jammy (3) Stdlib.Gc.3o.gz

Provided by: ocaml-man_4.13.1-3ubuntu1_all bug

NAME

       Stdlib.Gc - no description

Module

       Module   Stdlib.Gc

Documentation

       Module Gc
        : (module Stdlib__Gc)

       type stat = {
        minor_words : float ;  (* Number of words allocated in the minor heap since the program was started.
        *)
        promoted_words  :  float  ;   (*  Number  of  words  allocated  in  the minor heap that survived a minor
       collection and were moved to the major heap since the program was started.
        *)
        major_words : float ;  (* Number of words allocated in the major heap,  including  the  promoted  words,
       since the program was started.
        *)
        minor_collections : int ;  (* Number of minor collections since the program was started.
        *)
        major_collections : int ;  (* Number of major collection cycles completed since the program was started.
        *)
        heap_words : int ;  (* Total size of the major heap, in words.
        *)
        heap_chunks : int ;  (* Number of contiguous pieces of memory that make up the major heap.
        *)
        live_words : int ;  (* Number of words of live data in the major heap, including the header words.
        *)
        live_blocks : int ;  (* Number of live blocks in the major heap.
        *)
        free_words : int ;  (* Number of words in the free list.
        *)
        free_blocks : int ;  (* Number of blocks in the free list.
        *)
        largest_free : int ;  (* Size (in words) of the largest block in the free list.
        *)
        fragments : int ;  (* Number of wasted words due to fragmentation.  These are 1-words free blocks placed
       between two live blocks.  They are not available for allocation.
        *)
        compactions : int ;  (* Number of heap compactions since the program was started.
        *)
        top_heap_words : int ;  (* Maximum size reached by the major heap, in words.
        *)
        stack_size : int ;  (* Current size of the stack, in words.

       Since 3.12.0
        *)
        forced_major_collections : int ;  (* Number of forced full major collections completed since the program
       was started.

       Since 4.12.0
        *)
        }

       The memory management counters are returned in a stat record.

       The  total  amount  of  memory  allocated by the program since it was started is (in words) minor_words +
       major_words - promoted_words .  Multiply by the word size (4 on a 32-bit machine, 8 on a 64-bit  machine)
       to get the number of bytes.

       type control = {

       mutable  minor_heap_size : int ;  (* The size (in words) of the minor heap.  Changing this parameter will
       trigger a minor collection.  Default: 256k.
        *)

       mutable major_heap_increment : int ;  (* How much to add to the major heap when increasing  it.  If  this
       number is less than or equal to 1000, it is a percentage of the current heap size (i.e. setting it to 100
       will double the heap size at each increase). If it is more than 1000, it is a fixed number of words  that
       will be added to the heap. Default: 15.
        *)

       mutable  space_overhead  :  int  ;   (*  The major GC speed is computed from this parameter.  This is the
       memory that will be "wasted" because the GC does not  immediately  collect  unreachable  blocks.   It  is
       expressed as a percentage of the memory used for live data.  The GC will work more (use more CPU time and
       collect blocks more eagerly) if space_overhead is smaller.  Default: 120.
        *)

       mutable verbose : int ;  (* This value controls the GC messages on standard error output.  It is a sum of
       some of the following flags, to print messages on the corresponding events:

       - 0x001 Start and end of major GC cycle.

       - 0x002 Minor collection and major GC slice.

       - 0x004 Growing and shrinking of the heap.

       - 0x008 Resizing of stacks and memory manager tables.

       - 0x010 Heap compaction.

       - 0x020 Change of GC parameters.

       - 0x040 Computation of major GC slice size.

       - 0x080 Calling of finalisation functions.

       - 0x100 Bytecode executable and shared library search at start-up.

       - 0x200 Computation of compaction-triggering condition.

       - 0x400 Output GC statistics at program exit.  Default: 0.

