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.
25 *)
26 promoted_words : float ; (* Number of words allocated in the minor
27 heap that survived a minor collection and were moved to the major heap
28 since the program was started.
29 *)
30 major_words : float ; (* Number of words allocated in the major heap,
31 including the promoted words, since the program was started.
32 *)
33 minor_collections : int ; (* Number of minor collections since the
34 program was started.
35 *)
36 major_collections : int ; (* Number of major collection cycles com‐
37 pleted since the program was started.
38 *)
39 heap_words : int ; (* Total size of the major heap, in words.
40 *)
41 heap_chunks : int ; (* Number of contiguous pieces of memory that
42 make up the major heap.
43 *)
44 live_words : int ; (* Number of words of live data in the major heap,
45 including the header words.
46 *)
47 live_blocks : int ; (* Number of live blocks in the major heap.
48 *)
49 free_words : int ; (* Number of words in the free list.
50 *)
51 free_blocks : int ; (* Number of blocks in the free list.
52 *)
53 largest_free : int ; (* Size (in words) of the largest block in the
54 free list.
55 *)
56 fragments : int ; (* Number of wasted words due to fragmentation.
57 These are 1-words free blocks placed between two live blocks. They are
58 not available for allocation.
59 *)
60 compactions : int ; (* Number of heap compactions since the program
61 was started.
62 *)
63 top_heap_words : int ; (* Maximum size reached by the major heap, in
64 words.
65 *)
66 stack_size : int ; (* Current size of the stack, in words.
67 Since 3.12.0
70 *)
71 forced_major_collections : int ; (* Number of forced full major col‐
72 lections completed since the program was started.
73 Since 4.12.0
76 *)
77 }
78 The memory management counters are returned in a stat record.
81 The total amount of memory allocated by the program since it was
83 started is (in words) minor_words + major_words - promoted_words .
84 Multiply by the word size (4 on a 32-bit machine, 8 on a 64-bit ma‐
85 chine) to get the number of bytes.
86 type control = {
89 mutable minor_heap_size : int ; (* The size (in words) of the minor
91 heap. Changing this parameter will trigger a minor collection. De‐
92 fault: 256k.
93 *)
94 mutable major_heap_increment : int ; (* How much to add to the major
96 heap when increasing it. If this number is less than or equal to 1000,
97 it is a percentage of the current heap size (i.e. setting it to 100
98 will double the heap size at each increase). If it is more than 1000,
99 it is a fixed number of words that will be added to the heap. Default:
100 15.
101 *)
102 mutable space_overhead : int ; (* The major GC speed is computed from
104 this parameter. This is the memory that will be "wasted" because the
105 GC does not immediately collect unreachable blocks. It is expressed as
106 a percentage of the memory used for live data. The GC will work more
107 (use more CPU time and collect blocks more eagerly) if space_overhead
108 is smaller. Default: 120.
109 *)
110 mutable verbose : int ; (* This value controls the GC messages on
112 standard error output. It is a sum of some of the following flags, to
113 print messages on the corresponding events:
114 - 0x001 Start and end of major GC cycle.
116 - 0x002 Minor collection and major GC slice.
118 - 0x004 Growing and shrinking of the heap.
120 - 0x008 Resizing of stacks and memory manager tables.
122 - 0x010 Heap compaction.
124 - 0x020 Change of GC parameters.
126 - 0x040 Computation of major GC slice size.
128 - 0x080 Calling of finalisation functions.
130 - 0x100 Bytecode executable and shared library search at start-up.
132 - 0x200 Computation of compaction-triggering condition.
134 - 0x400 Output GC statistics at program exit. Default: 0.
136 *)
138 mutable max_overhead : int ; (* Heap compaction is triggered when the
140 estimated amount of "wasted" memory is more than max_overhead percent
141 of the amount of live data. If max_overhead is set to 0, heap com‐
142 paction is triggered at the end of each major GC cycle (this setting is
143 intended for testing purposes only). If max_overhead >= 1000000 , com‐
144 paction is never triggered. If compaction is permanently disabled, it
145 is strongly suggested to set allocation_policy to 2. Default: 500.
