Module Stdlib.Gc

Instructions: Use this module in your project

In the IDE (CLion, Visual Studio Code, Xcode, etc.) you use for your DkSDK project:

Add the following to your project's dependencies/CMakeLists.txt:

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DkSDKProject_DeclareAvailable(ocaml
    CONSTRAINT "= 4.14.0"
    FINDLIBS str unix runtime_events threads dynlink)
DkSDKProject_MakeAvailable(ocaml)

Add the Findlib::ocaml library to any desired targets in src/*/CMakeLists.txt:

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target_link_libraries(YourPackage_YourLibraryName
     # ... existing libraries, if any ...
     Findlib::ocaml)

Click your IDE's Build button

Not using DkSDK?

FIRST, do one or all of the following:

Run:
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```
opam install ocaml.4.14.0
```

Edit your dune-project and add:

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(package
  (name YourExistingPackage)
  (depends
  ; ... existing dependenices ...
  (ocaml (>= 4.14.0))))

Then run:

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dune build *.opam # if this fails, run: dune build

Edit your <package>.opam file and add:

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depends: [
  # ... existing dependencies ...
  "ocaml" {>= "4.14.0"}
]

Then run:

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opam install . --deps-only

FINALLY, add the library to any desired (library)and/or (executable) targets in your **/dune files:

Copy

(library
  (name YourLibrary)
  ; ... existing library options ...
  (libraries
    ; ... existing libraries ...
    ))

(executable
  (name YourExecutable)
  ; ... existing executable options ...
  (libraries
    ; ... existing libraries ...
    ))

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.
Note that "live" words refers to every word in the major heap that
isn't currently known to be collectable, which includes words that
have become unreachable by the program after the start of the
previous gc cycle. It is typically much simpler and more predictable
to call Gc.full_major (or
Gc.compact) then computing gc stats, as then
"live" words has the simple meaning of "reachable by the program".
One caveat is that a single call to
Gc.full_major will not reclaim values that have
a finaliser from Gc.finalise (this does not apply
to Gc.finalise_last). If this caveat
matters, simply call Gc.full_major twice
instead of once.
*)
   


live_blocks : int;
(*
Number of live blocks in the major heap.
See live_words for a caveat about what "live" means.
*)
   


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.

valstat : ``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.

valquick_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.

valcounters : ``unit-> float * float * float

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

valminor_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

valget : ``unit-> control

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

valset :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 }

valminor : ``unit-> unit

Trigger a minor collection.

valmajor_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).

valmajor : ``unit-> unit

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

valfull_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.

valcompact : ``unit-> unit

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

valprint_stat :out_channel -> unit

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

valallocated_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.

valget_minor_free : ``unit-> int

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

since 4.03.0

valget_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.

raises Invalid_argument

if n is negative, return 0 if n is larger than the smoothing window.
since 4.03.0

valget_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

valhuge_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 Stdlib.ref are guaranteed to be heap-allocated and non-constant except when the length argument is 0.

valfinalise_last : ``(``unit->unit)``-> 'a -> unit

same as 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 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 finalise are always called before the finalisation functions attached with finalise_last.

since 4.04

valfinalise_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.

valcreate_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.

valdelete_alarm :alarm -> unit

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

valeventlog_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.

valeventlog_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.

More from the DkSDK Book

1. DkSDK
1. 1. Package capnp
  1. 1. Module Capnp
    1. Module Capnp_unix
  1. Package cmdliner
  1. 1. Module Cmdliner
    1. Module Cmdliner_arg
    1. Module Cmdliner_base
    1. Module Cmdliner_cline
    1. Module Cmdliner_cmd
    1. Module Cmdliner_docgen
    1. Module Cmdliner_eval
    1. Module Cmdliner_info
    1. Module Cmdliner_manpage
    1. Module Cmdliner_msg
    1. Module Cmdliner_term
    1. Module Cmdliner_term_deprecated
    1. Module Cmdliner_trie
  1. Package fmt
  1. 1. Module Fmt
    1. Module Fmt_cli
    1. Module Fmt_tty
  1. Package logs
  1. 1. Module Logs
    1. Module Logs_cli
    1. Module Logs_fmt
    1. Module Logs_lwt
    1. Module Logs_threaded
  1. Package lwt
  1. 1. Module Lwt
    1. Module Lwt_bytes
    1. Module Lwt_condition
    1. Module Lwt_config
    1. Module Lwt_engine
    1. Module Lwt_features
    1. Module Lwt_fmt
    1. Module Lwt_gc
    1. Module Lwt_io
    1. Module Lwt_list
    1. Module Lwt_main
    1. Module Lwt_mutex
    1. Module Lwt_mvar
    1. Module Lwt_pool
    1. Module Lwt_pqueue
    1. Module Lwt_preemptive
    1. Module Lwt_process
    1. Module Lwt_result
    1. Module Lwt_seq
    1. Module Lwt_sequence
    1. Module Lwt_stream
    1. Module Lwt_switch
    1. Module Lwt_sys
    1. Module Lwt_throttle
    1. Module Lwt_timeout
    1. Module Lwt_unix
  1. Package mtime
  1. 1. Module Mtime
    1. Module Mtime_clock
  1. Package ocaml
  1. 1. Module Bigarray
    1. Module Condition
    1. Module Dynlink
    1. Module Event
    1. Module Mutex
    1. Module Profiling
    1. Module Semaphore
    1. Module Stdlib
    1. 1. ...
      1. Module Stdlib.Gc
      1. Module Gc.Memprof
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    1. Module Thread
    1. Module ThreadUnix
    1. Module Topdirs
    1. Module Unix
    1. Module UnixLabels
  1. Package
  1. Package result
  1. 1. Module Result
  1. Package stdint
  1. 1. Module Stdint