        *)

       mutable  max_overhead  :  int  ;   (*  Heap compaction is triggered when the estimated amount of "wasted"
       memory is more than max_overhead percent of the amount of live data.  If max_overhead is set to  0,  heap
       compaction  is triggered at the end of each major GC cycle (this setting is intended for testing purposes
       only).  If max_overhead >= 1000000 ,  compaction  is  never  triggered.   If  compaction  is  permanently
       disabled, it is strongly suggested to set allocation_policy to 2.  Default: 500.
        *)

       mutable  stack_limit  : int ;  (* The maximum size of the stack (in words).  This is only relevant to the
       byte-code runtime, as the native code runtime uses the operating system's stack.  Default: 1024k.
        *)

       mutable allocation_policy : int ;  (* The policy used for allocating in the major heap.  Possible  values
       are 0, 1 and 2.

       -0  is  the  next-fit  policy,  which  is usually fast but can result in fragmentation, increasing memory
       consumption.

       -1 is the first-fit policy, which avoids  fragmentation  but  has  corner  cases  (in  certain  realistic
       workloads) where it is sensibly slower.

       -2  is  the  best-fit policy, which is fast and avoids fragmentation. In our experiments it is faster and
       uses less memory than both next-fit and first-fit.  (since OCaml 4.10)

       The default is best-fit.

       On one example that was known to be bad for next-fit and first-fit, next-fit takes 28s  using  855Mio  of
       memory, first-fit takes 47s using 566Mio of memory, best-fit takes 27s using 545Mio of memory.

       Note:  If you change to next-fit, you may need to reduce the space_overhead setting, for example using 80
       instead of the default 120 which is tuned for best-fit. Otherwise, your program will need more memory.

       Note: changing the allocation policy at run-time forces a heap compaction, which is a  lengthy  operation
       unless the heap is small (e.g. at the start of the program).

       Default: 2.

       Since 3.11.0
        *)
        window_size  : int ;  (* The size of the window used by the major GC for smoothing out variations in its
       workload. This is an integer between 1 and 50.  Default: 1.

       Since 4.03.0
        *)
        custom_major_ratio : int ;  (* Target ratio of floating garbage  to  major  heap  size  for  out-of-heap
       memory  held by custom values located in the major heap. The GC speed is adjusted to try to use this much
       memory for dead values that are not yet collected. Expressed as a percentage  of  major  heap  size.  The
       default  value keeps the out-of-heap floating garbage about the same size as the in-heap overhead.  Note:
       this only applies to values allocated with caml_alloc_custom_mem (e.g. bigarrays).  Default: 44.

       Since 4.08.0
        *)
        custom_minor_ratio : int ;  (* Bound on floating garbage for out-of-heap memory held by custom values in
       the  minor  heap.  A  minor GC is triggered when this much memory is held by custom values located in the
       minor heap. Expressed as a percentage of minor heap size.  Note: this only applies  to  values  allocated
       with caml_alloc_custom_mem (e.g. bigarrays).  Default: 100.

       Since 4.08.0
        *)
        custom_minor_max_size  :  int ;  (* Maximum amount of out-of-heap memory for each custom value allocated
       in the minor heap. When a custom value is allocated on the minor heap  and  holds  more  than  this  many
       bytes,  only  this  value  is counted against custom_minor_ratio and the rest is directly counted against
       custom_major_ratio .  Note: this only  applies  to  values  allocated  with  caml_alloc_custom_mem  (e.g.
       bigarrays).  Default: 8192 bytes.

       Since 4.08.0
        *)
        }

       The  GC  parameters are given as a control record.  Note that these parameters can also be initialised by
       setting the OCAMLRUNPARAM environment variable.  See the documentation of ocamlrun .

       val stat : unit -> stat

       Return the current values of the memory management counters in a stat  record.   This  function  examines
       every heap block to get the statistics.

       val quick_stat : unit -> stat

       Same  as  stat  except  that  live_words  ,  live_blocks  , free_words , free_blocks , largest_free , and
       fragments are set to 0.  This function is much faster than stat because it does not need  to  go  through
       the heap.

       val counters : unit -> float * float * float

       Return (minor_words, promoted_words, major_words) .  This function is as fast as quick_stat .

       val minor_words : unit -> float

       Number  of  words  allocated  in the minor heap since the program was started. This number is accurate in
       byte-code programs, but only an approximation in programs compiled to native code.

