This chapter describes how the execution of Objective Caml
programs can be profiled, by recording how many times functions are
called, branches of conditionals are taken, ...
| 17.1 |
Compiling for profiling |
|
Before profiling an execution, the program must be compiled in
profiling mode, using the ocamlcp front-end to the ocamlc compiler
(see chapter 8). When compiling modules separately,
ocamlcp must be used when compiling the modules (production
of .cmo files), and can also be used (though this is not strictly
necessary) when linking them together.
Note
If a module (.ml file) doesn't have a corresponding
interface (.mli file), then compiling it with ocamlcp will produce
object files (.cmi and .cmo) that are not compatible with the ones
produced by ocamlc, which may lead to problems (if the .cmi or
.cmo is still around) when switching between profiling and
non-profiling compilations. To avoid this problem, you should always
have a .mli file for each .ml file.
Note
To make sure your programs can be compiled in
profiling mode, avoid using any identifier that begins with
__ocaml_prof.
The amount of profiling information can be controlled through the -p
option to ocamlcp, followed by one or several letters indicating which
parts of the program should be profiled:
-
a
- all options
- f
- function calls : a count point is set at the beginning of
function bodies
- i
- if ...then ...else ... : count points are set in
both then branch and else branch
- l
- while, for loops: a count point is set at the beginning of
the loop body
- m
- match branches: a count point is set at the beginning of the
body of each branch
- t
- try ...with ... branches: a count point is set at the
beginning of the body of each branch
For instance, compiling with ocamlcp -p film profiles function calls,
if...then...else..., loops and pattern matching.
Calling ocamlcp without the -p option defaults to -p fm, meaning
that only function calls and pattern matching are profiled.
Note: Due to the implementation of streams and stream patterns as
syntactic sugar, it is hard to predict what parts of stream expressions
and patterns will be profiled by a given flag. To profile a program with
streams, we recommend using ocamlcp -p a.
| 17.2 |
Profiling an execution |
|
Running a bytecode executable file that has been compiled with ocamlcp
records the execution counts for the specified parts of the program
and saves them in a file called ocamlprof.dump in the current directory.
The ocamlprof.dump file is written only if the program terminates
normally (by calling exit or by falling through). It is not written
if the program terminates with an uncaught exception.
If a compatible dump file already exists in the current directory, then the
profiling information is accumulated in this dump file. This allows, for
instance, the profiling of several executions of a program on
different inputs.
| 17.3 |
Printing profiling information |
|
The ocamlprof command produces a source listing of the program modules
where execution counts have been inserted as comments. For instance,
ocamlprof foo.ml
prints the source code for the foo module, with comments indicating
how many times the functions in this module have been called. Naturally,
this information is accurate only if the source file has not been modified
since the profiling execution took place.
The following options are recognized by ocamlprof:
-
-f dumpfile
-
Specifies an alternate dump file of profiling information
- -F string
-
Specifies an additional string to be output with profiling information.
By default, ocamlprof will annotate programs with comments of the form
(* n *) where n is the counter value for a profiling
point. With option -F s, the annotation will be
(* sn *).
Profiling with ocamlprof only records execution counts, not the actual
time spent into each function. There is currently no way to perform
time profiling on bytecode programs generated by ocamlc.
Native-code programs generated by ocamlopt can be profiled for time
and execution counts using the -p option and the standard Unix
profiler gprof. Just add the -p option when compiling and linking
the program:
ocamlopt -o myprog -p other-options files
./myprog
gprof myprog
Caml function names in the output of gprof have the following format:
Module-name_function-name_unique-number
Other functions shown are either parts of the Caml run-time system or
external C functions linked with the program.
The output of gprof is described in the Unix manual page for
gprof(1). It generally consists of two parts: a ``flat'' profile
showing the time spent in each function and the number of invocation
of each function, and a ``hierarchical'' profile based on the call
graph. Currently, only the Intel x86/Linux and Alpha/Digital Unix
ports of ocamlopt support the two profiles. On other platforms,
gprof will report only the ``flat'' profile with just time
information. When reading the output of gprof, keep in mind that
the accumulated times computed by gprof are based on heuristics and
may not be exact.