Profiling

Speed

Why is OCaml fast? Indeed, step back and ask is OCaml fast? How can we make programs faster? In this chapter we'll look at what actually happens when you compile your OCaml programs down to machine code. This will help in understanding why OCaml is not just a great language for programming, but is also a very fast language indeed. And it'll help you to help the compiler write better machine code for you. It's also (I believe anyway) a good thing for programmers to have some idea of what happens between you typing ocamlopt and getting a binary you can run.

But you will need to know some assembler to get the most out of this section. Don't be afraid! I'll help you out by translating the assembler into a C-like pseudocode (after all C is just a portable assembly language).

Basics of Assembly Language

The examples I give in this chapter are all compiled on an x86 Linux machine. The x86 is, of course, a 32 bit machine, so an x86 "word" is 4 bytes (= 32 bits) long. At this level OCaml deals mostly with word-sized objects, so you'll need to remember to multiply by four to get the size in bytes.

To refresh your memory, the x86 has only a small number of general purpose registers, each of which can store one word. The Linux assembler puts % in front of register names. The registers are: %eax, %ebx, %ecx, %edx, %esi, %edi, %ebp (special register used for stack frames), and %esp (the stack pointer).

The Linux assembler (in common with other Unix assemblers, but opposite to MS-derived assemblers) writes moves to and from registers/memory as:

movl from, to

So movl %ebx, %eax means "copy the contents of register %ebx into register %eax" (not the other way round).

Almost all of the assembly language that we will look at is going to be dominated not by machine code instructions like movl but by what are known as assembler directives. These directives begin with a . (period) and they literally direct the assembler to do something. Here are the common ones for the Linux assembler:

.text

Text is the Unix way of saying "program code". The text segment simply means the part of the executable where program code is stored. The .text directive switches the assembler so it starts writing into the text segment.

.data

Similarly, the .data directive switches the assembler so it starts writing into the data segment (part) of the executable.

  .globl foo
foo:

This declares a global symbol called foo. It means the address of the next thing to come can be named foo. Writing just foo: without the preceding .globl directive declares a local symbol (local to just the current file).

.long 12345
.byte 9
.ascii "hello"
.space 4

.long writes a word (4 bytes) to the current segment. .byte writes a single byte. .ascii writes a string of bytes (NOT nul-terminated). .space writes the given number of zero bytes. Normally you use these in the data segment.

The “hello, world” Program

Enough assembler. Put the following program into a file called smallest.ml:

print_string "hello, world\n"

And compile it to a native code executable using:

ocamlopt -S smallest.ml -o smallest

The -S (capital S) tells the compiler to leave the assembly language file (called smallest.s - lowercase s) instead of deleting it.

Here are the edited highlights of the smallest.s file with my comments added. First of all the data segment:

    .data
    .long   4348                     ; header for string
    .globl  Smallest__1
lest__1:
    .ascii  "hello, world\12"        ; string
    .space  2                        ; padding ..
    .byte   2                        ;  .. after string

Next up the text (program code) segment:

    .text
    .globl  Smallest__entry          ; entry point to the program
lest__entry:

    ; this is equivalent to the C pseudo-code:
    ; Pervasives.output_string (stdout, &Smallest__1)

    movl    $Smallest__1, %ebx
    movl    Pervasives + 92, %eax    ; Pervasives + 92 == stdout
    call    Pervasives__output_string_212

    ; return 1

    movl    $1, %eax
    ret

In C everything has to be inside a function. Think about how you can't just write printf ("hello, world\n"); in C, but instead you have to put it inside main () { ... }. In OCaml you are allowed to have commands at the top level, not inside a function. But when we translate this into assembly language, where do we put those commands? There needs to be some way to call those commands from outside, so they need to be labelled in some way. As you can see from the code snippet, OCaml solves this by taking the filename (smallest.ml), capitalising it and adding __entry, thus making up a symbol called Smallest__entry to refer to the top level commands in this file.

