Working with the Garbage Collector in OCaml
Learn how to handle command line arguments in ocaml
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 .