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This file is part of Logtalk
Copyright 1998-2019 Paulo Moura

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License.


The ports_profiler.lgt source file defines a simple predicate execution box model port profiler tool (inspired by the ECLiPSe port_profiler tool). The box model is the same used in the debugger tool.

The Logtalk predicate execution box model is an extended version of the original Byrd's four port model. Besides the standard call, exit, fail, and redo ports, Logtalk also defines two (post-)unification ports, fact and rule, and an exception port. This tool counts and reports the number of times each port is traversed during the execution of queries. It also distinguishes between deterministic exits (reported in the exit column in the profiling result tables) and exits that leave choice-points (reported in the *exit column).

API documentation

To consult this tool API documentation, open in a web browser the link:


For sample queries, please see the SCRIPT.txt file.


| ?- logtalk_load(ports_profiler(loader)).

Note that this tool cannot be loaded at the same time as other tools (e.g. the debugger) that also provide a debug handler, which must be unique in a running session.

Compiling source files for port profiling

To compile source files for port profiling, simply compile them in debug mode and with the source_data flag turned on. For example:

| ?- logtalk_load(my_source_file, [debug(on), source_data(on)]).

Alternatively, you can also simply turn on the debug and source_data flags globally before compiling your source files:

| ?- set_logtalk_flag(debug, on), set_logtalk_flag(source_data, on).

Be aware, however, that loader files (e.g. library loader files) may override default flag values and thus loaded files may not be compiled in debug mode.

Generating profiling data

After loading this tool and compiling the source files that you want to profile in debug mode, simply call the goals to be profiled.

Printing profiling data reports

After calling the goals that you want to profile, you can print a table with all profile data by typing:

| ?- ports_profiler::data.

To print a table with data for a single entity, use the query:

| ?- ports_profiler::data(Entity).

The profiling data can be reset using the query:

| ?- ports_profiler::reset.

To reset only the data about a specific entity, use the query:

| ?- ports_profiler::reset(Entity).

Interpreting profiling data

Some useful information that can be inferred from the profiling data include:

  • which predicates are called more often (from the call port)
  • unexpected failures (from the fail port)
  • unwanted non-determinism (from the *exit port)
  • performance issues due to backtracking (from the *exit and redo ports)
  • predicates acting like a generator of possible solutions (from the *exit and redo ports)
  • inefficient indexing of predicate clauses (from the fact, rule, and call ports)

The profiling data should be analyzed taking into account the expected behavior for the profiled predicates.

Known issues

Determinism information is currently not available when using Lean Prolog or Quintus Prolog as backend compilers.

Other notes

All source files are formatted using tabs (the recommended setting is a tab width equivalent to 4 spaces).