Now look at the code that OCaml has generated. The original code said print_string "hello, world\n", but OCaml has instead compiled the equivalent of Pervasives.output_string stdout "hello, world\n". Why? If you look into pervasives.ml you'll see why:

let print_string s = output_string stdout s

OCaml has inlined this function. Inlining - taking a function and expanding it from its definition - is sometimes a performance win, because it avoids the overhead of an extra function call, and it can lead to more opportunities for the optimiser to do its thing. Sometimes inlining is not good, because it can lead to code bloating, and thus destroys the good work done by the processor cache (and besides function calls are actually not very expensive at all on modern processors). OCaml will inline simple calls like this, because they are essentially risk free, almost always leading to a small performance gain.

What else can we notice about this? The calling convention seems to be that the first two arguments are passed in the %eax and %ebx registers respectively. Other arguments are probably passed on the stack, but we'll see about that later.

C programs have a simple convention for storing strings, known as ASCIIZ. This just means that the string is stored in ASCII, followed by a trailing NUL (\0) character. OCaml stores strings in a different way, as we can see from the data segment above. This string is stored like this:

4 byte header: 4348
the string:    h e l l o , SP w o r l d \n
padding:       \0 \0 \002

Firstly the padding brings the total length of the string up to a whole number of words (4 words, 16 bytes in this example). The padding is carefully designed so that you can work out the actual length of the string in bytes, provided that you know the total number of words allocated to the string. The encoding for this is unambiguous (which you can prove to yourself).

One nice feature of having strings with an explicit length is that you can represent strings containing ASCII NUL (\0) characters in them, something which is difficult to do in C. However, the flip side is that you need to be aware of this if you pass an OCaml string to some C native code: if it contains ASCII NUL, then the C str* functions will fail on it.

Secondly we have the header. Every boxed (allocated) object in OCaml has a header which tells the garbage collector about how large the object is in words, and something about what the object contains. Writing the number 4348 in binary:

length of the object in words:  0000 0000 0000 0000 0001 00 (4 words)
color (used by GC):             00
tag:                            1111 1100 (String_tag)

See /usr/include/caml/mlvalues.h for more information about the format of heap allocated objects in OCaml.

One unusual thing is that the code passes a pointer to the start of the string (ie. the word immediately after the header) to Pervasives.output_string. This means that output_string must subtract 4 from the pointer to get at the header to determine the length of the string.

What have I missed out from this simple example? Well, the text segment above is not the whole story. It would be really nice if OCaml translated that simple hello world program into just the five lines of assembler shown above. But there is the question of what actually calls Smallest__entry in the real program. For this OCaml includes a whole load of bootstrapping code which does things like starting up the garbage collector, allocating and initialising memory, calling initialisers in libraries and so on. OCaml links all of this code statically to the final executable, which is why the program I end up with (on Linux) weighs in at a portly 95,442 bytes. Nevertheless the start-up time for the program is still unmeasurably small (under a millisecond), compared to several seconds for starting up a reasonable Java program and a second or so for a Perl script.

Tail Recursion

We mentioned in chapter 6 that OCaml can turn tail-recursive function calls into simple loops. Is this actually true? Let's look at what simple tail recursion compiles to:

let rec loop () =
  print_string "I go on forever ...";
  loop ()

let () = loop ()

The file is called tail.ml, so following OCaml's usual procedure for naming functions, our function will be called Tail__loop_nnn (where nnn is some unique number which OCaml appends to distinguish identically named functions from one another).

Here is the assembler for just the loop function defined above:

        .text
        .globl  Tail__loop_56
Tail__loop_56:
.L100:
        ; Print the string
        movl    $Tail__2, %ebx
        movl    Pervasives + 92, %eax
        call    Pervasives__output_string_212
.L101:
        ; The following movl is in fact obsolete:
        movl    $1, %eax
        ; Jump back to the .L100 label above (ie. loop forever)
        jmp     .L100

So that's pretty conclusive. Calling Tail__loop_56 will first print the string, and then jump back to the top, then print the string, and jump back, and so on forever. It's a simple loop, not a recursive function call, so it doesn't use any stack space.

