+the number of clients in an application (for hosts), and so on.
+
+\li <b>\c TRACE_[host|link]_variable_[set|add|sub] (const char *[host|link], const char *variable, double value)</b>:
+Set the value of a given user variable for a given host/link. The value
+of this variable is always associated to the host/link. The host/link
+parameters should be its name as the one listed in the platform file.
+
+\li <b>\c TRACE_[host|link]_variable_[set|add|sub]_with_time (double time, const char *[host|link], const char *variable, double value)</b>:
+Same as TRACE_[host|link]_variable_[set|add|sub], but let user specify
+the time used to trace it. Users can specify a time that is not the
+simulated clock time as defined by the core simulator. This allows
+a fine-grain control of time definition, but should be used with
+caution since the trace can be inconsistent if resource utilization
+traces are also traced.
+
+\li <b>\c TRACE_link_srcdst_variable_[set|add|sub] (const char *src, const char *dst, const char *variable, double value)</b>:
+Same as TRACE_link_variable_[set|add|sub], but now users specify a source and
+destination hosts (as the names from the platform file). The tracing library
+will get the corresponding route that connects those two hosts (src and dst) and
+[set|add|sub] the value's variable for all the links of the route.
+
+\li <b>\c TRACE_link_srcdst_variable_[set|add|sub]_with_time (double time, const char *src, const char *dst, const char *variable, double value)</b>:
+Same as TRACE_link_srcdst_variable_[set|add|sub], but user specify a time different from the simulated time.
+
+\subsubsection faq_tracing_options Tracing configuration Options
+
+These are the options accepted by the tracing system of SimGrid:
+
+\li <b>\c
+tracing
+</b>:
+ Safe switch. It activates (or deactivates) the tracing system.
+ No other tracing options take effect if this one is not activated.
+
+\li <b>\c
+tracing/platform
+</b>:
+ Register the simulation platform in the trace file.
+
+\li <b>\c
+tracing/onelink_only
+</b>:
+ By default, the tracing system uses all routes in the platform file
+ to re-create a "graph" of the platform and register it in the trace file.
+ This option let the user tell the tracing system to use only the routes
+ that are composed with just one link.
+
+\li <b>\c
+tracing/categorized
+</b>:
+ It activates the categorized resource utilization tracing. It should
+ be enabled if tracing categories are used by this simulator.
+
+\li <b>\c
+tracing/uncategorized
+</b>:
+ It activates the uncategorized resource utilization tracing. Use it if
+ this simulator do not use tracing categories and resource use have to be
+ traced.
+
+\li <b>\c
+tracing/filename
+</b>:
+ A file with this name will be created to register the simulation. The file
+ is in the Paje format and can be analyzed using Triva or Paje visualization
+ tools. More information can be found in these webpages:
+ <a href="http://triva.gforge.inria.fr/">http://triva.gforge.inria.fr/</a>
+ <a href="http://paje.sourceforge.net/">http://paje.sourceforge.net/</a>
+
+\li <b>\c
+tracing/smpi
+</b>:
+ This option only has effect if this simulator is SMPI-based. Traces the MPI
+ interface and generates a trace that can be analyzed using Gantt-like
+ visualizations. Every MPI function (implemented by SMPI) is transformed in a
+ state, and point-to-point communications can be analyzed with arrows.
+
+\li <b>\c
+tracing/smpi/group
+</b>:
+ This option only has effect if this simulator is SMPI-based. The processes
+ are grouped by the hosts where they were executed.
+
+\li <b>\c
+tracing/msg/task
+</b>:
+ This option only has effect if this simulator is MSG-based. It traces the
+ behavior of all categorized MSG tasks, grouping them by hosts.
+
+\li <b>\c
+tracing/msg/process
+</b>:
+ This option only has effect if this simulator is MSG-based. It traces the
+ behavior of all categorized MSG processes, grouping them by hosts. This option
+ can be used to track process location if this simulator has process migration.
+
+
+\li <b>\c
+triva/categorized:graph_categorized.plist
+</b>:
+ This option generates a graph configuration file for Triva considering
+ categorized resource utilization.
+
+\li <b>\c
+triva/uncategorized:graph_uncategorized.plist
+</b>:
+ This option generates a graph configuration file for Triva considering
+ uncategorized resource utilization.