9.2 Execution Profiling

Execution profiling is a common aid for improving software performance. The SICStus Prolog compiler has the capability of instrumenting compiled code with counters, which are initially zero and incremented whenever the flow of control passes a given point in the compiled code. This way the number of calls, backtracks, choicepoints created, etc., can be counted for the instrumented predicates, and an estimate of the time spent in individual clauses and disjuncts can be calculated.

Gauge is a graphical user interface for inspecting execution profiles. It is available as a library module (see lib-gauge).

The original version of the profiling package was written by M.M. Gorlick and C.F. Kesselman at the Aerospace Corporation [Gorlick & Kesselman 87].

Only compiled code can be instrumented. To get an execution profile of a program, the compiler must first be told to produce instrumented code. This is done by issuing the query:

     | ?- prolog_flag(compiling,_,profiledcode).

after which the program to be analyzed can be compiled as usual. Any new compiled code will be instrumented while the compiling Prolog flag has the value profiledcode.

The profiling data is generated by simply running the program. The predicate profile_data/4 (see below) makes available a selection of the data as a Prolog term. The predicate profile_reset/1 zeroes the profiling counters for a selection of the currently instrumented predicates. For more information, see the respective reference page.

profile_data(:Spec,?Selection,?Resolution,-Data) development

Data is the profiling data collected from the instrumented predicates covered by Spec with selection and resolution Selection and Resolution respectively.

profile_reset(:Spec) development

The profiling counters for the instrumented predicates covered by Spec are zeroed.