Digression: Where Are the Types?

OCaml is statically typed as we've said before on many occasions, so at compile time, OCaml knows that the type of loop is unit -> unit. It knows that the type of "hello, world\n" is string. It doesn't make any attempt to communicate this fact to the output_string function. output_string is expecting a channel and a string as arguments, and indeed that's what it gets. What would happen if we passed, say, an int instead of a string?

This is essentially an impossible condition. Because OCaml knows the types at compile time, it doesn't need to deal with types or check types at run time. There is no way, in pure OCaml, to "trick" the compiler into generating a call to Pervasives.output_string stdout 1. Such an error would be flagged at compile time, by type inference, and so could never be compiled.

The upshot is that by the time we have compiled OCaml code to assembler type information mostly isn't required, certainly in the cases we've looked at above where the type is fully known at compile time, and there is no polymorphism going on.

Fully knowing all your types at compile time is a major performance win because it totally avoids any dynamic type checking at run time. Compare this to a Java method invocation for example: obj.method (). This is an expensive operation because you need to find the concrete class that obj is an instance of, and then look up the method, and you need to do all of this potentially every time you call any method. Casting objects is another case where you need to do a considerable amount of work at run time in Java. None of this is allowed with OCaml's static types.

Polymorphic Types

As you might have guessed from the discussion above, polymorphism, which is where the compiler doesn't have a fully known type for a function at compile time, might have an impact on performance. Suppose we require a function to work out the maximum of two integers. Our first attempt is:

# let max a b =
  if a > b then a else b;;
val max : 'a -> 'a -> 'a = <fun>

Simple enough, but recall that the > (greater than) operator in OCaml is polymorphic. It has type 'a -> 'a -> bool, and this means that the max function we defined above is going to be polymorphic:

# let max a b =
  if a > b then a else b;;
val max : 'a -> 'a -> 'a = <fun>

This is indeed reflected in the code that OCaml generates for this function, which is pretty complex:

        .text
        .globl  Max__max_56
Max__max_56:

        ; Reserve two words of stack space.

        subl    $8, %esp

        ; Save the first and second arguments (a and b) on the stack.

        movl    %eax, 4(%esp)
        movl    %ebx, 0(%esp)

        ; Call the C "greaterthan" function (in the OCaml library).

        pushl   %ebx
        pushl   %eax
        movl    $greaterthan, %eax
        call    caml_c_call
.L102:
        addl    $8, %esp

        ; If the C "greaterthan" function returned 1, jump to .L100

        cmpl    $1, %eax
        je      .L100

        ; Returned 0, so get argument a which we previously saved on
        ; the stack and return it.

        movl    4(%esp), %eax
        addl    $8, %esp
        ret

        ; Returned 1, so get argument b which we previously saved on
        ; the stack and return it.

.L100:
        movl    0(%esp), %eax
        addl    $8, %esp
        ret

Basically the > operation is done by calling a C function from the OCaml library. This is obviously not going to be very efficient, nothing like as efficient as if we could generate some quick inline assembly language for doing the >.

This is not a complete dead loss by any means. All we need to do is to hint to the OCaml compiler that the arguments are in fact integers. Then OCaml will generate a specialised version of max which only works on int arguments:

# let max (a : int) (b : int) =
  if a > b then a else b;;
val max : int -> int -> int = <fun>

Here is the assembly code generated for this function:

        .text
        .globl  Max_int__max_56
Max_int__max_56:

        ; Single assembly instruction "cmpl" for performing the > op.
        cmpl    %ebx, %eax

        ; If %ebx > %eax, jump to .L100
        jle     .L100
        ; Just return argument a.
        ret
        ; Return argument b.

.L100:
        movl    %ebx, %eax
        ret

That's just 5 lines of assembler, and is about as simple as you can make it.

