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 custom_major_ratio : int ; (* Target ratio of floating garbage to
166 major heap size for out-of-heap memory held by custom values located in
167 the major heap. The GC speed is adjusted to try to use this much memory
168 for dead values that are not yet collected. Expressed as a percentage
169 of major heap size. The default value keeps the out-of-heap floating
170 garbage about the same size as the in-heap overhead. Note: this only
171 applies to values allocated with caml_alloc_custom_mem (e.g. bigar‐
172 rays). Default: 44.
173 Since 4.08.0
176 *)
177 custom_minor_ratio : int ; (* Bound on floating garbage for
178 out-of-heap memory held by custom values in the minor heap. A minor GC
179 is triggered when this much memory is held by custom values located in
180 the minor heap. Expressed as a percentage of minor heap size. Note:
181 this only applies to values allocated with caml_alloc_custom_mem (e.g.
182 bigarrays). Default: 100.
183 Since 4.08.0
186 *)
187 custom_minor_max_size : int ; (* Maximum amount of out-of-heap memory
188 for each custom value allocated in the minor heap. When a custom value
189 is allocated on the minor heap and holds more than this many bytes,
190 only this value is counted against custom_minor_ratio and the rest is
191 directly counted against custom_major_ratio . Note: this only applies
192 to values allocated with caml_alloc_custom_mem (e.g. bigarrays).
193 Default: 8192 bytes.
194 Since 4.08.0
197 *)
198 }
199 The GC parameters are given as a control record. Note that these
202 parameters can also be initialised by setting the OCAMLRUNPARAM envi‐
203 ronment variable. See the documentation of ocamlrun .
204 val stat : unit -> stat
208 Return the current values of the memory management counters in a stat
210 record. This function examines every heap block to get the statistics.
211 val quick_stat : unit -> stat
215 Same as stat except that live_words , live_blocks , free_words ,
217 free_blocks , largest_free , and fragments are set to 0. This function
218 is much faster than stat because it does not need to go through the
219 heap.
220 val counters : unit -> float * float * float
224 Return (minor_words, promoted_words, major_words) . This function is
226 as fast as quick_stat .
227 val minor_words : unit -> float
231 Number of words allocated in the minor heap since the program was
233 started. This number is accurate in byte-code programs, but only an
234 approximation in programs compiled to native code.
235 In native code this function does not allocate.
237 Since 4.04
240 val get : unit -> control
244 Return the current values of the GC parameters in a control record.
246 val set : control -> unit
250 set r changes the GC parameters according to the control record r .
253 The normal usage is: Gc.set { (Gc.get()) with Gc.verbose = 0x00d }
254 val minor : unit -> unit
259 Trigger a minor collection.
261 val major_slice : int -> int
265 major_slice n Do a minor collection and a slice of major collection. n
268 is the size of the slice: the GC will do enough work to free (on aver‐
269 age) n words of memory. If n = 0, the GC will try to do enough work to
270 ensure that the next automatic slice has no work to do. This function
271 returns an unspecified integer (currently: 0).
272 val major : unit -> unit
276 Do a minor collection and finish the current major collection cycle.
278 val full_major : unit -> unit
282 Do a minor collection, finish the current major collection cycle, and
284 perform a complete new cycle. This will collect all currently unreach‐
285 able blocks.
286 val compact : unit -> unit
290 Perform a full major collection and compact the heap. Note that heap
292 compaction is a lengthy operation.
293 val print_stat : out_channel -> unit
297 Print the current values of the memory management counters (in
299 human-readable form) into the channel argument.
300 val allocated_bytes : unit -> float
304 Return the total number of bytes allocated since the program was
306 started. It is returned as a float to avoid overflow problems with int
307 on 32-bit machines.
308 val get_minor_free : unit -> int
312 Return the current size of the free space inside the minor heap.
314 Since 4.03.0
317 val get_bucket : int -> int
321 get_bucket n returns the current size of the n -th future bucket of the
324 GC smoothing system. The unit is one millionth of a full GC. Raise
325 Invalid_argument if n is negative, return 0 if n is larger than the
326 smoothing window.
327 Since 4.03.0
330 val get_credit : unit -> int
334 get_credit () returns the current size of the "work done in advance"
337 counter of the GC smoothing system. The unit is one millionth of a full
338 GC.
339 Since 4.03.0
342 val huge_fallback_count : unit -> int
346 Return the number of times we tried to map huge pages and had to fall
348 back to small pages. This is always 0 if OCAMLRUNPARAM contains H=1 .
