<!doctype html public "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd"> <html> <head> <title>Profiling programs</title> <link rel=stylesheet href="../styles.css" type="text/css"> </head> <body> <hr /> <h1><a class="back" title="Return to index" href="index.html#profiling">Profiling programs</a></h1> <p> In this example, we will illustrate the use of: </p> <ul> <li>events</li> <li>monitors</li> </ul> <p> by defining a simple profiler that prints the starting and ending time for processing a message sent to an object. </p> <hr /> <h2><a class="back" title="Return to index" name="events" href="index.html#profiling_events">Messages as events</a></h2> <p> In a pure object-oriented system, all computations start by sending messages to objects. We can thus define an <em>event</em> as the sending of a message to an object. An event can then be specified by the tuple <code>(Object, Message, Sender)</code>. This definition can be refined by interpreting the sending of a message and the return of the control to the object that has sent the message as two distinct events. We call these events respectively <code>before</code> and <code>after</code>. Therefore, we end up by representing an event by the tuple <code>(Event, Object, Message, Sender)</code>. For instance, if we send the message: </p> <pre> | ?- foo::bar(X). X = 1 yes </pre> <p> the two corresponding events will be: </p> <pre> (before, foo, bar(X), user) (after, foo, bar(1), user) </pre> <p> Note that the second event is only generated if the message succeeds. If the message as a goal have multiple solutions, then one <code>after</code> event will be generated for each solution. </p> <p> Events are automatically generated by the message sending mechanisms for each public message sent using the <a title="Consult reference manual" href="../refman/control/to_object2.html"><code>::/2</code></a> operator. </p> <hr /> <h2><a class="back" title="Return to index" name="monitors" href="index.html#profiling_monitors">Profilers as monitors</a></h2> <p> A monitor is an object that reacts whenever a spied event occurs. The monitor actions are defined by two event handlers: <a title="Consult reference manual" href="../refman/methods/before3.html"><code>before/3</code></a> for <code>before</code> events and <a title="Consult reference manual" href="../refman/methods/after3.html"><code>after/3</code></a> for <code>after</code> events. These predicates are automatically called by the message sending mechanisms when an event registered for the monitor occurs. </p> <p> In our example, we need a way to get the current time before and after we process a message. We will assume that we have a <code>time</code> object implementing a <code>cpu_time/1</code> predicate that returns the current CPU time for the Prolog session: </p> <pre> :- object(time). :- public(cpu_time/1). :- mode(cpu_time(-number), one). ... :- end_object. </pre> <p> Our profiler will be named <code>stop_watch</code>. It must define event handlers for the <code>before</code> and <code>after</code> events that will print the event description (object, message, and sender) and the current time: </p> <pre> :- object(stop_watch). :- uses(time). before(Object, Message, Sender) :- write(Object), write(' <-- '), writeq(Message), write(' from '), write(Sender), nl, write('STARTING at '), time::cpu_time(Seconds), write(Seconds), write(' seconds'), nl. after(Object, Message, Sender) :- write(Object), write(' <-- '), writeq(Message), write(' from '), write(Sender), nl, write('ENDING at '), time::cpu_time(Seconds), write(Seconds), write(' seconds'), nl. :- end_object. </pre> <p> After compiling and loading the <code>stop_watch</code> object (and the objects that we want to profile), we can use the <a title="Consult reference manual" href="../refman/builtins/define_events5.html"><code>define_events/5</code></a> built-in predicate to set up our profiler. For example, to profile all messages that are sent to the object <code>foo</code>, we need to call the goal: </p> <pre> | ?- define_events(_, foo, _, _, stop_watch). yes </pre> <p> This call will register <code>stop_watch</code> as a monitor to all messages sent to object <code>foo</code>, for both <code>before</code> and <code>after</code> events. Note that we say "as a monitor", not "the monitor": we can have any number of monitors over the same events. </p> <p> From now on, every time we sent a message to <code>foo</code>, the <code>stop_watch</code> monitor will print the starting and ending times for the message execution. For instance: </p> <pre> | ?- foo::bar(X). foo <-- bar(X) from user STARTING at 12.87415 seconds foo <-- bar(1) from user ENDING at 12.87419 seconds X = 1 yes </pre> <p> To stop profiling the messages sent to <code>foo</code> we use the <a title="Consult reference manual" href="../refman/builtins/abolish_events5.html"><code>abolish_events/5</code></a> built-in predicate: </p> <pre> | ?- abolish_events(_, foo, _, _, stop_watch). yes </pre> <p> This call will abolish all events defined over the object <code>foo</code> assigned to the <code>stop_watch</code> monitor. </p> <hr /> <h2><a class="back" title="Return to index" name="summary" href="index.html#profiling_summary">Summary</a></h2> <ul> <li>An event is defined as the sending of a (public) message to an object.</li> </ul> <ul> <li>There are two kinds of events: <code>before</code> events, generated before a message is processed, and <code>after</code> events, generated after the message processing completed successfully.</li> </ul> <ul> <li>Any object can be declared as a monitor to any event.</li> </ul> <ul> <li>A monitor defines event handlers, the predicates <a title="Consult reference manual" href="../refman/methods/before3.html"><code>before/3</code></a> and <a title="Consult reference manual" href="../refman/methods/after3.html"><code>after/3</code></a>, that are automatically called by the runtime engine when a spied event occurs.</li> </ul> <ul> <li>Three built-in predicates, <a title="Consult reference manual" href="../refman/builtins/define_events5.html"><code>define_events/5</code></a>, <a title="Consult reference manual" href="../refman/builtins/current_event5.html"><code>current_event/5</code></a>, and <a title="Consult reference manual" href="../refman/builtins/abolish_events5.html"><code>abolish_events/5</code></a>, enables us define, query, and abolish both events and monitors.</li> </ul> <hr /> <p class="center"> <strong><a href="reflection.html">Previous</a> | <a href="index.html">Next</a> | <a href="index.html">Table of Contents</a> | <a href="../bibliography.html">Bibliography</a> | <a href="../glossary.html">Glossary</a></strong> </p> <p class="center"> Last updated on: July 4, 2000 </p> <hr /> </body> </html>