X-Git-Url: http://info.iut-bm.univ-fcomte.fr/pub/gitweb/simgrid.git/blobdiff_plain/7207080bb744f50a6d8c418f25d82ad69e9b4f44..2ba8549e0a8371efc09c66bad197097f88d93366:/docs/source/Configuring_SimGrid.rst

diff --git a/docs/source/Configuring_SimGrid.rst b/docs/source/Configuring_SimGrid.rst
index 993f35cf79..d420c7155b 100644
--- a/docs/source/Configuring_SimGrid.rst
+++ b/docs/source/Configuring_SimGrid.rst
@@ -5,7 +5,7 @@ Configuring SimGrid
 
 .. raw:: html
 
-   <object id="TOC" data="graphical-toc.svg" width="100%" type="image/svg+xml"></object>
+   <object id="TOC" data="graphical-toc.svg" type="image/svg+xml"></object>
    <script>
    window.onload=function() { // Wait for the SVG to be loaded before changing it
      var elem=document.querySelector("#TOC").contentDocument.getElementById("ConfigBox")
@@ -38,11 +38,11 @@ type the following on the command-line:
 
 Several ``--cfg`` command line arguments can naturally be used. If you
 need to include spaces in the argument, don't forget to quote the
-argument. You can even escape the included quotes (write @' for ' if
-you have your argument between ').
+argument. You can even escape the included quotes (write ``@'`` for ``'`` if
+you have your argument between simple quotes).
 
 Another solution is to use the ``<config>`` tag in the platform file. The
-only restriction is that this tag must occure before the first
+only restriction is that this tag must occur before the first
 platform element (be it ``<zone>``, ``<cluster>``, ``<peer>`` or whatever).
 The ``<config>`` tag takes an ``id`` attribute, but it is currently
 ignored so you don't really need to pass it. The important part is that
@@ -67,7 +67,7 @@ with :cpp:func:`simgrid::s4u::Engine::set_config` or :cpp:func:`MSG_config`.
    int main(int argc, char *argv[]) {
      simgrid::s4u::Engine e(&argc, argv);
 
-     e->set_config("Item:Value");
+     simgrid::s4u::Engine::set_config("Item:Value");
 
      // Rest of your code
    }
@@ -81,13 +81,12 @@ Existing Configuration Items
   The full list can be retrieved by passing ``--help`` and
   ``--help-cfg`` to an executable that uses SimGrid. Try passing
   ``help`` as a value to get the list of values accepted by a given
-   option. For example, ``--cfg=plugin:help`` will give you the list
-   of plugins available in your installation of SimGrid.
+  option. For example, ``--cfg=plugin:help`` will give you the list
+  of plugins available in your installation of SimGrid.
 
 - **contexts/factory:** :ref:`cfg=contexts/factory`
 - **contexts/guard-size:** :ref:`cfg=contexts/guard-size`
 - **contexts/nthreads:** :ref:`cfg=contexts/nthreads`
-- **contexts/parallel-threshold:** :ref:`cfg=contexts/parallel-threshold`
 - **contexts/stack-size:** :ref:`cfg=contexts/stack-size`
 - **contexts/synchro:** :ref:`cfg=contexts/synchro`
 
@@ -112,7 +111,6 @@ Existing Configuration Items
 - **model-check/checkpoint:** :ref:`cfg=model-check/checkpoint`
 - **model-check/communications-determinism:** :ref:`cfg=model-check/communications-determinism`
 - **model-check/dot-output:** :ref:`cfg=model-check/dot-output`
-- **model-check/hash:** :ref:`cfg=model-checker/hash`
 - **model-check/max-depth:** :ref:`cfg=model-check/max-depth`
 - **model-check/property:** :ref:`cfg=model-check/property`
 - **model-check/reduction:** :ref:`cfg=model-check/reduction`
@@ -125,6 +123,8 @@ Existing Configuration Items
 - **network/bandwidth-factor:** :ref:`cfg=network/bandwidth-factor`
 - **network/crosstraffic:** :ref:`cfg=network/crosstraffic`
 - **network/latency-factor:** :ref:`cfg=network/latency-factor`
+- **network/loopback-lat:** :ref:`cfg=network/loopback`
+- **network/loopback-bw:** :ref:`cfg=network/loopback`
 - **network/maxmin-selective-update:** :ref:`Network Optimization Level <options_model_optim>`
 - **network/model:** :ref:`options_model_select`
 - **network/optim:** :ref:`Network Optimization Level <options_model_optim>`
@@ -132,6 +132,7 @@ Existing Configuration Items
 - **network/weight-S:** :ref:`cfg=network/weight-S`
 
 - **ns3/TcpModel:** :ref:`options_pls`
+- **ns3/seed:** :ref:`options_pls`
 - **path:** :ref:`cfg=path`
 - **plugin:** :ref:`cfg=plugin`
 
@@ -141,6 +142,7 @@ Existing Configuration Items
 
 - **For collective operations of SMPI,** please refer to Section :ref:`cfg=smpi/coll-selector`
 - **smpi/async-small-thresh:** :ref:`cfg=smpi/async-small-thresh`
+- **smpi/buffering:** :ref:`cfg=smpi/buffering`
 - **smpi/bw-factor:** :ref:`cfg=smpi/bw-factor`
 - **smpi/coll-selector:** :ref:`cfg=smpi/coll-selector`
 - **smpi/comp-adjustment-file:** :ref:`cfg=smpi/comp-adjustment-file`
@@ -183,7 +185,7 @@ Configuring the Platform Models
 Choosing the Platform Models
 ............................
 
