MLton 20210117

# ProfilingAllocation

With MLton and mlprof, you can profile your program to find out how many bytes each function allocates. To do so, compile your program with -profile alloc. For example, suppose that list-rev.sml is the following.

    fun append (l1, l2) =
       case l1 of
          [] => l2
        | x :: l1 => x :: append (l1, l2)

    fun rev l =
       case l of
          [] => []
        | x :: l => append (rev l, [x])

    val l = List.tabulate (1000, fn i => i)
    val _ = 1 + hd (rev l)

Compile and run list-rev as follows.

    % mlton -profile alloc list-rev.sml
    % ./list-rev
    % mlprof -show-line true list-rev mlmon.out
    6,030,136 bytes allocated (108,336 bytes by GC)
           function          cur
    ----------------------- -----
    append  list-rev.sml: 1 97.6%
    <gc>                     1.8%
    <main>                   0.4%
    rev  list-rev.sml: 6     0.2%

The data shows that most of the allocation is done by the append function defined on line 1 of list-rev.sml. The table also shows how special functions like gc and main are handled: they are printed with surrounding brackets. C functions are displayed similarly. In this example, the allocation done by the garbage collector is due to stack growth, which is usually the case.

The run-time performance impact of allocation profiling is noticeable, because it inserts additional C calls for object allocation.

Compile with -profile alloc -profile-branch true to find out how much allocation is done in each branch of a function; see ProfilingCounts for more details on -profile-branch.