Batteries user guide : BatGc

# Module BatGc

    module BatGc: sig .. end

Memory management control and statistics; finalised values.

This module extends Stdlib's Gc module, go there for documentation on the rest of the functions and types.
Author(s): Damien Doligez (Base module), David Teller

    type stat = Gc.stat = {

`` | `minor_words : `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. | `*)`
`` | `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. | `*)`

}
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 = Gc.control = {

`` | `mutable minor_heap_size : `int`;` | `(*` | The size (in words) of the minor heap. Changing this parameter will trigger a minor collection. Default: 32k. | `*)`
`` | `mutable major_heap_increment : `int`;` | `(*` | The minimum number of words to add to the major heap when increasing it. Default: 124k. | `*)`
`` | `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 immediatly 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: 80. | `*)`
`` | `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 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 search at start-up. `0x200` Computation of compaction triggering condition. 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. 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: 256k. | `*)`
`` | `mutable allocation_policy : `int`;` | `(*` | The policy used for allocating in the heap. Possible values are 0 and 1. 0 is the next-fit policy, which is quite fast but can result in fragmentation. 1 is the first-fit policy, which can be slower in some cases but can be better for programs with fragmentation problems. Default: 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 at `quick_stat`.

    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

Do a minor collection and a slice of major collection. The argument is the size of the slice, 0 to use the automatically-computed slice size. In all cases, the result is the computed slice size.

    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 : 'a BatInnerIO.output -> 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 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 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.

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 x -> ...) v`

Instead you should write:

- `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 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. You should also be aware that compiler optimisations may duplicate some immutable values, for example floating-point numbers when stored into arrays, so they can be finalised and collected while another copy is still in use by the program.

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

    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 = Gc.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.