. If n = 0, the GC will try to do enough work to
237 ensure that the next automatic slice has no work to do. This function
238 returns an unspecified integer (currently: 0).
239 val major : unit -> unit
243 Do a minor collection and finish the current major collection cycle.
245 val full_major : unit -> unit
249 Do a minor collection, finish the current major collection cycle, and
251 perform a complete new cycle. This will collect all currently unreach‐
252 able blocks.
253 val compact : unit -> unit
257 Perform a full major collection and compact the heap. Note that heap
259 compaction is a lengthy operation.
260 val print_stat : Pervasives.out_channel -> unit
264 Print the current values of the memory management counters (in
266 human-readable form) into the channel argument.
267 val allocated_bytes : unit -> float
271 Return the total number of bytes allocated since the program was
273 started. It is returned as a float to avoid overflow problems with int
274 on 32-bit machines.
275 val get_minor_free : unit -> int
279 Return the current size of the free space inside the minor heap.
281 Since 4.03.0
284 val get_bucket : int -> int
288 get_bucket n returns the current size of the n -th future bucket of the
291 GC smoothing system. The unit is one millionth of a full GC. Raise
292 Invalid_argument if n is negative, return 0 if n is larger than the
293 smoothing window.
294 Since 4.03.0
297 val get_credit : unit -> int
301 get_credit () returns the current size of the "work done in advance"
304 counter of the GC smoothing system. The unit is one millionth of a full
305 GC.
306 Since 4.03.0
309 val huge_fallback_count : unit -> int
313 Return the number of times we tried to map huge pages and had to fall
315 back to small pages. This is always 0 if OCAMLRUNPARAM contains H=1 .
316 Since 4.03.0
319 val finalise : ('a -> unit) -> 'a -> unit
323 finalise f v registers f as a finalisation function for v . v must be
326 heap-allocated. f will be called with v as argument at some point
327 between the first time v becomes unreachable (including through weak
328 pointers) and the time v is collected by the GC. Several functions can
329 be registered for the same value, or even several instances of the same
330 function. Each instance will be called once (or never, if the program
331 terminates before v becomes unreachable).
332 The GC will call the finalisation functions in the order of dealloca‐
334 tion. When several values become unreachable at the same time (i.e.
335 during the same GC cycle), the finalisation functions will be called in
336 the reverse order of the corresponding calls to finalise . If finalise
337 is called in the same order as the values are allocated, that means
338 each value is finalised before the values it depends upon. Of course,
339 this becomes false if additional dependencies are introduced by assign‐
340 ments.
341 In the presence of multiple OCaml threads it should be assumed that any
343 particular finaliser may be executed in any of the threads.
344 Anything reachable from the closure of finalisation functions is con‐
346 sidered reachable, so the following code will not work as expected:
347 - let v = ... in Gc.finalise (fun _ -> ...v...) v
349 Instead you should make sure that v is not in the closure of the final‐
351 isation function by writing:
352 - let f = fun x -> ... let v = ... in Gc.finalise f v
354 The f function can use all features of OCaml, including assignments
356 that make the value reachable again. It can also loop forever (in this
357 case, the other finalisation functions will not be called during the
358 execution of f, unless it calls finalise_release ). It can call
359 finalise on v or other values to register other functions or even
360 itself. It can raise an exception; in this case the exception will
361 interrupt whatever the program was doing when the function was called.
362 finalise will raise Invalid_argument if v is not guaranteed to be
365 heap-allocated. Some examples of values that are not heap-allocated
366 are integers, constant constructors, booleans, the empty array, the
367 empty list, the unit value. The exact list of what is heap-allocated
368 or not is implementation-dependent. Some constant values can be
369 heap-allocated but never deallocated during the lifetime of the pro‐
370 gram, for example a list of integer constants; this is also implementa‐
371 tion-dependent. Note that values of types float are sometimes allo‐
372 cated and sometimes not, so finalising them is unsafe, and finalise
373 will also raise Invalid_argument for them. Values of type 'a Lazy.t
374 (for any 'a ) are like float in this respect, except that the compiler
375 sometimes optimizes them in a way that prevents finalise from detecting
376 them. In this case, it will not raise Invalid_argument , but you should
377 still avoid calling finalise on lazy values.
378 The results of calling String.make , Bytes.make , Bytes.create ,
380 Array.make , and Pervasives.ref are guaranteed to be heap-allocated and
381 non-constant except when the length argument is 0 .
382 val finalise_last : (unit -> unit) -> 'a -> unit
386 same as Gc.finalise except the value is not given as argument. So you
388 can't use the given value for the computation of the finalisation func‐
389 tion. The benefit is that the function is called after the value is
390 unreachable for the last time instead of the first time. So contrary to
391 Gc.finalise the value will never be reachable again or used again. In
392 particular every weak pointer and ephemeron that contained this value
393 as key or data is unset before running the finalisation function. More‐
394 over the finalisation function attached with `GC.finalise` are always
395 called before the finalisation function attached with
396 `GC.finalise_last`.
397 Since 4.04
400 val finalise_release : unit -> unit
404 A finalisation function may call finalise_release to tell the GC that
406 it can launch the next finalisation function without waiting for the
407 current one to return.
408 type alarm
411 An alarm is a piece of data that calls a user function at the end of
414 each major GC cycle. The following functions are provided to create
415 and delete alarms.
416 val create_alarm : (unit -> unit) -> alarm
420 create_alarm f will arrange for f to be called at the end of each major
423 GC cycle, starting with the current cycle or the next one. A value of
424 type alarm is returned that you can use to call delete_alarm .
425 val delete_alarm : alarm -> unit
429 delete_alarm a will stop the calls to the function associated to a .
432 Calling delete_alarm a again has no effect.
433OCamldoc 2019-02-02 Gc(3)