146 *)
147 mutable stack_limit : int ; (* The maximum size of the stack (in
149 words). This is only relevant to the byte-code runtime, as the native
150 code runtime uses the operating system's stack. Default: 1024k.
151 *)
152 mutable allocation_policy : int ; (* The policy used for allocating in
154 the major heap. Possible values are 0, 1 and 2.
155 -0 is the next-fit policy, which is usually fast but can result in
158 fragmentation, increasing memory consumption.
159 -1 is the first-fit policy, which avoids fragmentation but has corner
162 cases (in certain realistic workloads) where it is sensibly slower.
163 -2 is the best-fit policy, which is fast and avoids fragmentation. In
166 our experiments it is faster and uses less memory than both next-fit
167 and first-fit. (since OCaml 4.10)
168 The default is best-fit.
170 On one example that was known to be bad for next-fit and first-fit,
172 next-fit takes 28s using 855Mio of memory, first-fit takes 47s using
173 566Mio of memory, best-fit takes 27s using 545Mio of memory.
174 Note: If you change to next-fit, you may need to reduce the space_over‐
176 head setting, for example using 80 instead of the default 120 which is
177 tuned for best-fit. Otherwise, your program will need more memory.
178 Note: changing the allocation policy at run-time forces a heap com‐
180 paction, which is a lengthy operation unless the heap is small (e.g. at
181 the start of the program).
182 Default: 2.
184 Since 3.11.0
187 *)
188 window_size : int ; (* The size of the window used by the major GC
189 for smoothing out variations in its workload. This is an integer be‐
190 tween 1 and 50. Default: 1.
191 Since 4.03.0
194 *)
195 custom_major_ratio : int ; (* Target ratio of floating garbage to ma‐
196 jor heap size for out-of-heap memory held by custom values located in
197 the major heap. The GC speed is adjusted to try to use this much memory
198 for dead values that are not yet collected. Expressed as a percentage
199 of major heap size. The default value keeps the out-of-heap floating
200 garbage about the same size as the in-heap overhead. Note: this only
201 applies to values allocated with caml_alloc_custom_mem (e.g. bigar‐
202 rays). Default: 44.
203 Since 4.08.0
206 *)
207 custom_minor_ratio : int ; (* Bound on floating garbage for
208 out-of-heap memory held by custom values in the minor heap. A minor GC
209 is triggered when this much memory is held by custom values located in
210 the minor heap. Expressed as a percentage of minor heap size. Note:
211 this only applies to values allocated with caml_alloc_custom_mem (e.g.
212 bigarrays). Default: 100.
213 Since 4.08.0
216 *)
217 custom_minor_max_size : int ; (* Maximum amount of out-of-heap memory
218 for each custom value allocated in the minor heap. When a custom value
219 is allocated on the minor heap and holds more than this many bytes,
220 only this value is counted against custom_minor_ratio and the rest is
221 directly counted against custom_major_ratio . Note: this only applies
222 to values allocated with caml_alloc_custom_mem (e.g. bigarrays). De‐
223 fault: 8192 bytes.
224 Since 4.08.0
227 *)
228 }
229 The GC parameters are given as a control record. Note that these pa‐
232 rameters can also be initialised by setting the OCAMLRUNPARAM environ‐
233 ment variable. See the documentation of ocamlrun .
234 val stat : unit -> stat
238 Return the current values of the memory management counters in a stat
240 record. This function examines every heap block to get the statistics.
241 val quick_stat : unit -> stat
245 Same as stat except that live_words , live_blocks , free_words ,
247 free_blocks , largest_free , and fragments are set to 0. This function
248 is much faster than stat because it does not need to go through the
249 heap.
250 val counters : unit -> float * float * float
254 Return (minor_words, promoted_words, major_words) . This function is
256 as fast as quick_stat .