       In native code this function does not allocate.

       Since 4.04

       val get : unit -> control

       Return the current values of the GC parameters in a control record.

       val set : control -> unit

       set r changes the GC parameters according to the control record r  .   The  normal  usage  is:  Gc.set  {
       (Gc.get()) with Gc.verbose = 0x00d }

       val minor : unit -> unit

       Trigger a minor collection.

       val major_slice : int -> int

       major_slice  n Do a minor collection and a slice of major collection.  n is the size of the slice: the GC
       will do enough work to free (on average) n words of memory. If n = 0, the GC will try to do  enough  work
       to  ensure that the next automatic slice has no work to do.  This function returns an unspecified integer
       (currently: 0).

       val major : unit -> unit

       Do a minor collection and finish the current major collection cycle.

       val full_major : unit -> unit

       Do a minor collection, finish the current major collection cycle, and perform a complete new cycle.  This
       will collect all currently unreachable blocks.

       val compact : unit -> unit

       Perform a full major collection and compact the heap.  Note that heap compaction is a lengthy operation.

       val print_stat : out_channel -> unit

       Print  the  current  values  of  the memory management counters (in human-readable form) into the channel
       argument.

       val allocated_bytes : unit -> float

       Return the total number of bytes allocated since the program was started.  It is returned as a  float  to
       avoid overflow problems with int on 32-bit machines.

       val get_minor_free : unit -> int

       Return the current size of the free space inside the minor heap.

       Since 4.03.0

       val get_bucket : int -> int

       get_bucket  n returns the current size of the n -th future bucket of the GC smoothing system. The unit is
       one millionth of a full GC.

       Since 4.03.0

       Raises Invalid_argument if n is negative, return 0 if n is larger than the smoothing window.

       val get_credit : unit -> int

       get_credit () returns the current size of the "work done in advance" counter of the GC smoothing  system.
       The unit is one millionth of a full GC.

       Since 4.03.0

       val huge_fallback_count : unit -> int

       Return the number of times we tried to map huge pages and had to fall back to small pages. This is always
       0 if OCAMLRUNPARAM contains H=1 .

       Since 4.03.0

       val finalise : ('a -> unit) -> 'a -> unit

       finalise f v registers f as a finalisation function for v .  v must be heap-allocated.  f will be  called
       with  v  as  argument  at some point between the first time v becomes unreachable (including through weak
       pointers) and the time v is collected by the GC. Several functions can be registered for the same  value,
       or  even  several  instances  of  the same function.  Each instance will be called once (or never, if the
       program terminates before v becomes unreachable).

       The GC will call the finalisation functions in the order of deallocation.   When  several  values  become
       unreachable  at  the same time (i.e. during the same GC cycle), the finalisation functions will be called
       in the reverse order of the corresponding calls to finalise .  If finalise is called in the same order as
       the  values  are  allocated,  that  means  each value is finalised before the values it depends upon.  Of
       course, this becomes false if additional dependencies are introduced by assignments.

       In the presence of multiple OCaml threads it should be assumed  that  any  particular  finaliser  may  be
       executed in any of the threads.

