1Stdlib.Gc(3) OCaml library Stdlib.Gc(3)
2
6 Stdlib.Gc - no description
7
9 Module Stdlib.Gc
10
12 Module Gc
13 : (module Stdlib__Gc)
14 type stat = {
22 minor_words : float ; (* Number of words allocated in the minor heap
23 since the program was started.
24 *)
25 promoted_words : float ; (* Number of words allocated in the minor
26 heap that survived a minor collection and were moved to the major heap
27 since the program was started.
28 *)
29 major_words : float ; (* Number of words allocated in the major heap,
30 including the promoted words, since the program was started.
31 *)
32 minor_collections : int ; (* Number of minor collections since the
33 program was started.
34 *)
35 major_collections : int ; (* Number of major collection cycles com‐
36 pleted since the program was started.
37 *)
38 heap_words : int ; (* Total size of the major heap, in words.
39 *)
40 heap_chunks : int ; (* Number of contiguous pieces of memory that
41 make up the major heap.
42 *)
43 live_words : int ; (* Number of words of live data in the major heap,
44 including the header words.
45 *)
46 live_blocks : int ; (* Number of live blocks in the major heap.
47 *)
48 free_words : int ; (* Number of words in the free list.
49 *)
50 free_blocks : int ; (* Number of blocks in the free list.
51 *)
52 largest_free : int ; (* Size (in words) of the largest block in the
53 free list.
54 *)
55 fragments : int ; (* Number of wasted words due to fragmentation.
56 These are 1-words free blocks placed between two live blocks. They are
57 not available for allocation.
58 *)
59 compactions : int ; (* Number of heap compactions since the program
60 was started.
61 *)
62 top_heap_words : int ; (* Maximum size reached by the major heap, in
63 words.
64 *)
65 stack_size : int ; (* Current size of the stack, in words.
66 Since 3.12.0
69 *)
70 forced_major_collections : int ; (* Number of forced full major col‐
71 lections completed since the program was started.
72 Since 4.12.0
75 *)
76 }
77 The memory management counters are returned in a stat record.
80 The total amount of memory allocated by the program since it was
82 started is (in words) minor_words + major_words - promoted_words .
83 Multiply by the word size (4 on a 32-bit machine, 8 on a 64-bit ma‐
84 chine) to get the number of bytes.
85 type control = {
88 mutable minor_heap_size : int ; (* The size (in words) of the minor
90 heap. Changing this parameter will trigger a minor collection. De‐
91 fault: 256k.
92 *)
93 mutable major_heap_increment : int ; (* How much to add to the major
95 heap when increasing it. If this number is less than or equal to 1000,
96 it is a percentage of the current heap size (i.e. setting it to 100
97 will double the heap size at each increase). If it is more than 1000,
98 it is a fixed number of words that will be added to the heap. Default:
99 15.
100 *)
101 mutable space_overhead : int ; (* The major GC speed is computed from
103 this parameter. This is the memory that will be "wasted" because the
104 GC does not immediately collect unreachable blocks. It is expressed as
105 a percentage of the memory used for live data. The GC will work more
106 (use more CPU time and collect blocks more eagerly) if space_overhead
107 is smaller. Default: 120.
108 *)
109 mutable verbose : int ; (* This value controls the GC messages on
111 standard error output. It is a sum of some of the following flags, to
112 print messages on the corresponding events:
113 - 0x001 Start and end of major GC cycle.
115 - 0x002 Minor collection and major GC slice.
117 - 0x004 Growing and shrinking of the heap.
119 - 0x008 Resizing of stacks and memory manager tables.
121 - 0x010 Heap compaction.
123 - 0x020 Change of GC parameters.
125 - 0x040 Computation of major GC slice size.
127 - 0x080 Calling of finalisation functions.
129 - 0x100 Bytecode executable and shared library search at start-up.
131 - 0x200 Computation of compaction-triggering condition.
133 - 0x400 Output GC statistics at program exit. Default: 0.