What about this code:

# let max a b =
  if a > b then a else b;;
val max : 'a -> 'a -> 'a = <fun>
# let () = print_int (max 2 3);;
3

Is OCaml going to be smart enough to inline the max function and specialise it to work on integers? Disappointingly the answer is no. OCaml still has to generate the external Max.max symbol (because this is a module, and so that function might be called from outside the module), and it doesn't inline the function.

Here's another variation:

# let max a b =
  if a > b then a else b in
  print_int (max 2 3);;
3
- : unit = ()

Disappointingly although the definition of max in this code is local (it can't be called from outside the module), OCaml still doesn't specialise the function.

Lesson: if you have a function which is unintentionally polymorphic then you can help the compiler by specifying types for one or more of the arguments.

The Representation of Integers, Tag Bits, Heap-Allocated Values

There are a number of peculiarities about integers in OCaml. One of these is that integers are 31 bit entities, not 32 bit entities. What happens to the "missing" bit?

Write this to int.ml:

print_int 3

and compile with ocamlopt -S int.ml -o int to generate assembly language in int.s. Recall from the discussion above that we are expecting OCaml to inline the print_int function as output_string (string_of_int 3), and examining the assembly language output we can see that this is exactly what OCaml does:

        .text
        .globl  Int__entry
Int__entry:

        ; Call Pervasives.string_of_int (3)

        movl    $7, %eax
        call    Pervasives__string_of_int_152

        ; Call Pervasives.output_string (stdout, result_of_previous)

        movl    %eax, %ebx
        movl    Pervasives + 92, %eax
        call    Pervasives__output_string_212

The important code is movl $7, %eax. It shows two things: Firstly the integer is unboxed (not allocated on the heap), but is instead passed directly to the function in the register %eax. This is fast. But secondly we see that the number being passed is 7, not 3.

This is a consequence of the representation of integers in OCaml. The bottom bit of the integer is used as a tag - we'll see what for next. The top 31 bits are the actual integer. The binary representation of 7 is 111, so the bottom tag bit is 1 and the top 31 bits form the number 11 in binary = 3. To get from the OCaml representation to the integer, divide by two and round down.

Why the tag bit at all? This bit is used to distinguish between integers and pointers to structures on the heap, and the distinction is only necessary if we are calling a polymorphic function. In the case above, where we are calling string_of_int, the argument can only ever be an int and so the tag bit would never be consulted. Nevertheless, to avoid having two internal representations for integers, all integers in OCaml carry around the tag bit.

A bit of background about pointers is required to understand why the tag bit is really necessary, and why it is where it is.

In the world of RISC chips like the Sparc, MIPS and Alpha, pointers have to be word-aligned. So on the older 32 bit Sparc, for example, it's not possible to create and use a pointer which isn't aligned to a multiple of 4 (bytes). Trying to use one generates a processor exception, which means basically your program segfaults. The reason for this is just to simplify memory access. It's just a lot simpler to design the memory subsystem of a CPU if you only need to worry about word-aligned access.

For historical reasons (because the x86 is derived from an 8 bit chip), the x86 has supported unaligned memory access, although if you align all memory accesses to multiples of 4, then things go faster.

Nevertheless, all pointers in OCaml are aligned - ie. multiples of 4 for 32 bit processors, and multiples of 8 for 64 bit processors. This means that the bottom bit of any pointer in OCaml will always be zero.

So you can see that by looking at the bottom bit of a register, you can immediately tell if it stores a pointer ("tag" bit is zero), or an integer (tag bit set to one).

Remember our polymorphic > function which caused us so much trouble in the previous section? We looked at the assembler and found out that OCaml compiles a call to a C function called greaterthan whenever it sees the polymorphic form of >. This function takes two arguments, in registers %eax and %ebx. But greaterthan can be called with integers, floats, strings, opaque objects ... How does it know what %eax and %ebx point to?

It uses the following decision tree:

• Tag bit is one: compare the two integers and return.
• Tag bit is zero: %eax and %ebx must point at two heap-allocated memory blocks. Look at the header word of the memory blocks, specifically the bottom 8 bits of the header word, which tag the content of the block.
  • String_tag: Compare two strings.
  • Double_tag: Compare two floats.
  • etc.

