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Landmarks: A Simple Profiling Library

Landmarks is a simple profiling library for OCaml. It provides primitives to
delimit portions of code and measure the performance of instrumented code at
runtime. The available measures are obtained by aggregating CPU cycles (using
the cpu's time stamp counter), applicative time (using Sys.time) and
allocated bytes (with Gc.allocated_bytes). The instrumentation of the code
may either done by hand, automatically or semi-automatically using a PPX

During the execution of your program, the traversal of instrumented code by
the control flow is recorded as a "callgraph" that carries the collected
measures. The results may be browsed either directly on the console, or
exported to json.

This tool is intended to be used as a way to find where the time is spent in
your programs (and not benchmark independent pieces of code like
Core_bench) while providing results
that only correspond to the instrumented portion of your OCaml code (contrary to
tools that directly work with the binary executable like
gprof or

For more information, you may browse the API.


  • With opam:

opam install landmarks


opam install landmarks-viewer

for installing the landmarks viewer.

  • Manually:

git clone
cd landmarks
dune build @install

and make uninstall to remove installed files.

Usage with dune

Simply use the library landmarks and the preprocessor landmarks.ppx to
benchmark your executables and libraries. For instance, the following dune
file builds the executable test using the landmarks library and its PPX.

 (name test)
 (libraries landmarks)
 (preprocess (pps landmarks.ppx))

You can find a sample program in the example directory.

Usage with ocamlfind

  • Compiling and linking:

  ocamlfind ocamlopt -c -package landmarks
  ocamlfind ocamlopt -o prog -package landmarks -linkpkg prog.cmx

You can replace "ocamlopt" by "ocamlc" to compile the program in

  • With the PPX extension:

  ocamlfind ocamlopt -c -package landmarks -package landmarks.ppx
  ocamlfind ocamlopt -o prog -package landmarks -linkpkg prog.cmx
  • Launching the viewer (when available):

x-www-browser "$(ocamlfind query landmarks-viewer)/landmarks_viewer.html"

You may want to replace "x-www-browser" with your system's way to
invoke your favorite web-browser from the command line. It has
to support javascript.

You can find a sample program in the example directory.

Benchmarking manually

There are three main primitives:

  val register: string -> landmark
  val enter: landmark -> unit
  val exit: landmark -> unit

The register function declares new landmarks and should be used at the
toplevel. The functions enter and exit are used to delimit the portion
of code attached to a landmark. At the end of the profiling, we retrieve for
each landmark the aggregated time information spent executing the corresponding
piece of code. During the execution, a trace of each visited landmark is also
recorded in order to build a "callgraph".

For example:

open Landmark

let loop = register "loop"
let sleep = register "sleep"
let main = register "main"

let zzz () =
  enter sleep;
    Unix.sleep 1;
  exit sleep

let () =
    start_profiling ();
    enter main;
      enter loop;
        for _ = 1 to 9 do
          zzz ()
      exit loop;
      zzz ();
    exit main;

(This file can be compiled with
ocamlfind ocamlc -o prog -package landmarks -package unix -linkpkg

The induced callgraph is:

- 100.00% : main
|   - 90.00% : loop
|   |   - 100.00% : sleep
|   - 10.00% : sleep

which can be paraphrased as:

  • 100% of time is spent inside the main landmark,

  • 90% of time spent inside the main landmark is spent in the loop landmark,

  • 10% of time spent inside the main landmark is spent in the sleep landmark,

  • 100% of the time spent in loop is spent in the sleep landmark.

The clock() function

The library provides a binding to the high-performance cycles
for x86 32 and 64
bits architectures (note that you may use the archive to
provide your own implementation). It is used to measure the time spent inside
instrumented code.

The PPX extension point

To avoid writing boilerplate code, you may use the ppx extension distributed
with this package. It allows the programmer to instrument expressions using
annotation and to automatically instrument top-level functions.


The value expr [@landmark "name"] is expanded into

  Landmark.enter __generated_landmark_1;
  let r =
    try expr with e -> Landmark.exit __generated_landmark_1; raise e
  Landmark.exit __generated_landmark_1;

and the declaration

  let __generated_landmark_1 = Landmark.register "name"

is appended at the top-level.