349 Since 4.03.0
352 val finalise : ('a -> unit) -> 'a -> unit
356 finalise f v registers f as a finalisation function for v . v must be
359 heap-allocated. f will be called with v as argument at some point
360 between the first time v becomes unreachable (including through weak
361 pointers) and the time v is collected by the GC. Several functions can
362 be registered for the same value, or even several instances of the same
363 function. Each instance will be called once (or never, if the program
364 terminates before v becomes unreachable).
365 The GC will call the finalisation functions in the order of dealloca‐
367 tion. When several values become unreachable at the same time (i.e.
368 during the same GC cycle), the finalisation functions will be called in
369 the reverse order of the corresponding calls to finalise . If finalise
370 is called in the same order as the values are allocated, that means
371 each value is finalised before the values it depends upon. Of course,
372 this becomes false if additional dependencies are introduced by assign‐
373 ments.
374 In the presence of multiple OCaml threads it should be assumed that any
376 particular finaliser may be executed in any of the threads.
377 Anything reachable from the closure of finalisation functions is con‐
379 sidered reachable, so the following code will not work as expected:
380 - let v = ... in Gc.finalise (fun _ -> ...v...) v
382 Instead you should make sure that v is not in the closure of the final‐
384 isation function by writing:
385 - let f = fun x -> ... let v = ... in Gc.finalise f v
387 The f function can use all features of OCaml, including assignments
389 that make the value reachable again. It can also loop forever (in this
390 case, the other finalisation functions will not be called during the
391 execution of f, unless it calls finalise_release ). It can call
392 finalise on v or other values to register other functions or even
393 itself. It can raise an exception; in this case the exception will
394 interrupt whatever the program was doing when the function was called.
395 finalise will raise Invalid_argument if v is not guaranteed to be
398 heap-allocated. Some examples of values that are not heap-allocated
399 are integers, constant constructors, booleans, the empty array, the
400 empty list, the unit value. The exact list of what is heap-allocated
401 or not is implementation-dependent. Some constant values can be
402 heap-allocated but never deallocated during the lifetime of the pro‐
403 gram, for example a list of integer constants; this is also implementa‐
404 tion-dependent. Note that values of types float are sometimes allo‐
405 cated and sometimes not, so finalising them is unsafe, and finalise
406 will also raise Invalid_argument for them. Values of type 'a Lazy.t
407 (for any 'a ) are like float in this respect, except that the compiler
408 sometimes optimizes them in a way that prevents finalise from detecting
409 them. In this case, it will not raise Invalid_argument , but you should
410 still avoid calling finalise on lazy values.
411 The results of calling String.make , Bytes.make , Bytes.create ,
413 Array.make , and ref are guaranteed to be heap-allocated and non-con‐
414 stant except when the length argument is 0 .
415 val finalise_last : (unit -> unit) -> 'a -> unit
419 same as Gc.finalise except the value is not given as argument. So you
421 can't use the given value for the computation of the finalisation func‐
422 tion. The benefit is that the function is called after the value is
423 unreachable for the last time instead of the first time. So contrary to
424 Gc.finalise the value will never be reachable again or used again. In
425 particular every weak pointer and ephemeron that contained this value
426 as key or data is unset before running the finalisation function. More‐
427 over the finalisation functions attached with Gc.finalise are always
428 called before the finalisation functions attached with Gc.finalise_last
429 .
430 Since 4.04
433 val finalise_release : unit -> unit
437 A finalisation function may call finalise_release to tell the GC that
439 it can launch the next finalisation function without waiting for the
440 current one to return.
441 type alarm
444 An alarm is a piece of data that calls a user function at the end of
447 each major GC cycle. The following functions are provided to create
448 and delete alarms.
449 val create_alarm : (unit -> unit) -> alarm
453 create_alarm f will arrange for f to be called at the end of each major
456 GC cycle, starting with the current cycle or the next one. A value of
457 type alarm is returned that you can use to call delete_alarm .
458 val delete_alarm : alarm -> unit
462 delete_alarm a will stop the calls to the function associated to a .
465 Calling delete_alarm a again has no effect.
466OCamldoc 2019-07-30 Gc(3)