Note that because > has type 'a -> 'a -> bool, both arguments must have the same type. The compiler should enforce this at compile time. I would assume that greaterthan probably contains code to sanity-check this at run time however.

Floats

Floats are, by default, boxed (allocated on the heap). Save this as float.ml and compile it with ocamlopt -S float.ml -o float:

print_float 3.0

The number is not passed directly to string_of_float in the %eax register as happened above with ints. Instead, it is created statically in the data segment:

        .data
        .long   2301
        .globl  Float__1
Float__1:
        .double 3.0

and a pointer to the float is passed in %eax instead:

        movl    $Float__1, %eax
        call    Pervasives__string_of_float_157

Note the structure of the floating point number: it has a header (2301), followed by the 8 byte (2 word) representation of the number itself. The header can be decoded by writing it as binary:

Length of the object in words:  0000 0000 0000 0000 0000 10 (8 bytes)
Color:                          00
Tag:                            1111 1101 (Double_tag)

string_of_float isn't polymorphic, but suppose we have a polymorphic function foo : 'a -> unit taking one polymorphic argument. If we call foo with %eax containing 7, then this is equivalent to foo 3, whereas if we call foo with %eax containing a pointer to Float__1 above, then this is equivalent to foo 3.0.

Arrays

I mentioned earlier that one of OCaml's targets was numerical computing. Numerical computing does a lot of work on vectors and matrices, which are essentially arrays of floats. As a special hack to make this go faster, OCaml implements arrays of unboxed floats. This means that in the special case where we have an object of type float array (array of floats), OCaml stores them the same way as in C:

double array[10];

... instead of having an array of pointers to ten separately allocated floats on the heap.

Let's see this in practice:

let a = Array.create 10 0.0;;
for i = 0 to 9 do
  a.(i) <- float_of_int i
done

I'm going to compile this code with the -unsafe option to remove bounds checking (simplifying the code for our exposition here). The first line, which creates the array, is compiled to a simple C call:

        pushl   $Arrayfloats__1     ; Boxed float 0.0
        pushl   $21                 ; The integer 10
        movl    $make_vect, %eax    ; Address of the C function to call
        call    caml_c_call
    ; ...
        movl    %eax, Arrayfloats   ; Store the resulting pointer to the
                                    ; array at this place on the heap.

The loop is compiled to this relatively simple assembly language:

        movl    $1, %eax            ; Let %eax = 0. %eax is going to store i.
        cmpl    $19, %eax           ; If %eax > 9, then jump out of the
        jg      .L100               ;   loop (to label .L100 at the end).

.L101:                              ; This is the start of the loop body.
        movl    Arrayfloats, %ecx   ; Address of the array to %ecx.

        movl    %eax, %ebx          ; Copy i to %ebx.
        sarl    $1, %ebx            ; Remove the tag bit from %ebx by
                                    ;   shifting it right 1 place. So %ebx
                                    ;   now contains the real integer i.

        pushl   %ebx                ; Convert %ebx to a float.
        fildl   (%esp)
        addl    $4, %esp

        fstpl   -4(%ecx, %eax, 4)   ; Store the float in the array at the ith
                                ; position.

        addl    $2, %eax            ; i := i + 1
        cmpl    $19, %eax           ; If i <= 9, loop around again.
        jle     .L101
.L100:

The important statement is the one which stores the float into the array. It is:

        fstpl   -4(%ecx, %eax, 4)

The assembler syntax is rather complex, but the bracketed expression -4(%ecx, %eax, 4) means "at the address %ecx + 4*%eax - 4". Recall that %eax is the OCaml representation of i, complete with tag bit, so it is essentially equal to i*2+1, so let's substitute that and multiply it out:

  %ecx + 4*%eax - 4
= %ecx + 4*(i*2+1) - 4
= %ecx + 4*i*2 + 4 - 4
= %ecx + 8*i

(Each float in the array is 8 bytes long.)

So arrays of floats are unboxed, as expected.

