Introduction

In this tutorial, we will explore how to use OCaml’s Gc module to interact with the garbage collector and how to write your own finalizers. We will also provide exercises to help you develop a better understanding.

The Gc Module

The Gc module contains useful functions for querying and controlling the garbage collector from OCaml programs.

Example: Running GC and Printing Statistics

The following program runs and then prints out GC statistics before quitting:

  let rec iterate r x_init i =
  if i = 1 then x_init
  else
    let x = iterate r x_init (i - 1) in
    r *. x *. (1.0 -. x)

let () =
  Random.self_init ();
  Graphics.open_graph " 640x480";
  for x = 0 to 640 do
    let r = 4.0 *. float_of_int x /. 640.0 in
    for i = 0 to 39 do
      let x_init = Random.float 1.0 in
      let x_final = iterate r x_init 500 in
      let y = int_of_float (x_final *. 480.) in
      Graphics.plot x y
    done
  done;
  Gc.print_stat stdout
  

When you run this program, it prints out GC statistics such as minor_words, major_words, minor_collections, and major_collections.

Enabling GC Debugging Messages

To print debugging messages when certain events occur (e.g., on every major collection), add the following code at the beginning of the program:

  Gc.set { (Gc.get ()) with Gc.verbose = 0x01 }
  

This will cause the GC to print a message at the start of every major collection.

Finalization and the Weak Module

You can write a finalizer function that gets called when an object is about to be freed by the GC.

The Weak module lets you create weak pointers, which hint to the garbage collector that the object may be collected at any time.

Example: In-Memory Object Database Cache

Let’s create an in-memory object database cache that uses finalizers and weak pointers.

Data Structure

Define the in-memory and on-disk formats:

  (* In-memory format. *)
type record = { mutable name : string; mutable address : string }

(* On-disk format. *)
let record_size = 256
let name_size = 64
let addr_size = 192
  

Low-Level Functions

Define functions to read, write, lock, and unlock records:

  let seek_record n fd = ignore (Unix.lseek fd (n * record_size) Unix.SEEK_SET)

let write_record record n fd =
  seek_record n fd;
  ignore (Unix.write fd (Bytes.of_string record.name) 0 name_size);
  ignore (Unix.write fd (Bytes.of_string record.address) 0 addr_size)

let read_record record n fd =
  seek_record n fd;
  ignore (Unix.read fd (Bytes.of_string record.name) 0 name_size);
  ignore (Unix.read fd (Bytes.of_string record.address) 0 addr_size)

let lock_record n fd =
  seek_record n fd;
  Unix.lockf fd Unix.F_LOCK record_size

let unlock_record n fd =
  seek_record n fd;
  Unix.lockf fd Unix.F_ULOCK record_size
  

Creating New Records

Define a function to create new, empty in-memory record objects:

  let new_record () =
  { name = String.make name_size ' '; address = String.make addr_size ' ' }
  

Fixed Number of Records

Set the total number of records:

  let nr_records = 10000
let diskfile = Unix.openfile "users.bin" [ Unix.O_RDWR; Unix.O_CREAT ] 0o666
  

Record Cache

Define the cache of records:

  let cache = Weak.create nr_records
  

Finalizer Function

Define the finalizer function:

  let finaliser n record =
  Printf.printf "*** objcache: finalising record %d\n%!" n;
  write_record record n diskfile;
  unlock_record n diskfile
  

Get Record Function

Define the function to get a record from the cache or disk:

  let get_record n =
  match Weak.get cache n with
  | Some record ->
      Printf.printf "*** objcache: fetching record %d from memory cache\n%!" n;
      record
  | None ->
      Printf.printf "*** objcache: loading record %d from disk\n%!" n;
      let record = new_record () in
      Gc.finalise (finaliser n) record;
      lock_record n diskfile;
      read_record record n diskfile;
      Weak.set cache n (Some record);
      record
  

Sync Records Function

Define the function to synchronize records:

  let sync_records () =
  for i = 0 to nr_records - 1 do
    Weak.set cache i None
  done;
  Gc.full_major ()
  

Test Code

Download the complete program and test code objcache.ml, and compile it with:

  $ ocamlc unix.cma objcache.ml -o objcache
  

Conclusion

In this tutorial, we learned how to use the Gc module to interact with the garbage collector and how to write finalizers using the Weak module. We also implemented an in-memory object database cache as a practical example. By understanding and utilizing these concepts, you can manage memory more effectively in OCaml programs.

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Last updated 17 Aug 2024, 12:31 +0200 . history