135 *)
137 mutable max_overhead : int ; (* Heap compaction is triggered when the
139 estimated amount of "wasted" memory is more than max_overhead percent
140 of the amount of live data. If max_overhead is set to 0, heap com‐
141 paction is triggered at the end of each major GC cycle (this setting is
142 intended for testing purposes only). If max_overhead >= 1000000 , com‐
143 paction is never triggered. If compaction is permanently disabled, it
144 is strongly suggested to set allocation_policy to 2. Default: 500.
145 *)
146 mutable stack_limit : int ; (* The maximum size of the stack (in
148 words). This is only relevant to the byte-code runtime, as the native
149 code runtime uses the operating system's stack. Default: 1024k.
150 *)
151 mutable allocation_policy : int ; (* The policy used for allocating in
153 the major heap. Possible values are 0, 1 and 2.
154 -0 is the next-fit policy, which is usually fast but can result in
157 fragmentation, increasing memory consumption.
158 -1 is the first-fit policy, which avoids fragmentation but has corner
161 cases (in certain realistic workloads) where it is sensibly slower.
162 -2 is the best-fit policy, which is fast and avoids fragmentation. In
165 our experiments it is faster and uses less memory than both next-fit
166 and first-fit. (since OCaml 4.10)
167 The default is best-fit.
169 On one example that was known to be bad for next-fit and first-fit,
171 next-fit takes 28s using 855Mio of memory, first-fit takes 47s using
172 566Mio of memory, best-fit takes 27s using 545Mio of memory.
173 Note: If you change to next-fit, you may need to reduce the space_over‐
175 head setting, for example using 80 instead of the default 120 which is
176 tuned for best-fit. Otherwise, your program will need more memory.
177 Note: changing the allocation policy at run-time forces a heap com‐
179 paction, which is a lengthy operation unless the heap is small (e.g. at
180 the start of the program).
181 Default: 2.
183 Since 3.11.0
186 *)
187 window_size : int ; (* The size of the window used by the major GC
188 for smoothing out variations in its workload. This is an integer be‐
189 tween 1 and 50. Default: 1.
190 Since 4.03.0
193 *)
194 custom_major_ratio : int ; (* Target ratio of floating garbage to ma‐
195 jor heap size for out-of-heap memory held by custom values located in
196 the major heap. The GC speed is adjusted to try to use this much memory
197 for dead values that are not yet collected. Expressed as a percentage
198 of major heap size. The default value keeps the out-of-heap floating
199 garbage about the same size as the in-heap overhead. Note: this only
200 applies to values allocated with caml_alloc_custom_mem (e.g. bigar‐
201 rays). Default: 44.
202 Since 4.08.0
205 *)
206 custom_minor_ratio : int ; (* Bound on floating garbage for
207 out-of-heap memory held by custom values in the minor heap. A minor GC
208 is triggered when this much memory is held by custom values located in
209 the minor heap. Expressed as a percentage of minor heap size. Note:
210 this only applies to values allocated with caml_alloc_custom_mem (e.g.
211 bigarrays). Default: 100.
212 Since 4.08.0
215 *)
216 custom_minor_max_size : int ; (* Maximum amount of out-of-heap memory
217 for each custom value allocated in the minor heap. When a custom value
218 is allocated on the minor heap and holds more than this many bytes,
219 only this value is counted against custom_minor_ratio and the rest is
220 directly counted against custom_major_ratio . Note: this only applies
221 to values allocated with caml_alloc_custom_mem (e.g. bigarrays). De‐
222 fault: 8192 bytes.
223 Since 4.08.0
226 *)
227 }
228 The GC parameters are given as a control record. Note that these pa‐
231 rameters can also be initialised by setting the OCAMLRUNPARAM environ‐
232 ment variable. See the documentation of ocamlrun .
233 val stat : unit -> stat
237 Return the current values of the memory management counters in a stat
239 record. This function examines every heap block to get the statistics.
240 val quick_stat : unit -> stat
244 Same as stat except that live_words , live_blocks , free_words ,
246 free_blocks , largest_free , and fragments are set to 0. This function
247 is much faster than stat because it does not need to go through the
248 heap.
249 val counters : unit -> float * float * float
253 Return (minor_words, promoted_words, major_words) . This function is
255 as fast as quick_stat .