257 val minor_words : unit -> float
261 Number of words allocated in the minor heap since the program was
263 started. This number is accurate in byte-code programs, but only an ap‐
264 proximation in programs compiled to native code.
265 In native code this function does not allocate.
267 Since 4.04
270 val get : unit -> control
274 Return the current values of the GC parameters in a control record.
276 val set : control -> unit
280 set r changes the GC parameters according to the control record r .
283 The normal usage is: Gc.set { (Gc.get()) with Gc.verbose = 0x00d }
284 val minor : unit -> unit
289 Trigger a minor collection.
291 val major_slice : int -> int
295 major_slice n Do a minor collection and a slice of major collection. n
298 is the size of the slice: the GC will do enough work to free (on aver‐
299 age) n words of memory. If n = 0, the GC will try to do enough work to
300 ensure that the next automatic slice has no work to do. This function
301 returns an unspecified integer (currently: 0).
302 val major : unit -> unit
306 Do a minor collection and finish the current major collection cycle.
308 val full_major : unit -> unit
312 Do a minor collection, finish the current major collection cycle, and
314 perform a complete new cycle. This will collect all currently unreach‐
315 able blocks.
316 val compact : unit -> unit
320 Perform a full major collection and compact the heap. Note that heap
322 compaction is a lengthy operation.
323 val print_stat : out_channel -> unit
327 Print the current values of the memory management counters (in hu‐
329 man-readable form) into the channel argument.
330 val allocated_bytes : unit -> float
334 Return the total number of bytes allocated since the program was
336 started. It is returned as a float to avoid overflow problems with int
337 on 32-bit machines.
338 val get_minor_free : unit -> int
342 Return the current size of the free space inside the minor heap.
344 Since 4.03.0
347 val get_bucket : int -> int
351 get_bucket n returns the current size of the n -th future bucket of the
354 GC smoothing system. The unit is one millionth of a full GC.
355 Since 4.03.0
358 Raises Invalid_argument if n is negative, return 0 if n is larger than
361 the smoothing window.
362 val get_credit : unit -> int
366 get_credit () returns the current size of the "work done in advance"
369 counter of the GC smoothing system. The unit is one millionth of a full
370 GC.
371 Since 4.03.0
374 val huge_fallback_count : unit -> int
378 Return the number of times we tried to map huge pages and had to fall
380 back to small pages. This is always 0 if OCAMLRUNPARAM contains H=1 .
381 Since 4.03.0
384 val finalise : ('a -> unit) -> 'a -> unit
388 finalise f v registers f as a finalisation function for v . v must be
391 heap-allocated. f will be called with v as argument at some point be‐
392 tween the first time v becomes unreachable (including through weak
393 pointers) and the time v is collected by the GC. Several functions can
394 be registered for the same value, or even several instances of the same
395 function. Each instance will be called once (or never, if the program
396 terminates before v becomes unreachable).
397 The GC will call the finalisation functions in the order of dealloca‐
399 tion. When several values become unreachable at the same time (i.e.
400 during the same GC cycle), the finalisation functions will be called in
401 the reverse order of the corresponding calls to finalise . If finalise
402 is called in the same order as the values are allocated, that means
403 each value is finalised before the values it depends upon. Of course,
404 this becomes false if additional dependencies are introduced by assign‐
405 ments.
406 In the presence of multiple OCaml threads it should be assumed that any
408 particular finaliser may be executed in any of the threads.
409 Anything reachable from the closure of finalisation functions is con‐
411 sidered reachable, so the following code will not work as expected:
412 - let v = ... in Gc.finalise (fun _ -> ...v...) v
414 Instead you should make sure that v is not in the closure of the final‐
416 isation function by writing:
417 - let f = fun x -> ... let v = ... in Gc.finalise f v
419 The f function can use all features of OCaml, including assignments
421 that make the value reachable again. It can also loop forever (in this
422 case, the other finalisation functions will not be called during the
423 execution of f, unless it calls finalise_release ). It can call fi‐
424 nalise on v or other values to register other functions or even itself.
425 It can raise an exception; in this case the exception will interrupt
426 whatever the program was doing when the function was called.