Partially Applied Functions and Closures

How does OCaml compile functions which are only partially applied? Let's compile this code:

Array.map ((+) 2) [|1; 2; 3; 4; 5|]

If you recall the syntax, [| ... |] declares an array (in this case an int array), and ((+) 2) is a closure - "the function which adds 2 to things".

Compiling this code reveals some interesting new features. Firstly the code which allocates the array:

        movl    $24, %eax           ; Allocate 5 * 4 + 4 = 24 bytes of memory.
        call    caml_alloc

        leal    4(%eax), %eax       ; Let %eax point to 4 bytes into the
                                    ;   allocated memory.

All heap allocations have the same format: 4 byte header + data. In this case the data is 5 integers, so we allocate 4 bytes for the header plus 5 * 4 bytes for the data. We update the pointer to point at the first data word, ie. 4 bytes into the allocated memory block.

Next OCaml generates code to initialise the array:

        movl    $5120, -4(%eax)
        movl    $3, (%eax)
        movl    $5, 4(%eax)
        movl    $7, 8(%eax)
        movl    $9, 12(%eax)
        movl    $11, 16(%eax)

The header word is 5120, which if you write it in binary means a block containing 5 words, with tag zero. The tag of zero means it's a "structured block" a.k.a. an array. We also copy the numbers 1, 2, 3, 4 and 5 to the appropriate places in the array. Notice the OCaml representation of integers is used. Because this is a structured block, the garbage collector will scan each word in this block, and the GC needs to be able to distinguish between integers and pointers to other heap-allocated blocks (the GC does not have access to type information about this array).

Next the closure ((+) 2) is created. The closure is represented by this block allocated in the data segment:

        .data
        .long   3319
        .globl  Closure__1
Closure__1:
        .long   caml_curry2
        .long   5
        .long   Closure__fun_58

The header is 3319, indicating a Closure_tag with length 3 words. The 3 words in the block are the address of the function caml_curry2, the integer number 2 and the address of this function:

        .text
        .globl  Closure__fun_58
Closure__fun_58:

        ; The function takes two arguments, %eax and %ebx.
        ; This line causes the function to return %eax + %ebx - 1.

        lea     -1(%eax, %ebx), %eax
        ret

What does this function do? On the face of it, it adds the two arguments, and subtracts one. But remember that %eax and %ebx are in the OCaml representation for integers. Let's represent them as:

• %eax = 2 * a + 1
• %ebx = 2 * b + 1

where a and b are the actual integer arguments. So this function returns:

%eax + %ebx - 1
= 2 * a + 1 + 2 * b + 1 - 1
= 2 * a + 2 * b + 1
= 2 * (a + b) + 1

In other words, this function returns the OCaml integer representation of the sum a + b. This function is (+)!

(It's actually more subtle than this - to perform the mathematics quickly, OCaml uses the x86 addressing hardware in a way that probably wasn't intended by the designers of the x86.)

So back to our closure - we won't go into the details of the caml_curry2 function, but just say that this closure is the argument 2 applied to the function (+), waiting for a second argument. Just as expected.

The actual call to the Array.map function is quite difficult to understand, but the main points for our examination of OCaml is that the code:

• Does call Array.map with an explicit closure.
• Does not attempt to inline the call and turn it into a loop.

Calling Array.map in this way is undoubtedly slower than writing a loop over the array by hand. The overhead is mainly in the fact that the closure must be evaluated for each element of the array, and that isn't as fast as inlining the closure as a function (if this optimisation were even possible). However, if you had a more substantial closure than just ((+) 2), the overhead would be reduced. The FP version also saves expensive programmer time versus writing the imperative loop.

Profiling Tools

There are two types of profiling that you can do on OCaml programs:

1. Get execution counts for bytecode.
2. Get real profiling for native code.

The ocamlcp and ocamlprof programs perform profiling on bytecode. Here is an example:

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

And can be run and compiled with

$ ocamlcp -p a graphics.cma graphtest.ml -o graphtest
$ ./graphtest
$ ocamlprof graphtest.ml

The comments (* nnn *) are added by ocamlprof, showing how many times each part of the code was called.