It should be pointed out that this transformation does not preserve
tail-recursive calls (and also prevents some polymorphism generalization).
To get around these problems, it is recommended to use the other provided
extension around let ... in and let rec ... in:

let[@landmark] f = body

which is expanded in :

let __generated_landmark_2 = Landmark.register "f"
let f = body
let f x1 ... xn =
  Landmark.enter __generated_landmark_2;
  let r =
    try f x1 ... xn with e -> Landmark.exit __generated_landmark_2; raise e
  Landmark.exit __generated_landmark_2;

when the arity n of f is obtained by counting the shallow occurrences
of fun ... -> and function ... -> in body. Please note that when using
this annotation with let-rec bindings, only entry-point calls will be recorded.
For instance, in the following piece of code

  let () =
    let[@landmark] rec even n = (n = 0) || odd (n - 1)
    and[@landmark] odd n = (n = 1) || n > 0 && even (n - 1)
    in Printf.printf "'six is even' is %b\n" (even 6)

the landmark associated with "even" will be traversed exactly once (and not three
times !) whereas the control flow will not pass through the landmark associated
with "odd".

Automatic instrumentation

The structure annotations [@@@landmark "auto"] and [@@@landmark "auto-off"]
activate or deactivate the automatic instrumentation of top-level functions
in a module. In automatic mode, all functions declarations are implicitly
annotated. Automatic instrumentation can be enabled/disabled for all files via
option auto in OCAML_LANDMARKS, as detailed below.

The OCAML_LANDMARKS environment variable

The environment variable OCAML_LANDMARKS is read at two different stages:
when the ppx rewriter is executed, and when the landmarks module is loaded by
an instrumented program.
This variable is parsed as a comma-separated list of items of the form
option=argument or option, where option is:

  • During the ppx rewriter stage (at compilation time):

    • auto (no arguments): turns on the automatic instrumentation by default
      (behaves as if each module starts with annotation [@@@landmark "auto"]).

    • threads (no arguments): tells the ppx extension to use the
      Landmark_threads module instead of the module Landmark.

  • When loading an instrumented program (at runtime):

    • format with possible arguments: textual (default) or json. It controls
      the output format of the profiling which is either a console friendly
      representation or json encoding of the callgraph.

    • threshold with a number between 0.0 and 100.0 as argument (default: 1.0). If the threshold is not zero the textual output will hide nodes in the callgraph below this threshold (in percent of time of their parent). This option is meaningless for other formats.

    • output with possible argument: stderr (default), stdout, temporary,
      <file> (where <file> is the path of a file). It tells where to output the
      results of the profiling. With temporary it will print it in a temporary
      file (the name of this file will be printed on the standard error). You may
      also use temporary:<directory> to specify the directory where the files
      are generated.

    • debug with no argument. Activates a verbose mode that outputs traces on
      stderr each time the landmarks primitives are called.

    • time with no argument. Also collect Sys.time timestamps during profiling.

    • off with no argument. Disable profiling.

    • on with no argument. Enable profiling (default; may be omitted).

    • allocation with no argument. Also collect Gc.allocated_byte data.

Browsing the JSON export using the Web Viewer

You can either compile the web viewer on your computer or
browse it online.
You need to load the JSON files using the filepicker and then you can click
around to browse the callgraph.

Instrumenting with threads

The Landmark module is not thread-safe. If you have multiple threads,
you have to make sure that at most one thread is executing instrumented
code. For that you may use the Landmark_threads module (included
in the archive) that prevents non thread-safe
functions to execute in all threads but the one which started the

Instrumenting with OCAMLPARAM

A way to blindly instrument a project is to use ocaml's OCAMLPARAM
experimental feature, by setting the environment variable OCAMLPARAM with

I=$(ocamlfind query landmarks),cma=landmarks.cma,cmxa=landmarks.cmxa,ppx=$(ocamlfind query landmarks)/ppx_landmarks,_"

Known Issue

The annotation on expressions may temper with polymorphism (this is not
the case for the let-binding annotation). For instance, the following
piece of code will fail to compile:

  let test = (fun x -> x)[@landmark "test"]
  in test "string", test 1


This 'Landmarks' package is licensed by LexiFi under the terms of the
MIT license.