256 val minor_words : unit -> float
260 Number of words allocated in the minor heap since the program was
262 started. This number is accurate in byte-code programs, but only an ap‐
263 proximation in programs compiled to native code.
264 In native code this function does not allocate.
266 Since 4.04
269 val get : unit -> control
273 Return the current values of the GC parameters in a control record.
275 val set : control -> unit
279 set r changes the GC parameters according to the control record r .
282 The normal usage is: Gc.set { (Gc.get()) with Gc.verbose = 0x00d }
283 val minor : unit -> unit
288 Trigger a minor collection.
290 val major_slice : int -> int
294 major_slice n Do a minor collection and a slice of major collection. n
297 is the size of the slice: the GC will do enough work to free (on aver‐
298 age) n words of memory. If n = 0, the GC will try to do enough work to
299 ensure that the next automatic slice has no work to do. This function
300 returns an unspecified integer (currently: 0).
301 val major : unit -> unit
305 Do a minor collection and finish the current major collection cycle.
307 val full_major : unit -> unit
311 Do a minor collection, finish the current major collection cycle, and
313 perform a complete new cycle. This will collect all currently unreach‐
314 able blocks.
315 val compact : unit -> unit
319 Perform a full major collection and compact the heap. Note that heap
321 compaction is a lengthy operation.
322 val print_stat : out_channel -> unit
326 Print the current values of the memory management counters (in hu‐
328 man-readable form) into the channel argument.
329 val allocated_bytes : unit -> float
333 Return the total number of bytes allocated since the program was
335 started. It is returned as a float to avoid overflow problems with int
336 on 32-bit machines.
337 val get_minor_free : unit -> int
341 Return the current size of the free space inside the minor heap.
343 Since 4.03.0
346 val get_bucket : int -> int
350 get_bucket n returns the current size of the n -th future bucket of the
353 GC smoothing system. The unit is one millionth of a full GC.
354 Since 4.03.0
357 Raises Invalid_argument if n is negative, return 0 if n is larger than
360 the smoothing window.
361 val get_credit : unit -> int
365 get_credit () returns the current size of the "work done in advance"
368 counter of the GC smoothing system. The unit is one millionth of a full
369 GC.
370 Since 4.03.0
373 val huge_fallback_count : unit -> int
377 Return the number of times we tried to map huge pages and had to fall
379 back to small pages. This is always 0 if OCAMLRUNPARAM contains H=1 .
380 Since 4.03.0
383 val finalise : ('a -> unit) -> 'a -> unit
387 finalise f v registers f as a finalisation function for v . v must be
390 heap-allocated. f will be called with v as argument at some point be‐
391 tween the first time v becomes unreachable (including through weak
392 pointers) and the time v is collected by the GC. Several functions can
393 be registered for the same value, or even several instances of the same
394 function. Each instance will be called once (or never, if the program
395 terminates before v becomes unreachable).
396 The GC will call the finalisation functions in the order of dealloca‐
398 tion. When several values become unreachable at the same time (i.e.
399 during the same GC cycle), the finalisation functions will be called in
400 the reverse order of the corresponding calls to finalise . If finalise
401 is called in the same order as the values are allocated, that means
402 each value is finalised before the values it depends upon. Of course,
403 this becomes false if additional dependencies are introduced by assign‐
404 ments.
405 In the presence of multiple OCaml threads it should be assumed that any
407 particular finaliser may be executed in any of the threads.
408 Anything reachable from the closure of finalisation functions is con‐
410 sidered reachable, so the following code will not work as expected:
411 - let v = ... in Gc.finalise (fun _ -> ...v...) v
413 Instead you should make sure that v is not in the closure of the final‐
415 isation function by writing:
416 - let f = fun x -> ... let v = ... in Gc.finalise f v
418 The f function can use all features of OCaml, including assignments
420 that make the value reachable again. It can also loop forever (in this
421 case, the other finalisation functions will not be called during the
422 execution of f, unless it calls finalise_release ). It can call fi‐
423 nalise on v or other values to register other functions or even itself.