       Anything  reachable  from the closure of finalisation functions is considered reachable, so the following
       code will not work as expected:

       - let v = ... in Gc.finalise (fun _ -> ...v...) v

       Instead you should make sure that v is not in the closure of the finalisation function by writing:

       - let f = fun x -> ...  let v = ... in Gc.finalise f v

       The f function can use all features of OCaml, including assignments that make the value reachable  again.
       It  can  also  loop forever (in this case, the other finalisation functions will not be called during the
       execution of f, unless it calls finalise_release ).  It can  call  finalise  on  v  or  other  values  to
       register  other  functions  or  even  itself.  It can raise an exception; in this case the exception will
       interrupt whatever the program was doing when the function was called.

       finalise will raise Invalid_argument if v is not guaranteed  to  be  heap-allocated.   Some  examples  of
       values  that  are  not heap-allocated are integers, constant constructors, booleans, the empty array, the
       empty list, the unit value.  The exact list of what is heap-allocated or not is implementation-dependent.
       Some  constant values can be heap-allocated but never deallocated during the lifetime of the program, for
       example a list of integer constants; this is also implementation-dependent.  Note that  values  of  types
       float  are  sometimes  allocated  and sometimes not, so finalising them is unsafe, and finalise will also
       raise Invalid_argument for them. Values of type 'a Lazy.t (for any 'a ) are like float in  this  respect,
       except that the compiler sometimes optimizes them in a way that prevents finalise from detecting them. In
       this case, it will not raise Invalid_argument , but you should  still  avoid  calling  finalise  on  lazy
       values.

       The  results  of calling String.make , Bytes.make , Bytes.create , Array.make , and ref are guaranteed to
       be heap-allocated and non-constant except when the length argument is 0 .

       val finalise_last : (unit -> unit) -> 'a -> unit

       same as Gc.finalise except the value is not given as argument. So you can't use the given value  for  the
       computation  of  the finalisation function. The benefit is that the function is called after the value is
       unreachable for the last time instead of the first time. So contrary to Gc.finalise the value will  never
       be  reachable  again  or  used  again. In particular every weak pointer and ephemeron that contained this
       value as key or data is unset  before  running  the  finalisation  function.  Moreover  the  finalisation
       functions  attached  with  Gc.finalise  are always called before the finalisation functions attached with
       Gc.finalise_last .

       Since 4.04

       val finalise_release : unit -> unit

       A finalisation function may call finalise_release to tell the GC that it can launch the next finalisation
       function without waiting for the current one to return.

       type alarm

       An  alarm is a piece of data that calls a user function at the end of each major GC cycle.  The following
       functions are provided to create and delete alarms.

       val create_alarm : (unit -> unit) -> alarm

       create_alarm f will arrange for f to be called at the end of each  major  GC  cycle,  starting  with  the
       current cycle or the next one.  A value of type alarm is returned that you can use to call delete_alarm .

       val delete_alarm : alarm -> unit

       delete_alarm  a will stop the calls to the function associated to a . Calling delete_alarm a again has no
       effect.

       val eventlog_pause : unit -> unit

       eventlog_pause () will pause the collection of traces in  the  runtime.   Traces  are  collected  if  the
       program   is   linked   to   the   instrumented   runtime  and  started  with  the  environment  variable
       OCAML_EVENTLOG_ENABLED.  Events are flushed to disk after pausing, and no new  events  will  be  recorded
       until eventlog_resume is called.

       val eventlog_resume : unit -> unit

       eventlog_resume  ()  will  resume  the  collection of traces in the runtime.  Traces are collected if the
       program  is  linked  to  the  instrumented  runtime   and   started   with   the   environment   variable
       OCAML_EVENTLOG_ENABLED.   This  call  can  be  used  after calling eventlog_pause , or if the program was
       started with OCAML_EVENTLOG_ENABLED=p. (which pauses the collection of traces before the first event.)

       module Memprof : sig end

       Memprof is a sampling engine for allocated memory words. Every allocated word has a probability of  being
       sampled  equal  to  a  configurable sampling rate. Once a block is sampled, it becomes tracked. A tracked
       block triggers a user-defined callback as soon as it is allocated, promoted or deallocated.

       Since blocks are composed of several words, a block can potentially be sampled several times. If a  block
       is  sampled  several  times,  then  each of the callback is called once for each event of this block: the
       multiplicity is given in the n_samples field of the allocation structure.

       This engine makes it possible to implement a low-overhead memory profiler as an OCaml library.

       Note: this API is EXPERIMENTAL. It may change without prior notice.