Profiling native code is done using your operating system's native support for profiling. In the case of Linux, we use gprof. An alternative is perf, as explained below.

We compile it using the -p option to ocamlopt which tells the compiler to include profiling information for gprof:

After running the program as normal, the profiling code dumps out a file gmon.out which we can interpret with gprof:

$ gprof ./a.out
Flat profile:

Each sample counts as 0.01 seconds.
  %   cumulative   self              self     total
 time   seconds   seconds    calls   s/call   s/call  name
 10.86      0.57     0.57     2109     0.00     0.00  sweep_slice
  9.71      1.08     0.51     1113     0.00     0.00  mark_slice
  7.24      1.46     0.38  4569034     0.00     0.00  Sieve__code_begin
  6.86      1.82     0.36  9171515     0.00     0.00  Stream__set_data_140
  6.57      2.17     0.34 12741964     0.00     0.00  fl_merge_block
  6.29      2.50     0.33  4575034     0.00     0.00  Stream__peek_154
  5.81      2.80     0.30 12561656     0.00     0.00  alloc_shr
  5.71      3.10     0.30     3222     0.00     0.00  oldify_mopup
  4.57      3.34     0.24 12561656     0.00     0.00  allocate_block
  4.57      3.58     0.24  9171515     0.00     0.00  modify
  4.38      3.81     0.23  8387342     0.00     0.00  oldify_one
  3.81      4.01     0.20 12561658     0.00     0.00  fl_allocate
  3.81      4.21     0.20  4569034     0.00     0.00  Sieve__filter_56
  3.62      4.40     0.19     6444     0.00     0.00  empty_minor_heap
  3.24      4.57     0.17     3222     0.00     0.00  oldify_local_roots
  2.29      4.69     0.12  4599482     0.00     0.00  Stream__slazy_221
  2.10      4.80     0.11  4597215     0.00     0.00  darken
  1.90      4.90     0.10  4596481     0.00     0.00  Stream__fun_345
  1.52      4.98     0.08  4575034     0.00     0.00  Stream__icons_207
  1.52      5.06     0.08  4575034     0.00     0.00  Stream__junk_165
  1.14      5.12     0.06     1112     0.00     0.00  do_local_roots

[ etc. ]

Using perf on Linux

Assuming perf is installed and your program is compiled into native code with -g (or ocamlbuild tag debug), you just need to type

perf record --call-graph=dwarf -- ./foo.native a b c d
perf report

The first command launches foo.native with arguments a b c d and records profiling information in perf.data; the second command starts an interactive program to explore the call graph. The option --call-graph=dwarf makes perf aware of the calling convention of OCaml (with old versions of perf, enabling frame pointers in OCaml might help; opam provides suitable compiler switches, such as 4.02.1+fp).

Using Instruments on macOS

macOS ships with a performance monitoring and debugging application called Instruments that comes with a CPU counter, a Time Profiler, and a System Trace templates.

Once you launch it and select the template you want, you must start recording before you launch your application.

As you launch your application, real-time results will appear listed in Instrument's browser:

From there, you can click on your program there and dig to see which functions are taking the longest to execute.

Summary

In summary here are some tips for getting the best performance out of your programs:

1. Write your program as simply as possible. If it takes too long to run, profile it to find out where it's spending its time and concentrate optimisations on just those areas.
2. Check for unintentional polymorphism, and add type hints for the compiler.
3. Closures are slower than simple function calls, but add to maintainability and readability.
4. As a last resort, rewrite hotspots in your program in C (but first check the assembly language produced by the OCaml compiler to see if you can do better than it).
5. Performance might depend on external factors (speed of your database queries? speed of the network?). If so then no amount of optimization will help you.

Further Reading

You can find out more about how OCaml represents different types by reading the ("Interfacing C with OCaml") chapter in the OCaml manual and also looking at the mlvalues.h header file.