424 It can raise an exception; in this case the exception will interrupt
425 whatever the program was doing when the function was called.
426 finalise will raise Invalid_argument if v is not guaranteed to be
429 heap-allocated. Some examples of values that are not heap-allocated
430 are integers, constant constructors, booleans, the empty array, the
431 empty list, the unit value. The exact list of what is heap-allocated
432 or not is implementation-dependent. Some constant values can be
433 heap-allocated but never deallocated during the lifetime of the pro‐
434 gram, for example a list of integer constants; this is also implementa‐
435 tion-dependent. Note that values of types float are sometimes allo‐
436 cated and sometimes not, so finalising them is unsafe, and finalise
437 will also raise Invalid_argument for them. Values of type 'a Lazy.t
438 (for any 'a ) are like float in this respect, except that the compiler
439 sometimes optimizes them in a way that prevents finalise from detecting
440 them. In this case, it will not raise Invalid_argument , but you should
441 still avoid calling finalise on lazy values.
442 The results of calling String.make , Bytes.make , Bytes.create , Ar‐
444 ray.make , and ref are guaranteed to be heap-allocated and non-constant
445 except when the length argument is 0 .
446 val finalise_last : (unit -> unit) -> 'a -> unit
450 same as Gc.finalise except the value is not given as argument. So you
452 can't use the given value for the computation of the finalisation func‐
453 tion. The benefit is that the function is called after the value is un‐
454 reachable for the last time instead of the first time. So contrary to
455 Gc.finalise the value will never be reachable again or used again. In
456 particular every weak pointer and ephemeron that contained this value
457 as key or data is unset before running the finalisation function. More‐
458 over the finalisation functions attached with Gc.finalise are always
459 called before the finalisation functions attached with Gc.finalise_last
460 .
461 Since 4.04
464 val finalise_release : unit -> unit
468 A finalisation function may call finalise_release to tell the GC that
470 it can launch the next finalisation function without waiting for the
471 current one to return.
472 type alarm
475 An alarm is a piece of data that calls a user function at the end of
478 each major GC cycle. The following functions are provided to create
479 and delete alarms.
480 val create_alarm : (unit -> unit) -> alarm
484 create_alarm f will arrange for f to be called at the end of each major
487 GC cycle, starting with the current cycle or the next one. A value of
488 type alarm is returned that you can use to call delete_alarm .
489 val delete_alarm : alarm -> unit
493 delete_alarm a will stop the calls to the function associated to a .
496 Calling delete_alarm a again has no effect.
497 val eventlog_pause : unit -> unit
501 eventlog_pause () will pause the collection of traces in the runtime.
504 Traces are collected if the program is linked to the instrumented run‐
505 time and started with the environment variable OCAML_EVENTLOG_ENABLED.
506 Events are flushed to disk after pausing, and no new events will be
507 recorded until eventlog_resume is called.
508 val eventlog_resume : unit -> unit
512 eventlog_resume () will resume the collection of traces in the runtime.
515 Traces are collected if the program is linked to the instrumented run‐
516 time and started with the environment variable OCAML_EVENTLOG_ENABLED.
517 This call can be used after calling eventlog_pause , or if the program
518 was started with OCAML_EVENTLOG_ENABLED=p. (which pauses the collection
519 of traces before the first event.)
520 module Memprof : sig end
523 Memprof is a sampling engine for allocated memory words. Every allo‐
527 cated word has a probability of being sampled equal to a configurable
528 sampling rate. Once a block is sampled, it becomes tracked. A tracked
529 block triggers a user-defined callback as soon as it is allocated, pro‐
530 moted or deallocated.
531 Since blocks are composed of several words, a block can potentially be
533 sampled several times. If a block is sampled several times, then each
534 of the callback is called once for each event of this block: the multi‐
535 plicity is given in the n_samples field of the allocation structure.
536 This engine makes it possible to implement a low-overhead memory pro‐
538 filer as an OCaml library.
539 Note: this API is EXPERIMENTAL. It may change without prior notice.
541OCamldoc 2022-02-04 Stdlib.Gc(3)