427 finalise will raise Invalid_argument if v is not guaranteed to be
430 heap-allocated. Some examples of values that are not heap-allocated
431 are integers, constant constructors, booleans, the empty array, the
432 empty list, the unit value. The exact list of what is heap-allocated
433 or not is implementation-dependent. Some constant values can be
434 heap-allocated but never deallocated during the lifetime of the pro‐
435 gram, for example a list of integer constants; this is also implementa‐
436 tion-dependent. Note that values of types float are sometimes allo‐
437 cated and sometimes not, so finalising them is unsafe, and finalise
438 will also raise Invalid_argument for them. Values of type 'a Lazy.t
439 (for any 'a ) are like float in this respect, except that the compiler
440 sometimes optimizes them in a way that prevents finalise from detecting
441 them. In this case, it will not raise Invalid_argument , but you should
442 still avoid calling finalise on lazy values.
443 The results of calling String.make , Bytes.make , Bytes.create , Ar‐
445 ray.make , and ref are guaranteed to be heap-allocated and non-constant
446 except when the length argument is 0 .
447 val finalise_last : (unit -> unit) -> 'a -> unit
451 same as Gc.finalise except the value is not given as argument. So you
453 can't use the given value for the computation of the finalisation func‐
454 tion. The benefit is that the function is called after the value is un‐
455 reachable for the last time instead of the first time. So contrary to
456 Gc.finalise the value will never be reachable again or used again. In
457 particular every weak pointer and ephemeron that contained this value
458 as key or data is unset before running the finalisation function. More‐
459 over the finalisation functions attached with Gc.finalise are always
460 called before the finalisation functions attached with Gc.finalise_last
461 .
462 Since 4.04
465 val finalise_release : unit -> unit
469 A finalisation function may call finalise_release to tell the GC that
471 it can launch the next finalisation function without waiting for the
472 current one to return.
473 type alarm
476 An alarm is a piece of data that calls a user function at the end of
479 each major GC cycle. The following functions are provided to create
480 and delete alarms.
481 val create_alarm : (unit -> unit) -> alarm
485 create_alarm f will arrange for f to be called at the end of each major
488 GC cycle, starting with the current cycle or the next one. A value of
489 type alarm is returned that you can use to call delete_alarm .
490 val delete_alarm : alarm -> unit
494 delete_alarm a will stop the calls to the function associated to a .
497 Calling delete_alarm a again has no effect.
498 val eventlog_pause : unit -> unit
502 eventlog_pause () will pause the collection of traces in the runtime.
505 Traces are collected if the program is linked to the instrumented run‐
506 time and started with the environment variable OCAML_EVENTLOG_ENABLED.
507 Events are flushed to disk after pausing, and no new events will be
508 recorded until eventlog_resume is called.
509 val eventlog_resume : unit -> unit
513 eventlog_resume () will resume the collection of traces in the runtime.
516 Traces are collected if the program is linked to the instrumented run‐
517 time and started with the environment variable OCAML_EVENTLOG_ENABLED.
518 This call can be used after calling eventlog_pause , or if the program
519 was started with OCAML_EVENTLOG_ENABLED=p. (which pauses the collection
520 of traces before the first event.)
521 module Memprof : sig end
524 Memprof is a sampling engine for allocated memory words. Every allo‐
528 cated word has a probability of being sampled equal to a configurable
529 sampling rate. Once a block is sampled, it becomes tracked. A tracked
530 block triggers a user-defined callback as soon as it is allocated, pro‐
531 moted or deallocated.
532 Since blocks are composed of several words, a block can potentially be
534 sampled several times. If a block is sampled several times, then each
535 of the callback is called once for each event of this block: the multi‐
536 plicity is given in the n_samples field of the allocation structure.
537 This engine makes it possible to implement a low-overhead memory pro‐
539 filer as an OCaml library.
540 Note: this API is EXPERIMENTAL. It may change without prior notice.
542OCamldoc 2022-02-04 Gc(3)