-SimGrid comes with several network, CPU and storage models built in,
+SimGrid comes with several network, CPU and disk models built in,
 and you can change the used model at runtime by changing the passed
 configuration. The three main configuration items are given below.
 For each of these items, passing the special ``help`` value gives you
@@ -217,21 +219,21 @@ models for all existing resources.
     poorly modeled. This model is described in `A Network Model for
     Simulation of Grid Application
     <https://hal.inria.fr/inria-00071989/document>`_.
-  - **NS3** (only available if you compiled SimGrid accordingly):
+  - **ns-3** (only available if you compiled SimGrid accordingly):
     Use the packet-level network
-    simulators as network models (see :ref:`pls_ns3`).
+    simulators as network models (see :ref:`model_ns3`).
     This model can be :ref:`further configured <options_pls>`.
 
 - ``cpu/model``: specify the used CPU model.  We have only one model
   for now:
 
-  - **Cas01:** Simplistic CPU model (time=size/power)
+  - **Cas01:** Simplistic CPU model (time=size/speed)
 
 - ``host/model``: The host concept is the aggregation of a CPU with a
   network card. Three models exists, but actually, only 2 of them are
   interesting. The "compound" one is simply due to the way our
   internal code is organized, and can easily be ignored. So at the
-  end, you have two host models: The default one allows to aggregate
+  end, you have two host models: The default one allows aggregation of
   an existing CPU model with an existing network model, but does not
   allow parallel tasks because these beasts need some collaboration
   between the network and CPU model. That is why, ptask_07 is used by
@@ -395,16 +397,26 @@ can be set to 0 (disable this feature) or 1 (enable it).
 
 Note that with the default host model this option is activated by default.
 
+.. _cfg=network/loopback:
+
+Configuring loopback link
+^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Several network model provide an implicit loopback link to account for local 
+communication on a host. By default it has a 10GBps bandwidth and a null latency.
+This can be changed with ``network/loopback-lat`` and ``network/loopback-bw`` 
+items.
+
 .. _cfg=smpi/async-small-thresh:
 
-Simulating Asyncronous Send
-^^^^^^^^^^^^^^^^^^^^^^^^^^^
+Simulating Asynchronous Send
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-(this configuration item is experimental and may change or disapear)
+(this configuration item is experimental and may change or disappear)
 
-It is possible to specify that messages below a certain size will be
+It is possible to specify that messages below a certain size (in bytes) will be
 sent as soon as the call to MPI_Send is issued, without waiting for
-the correspondant receive. This threshold can be configured through
+the correspondent receive. This threshold can be configured through
 the ``smpi/async-small-thresh`` item. The default value is 0. This
 behavior can also be manually set for mailboxes, by setting the
 receiving mode of the mailbox with a call to
@@ -412,28 +424,37 @@ receiving mode of the mailbox with a call to
 this mailbox will have this behavior regardless of the message size.
 
 This value needs to be smaller than or equals to the threshold set at
-@ref options_model_smpi_detached , because asynchronous messages are
-meant to be detached as well.
+:ref:`cfg=smpi/send-is-detached-thresh`, because asynchronous messages
+are meant to be detached as well.
 
 .. _options_pls:
 
-Configuring NS3
-^^^^^^^^^^^^^^^
+Configuring ns-3
+^^^^^^^^^^^^^^^^
 
-**Option** ``ns3/TcpModel`` **Default:** "default" (NS3 default)
+**Option** ``ns3/TcpModel`` **Default:** "default" (ns-3 default)
 
-When using NS3, there is an extra item ``ns3/TcpModel``, corresponding
+When using ns-3, there is an extra item ``ns3/TcpModel``, corresponding
 to the ``ns3::TcpL4Protocol::SocketType`` configuration item in
-NS3. The only valid values (enforced on the SimGrid side) are
-'default' (no change to the NS3 configuration), 'NewReno' or 'Reno' or
+ns-3. The only valid values (enforced on the SimGrid side) are
+'default' (no change to the ns-3 configuration), 'NewReno' or 'Reno' or
 'Tahoe'.
 
+**Option** ``ns3/seed`` **Default:** "" (don't set the seed in ns-3)
+
+This option is the random seed to provide to ns-3 with
+``ns3::RngSeedManager::SetSeed`` and ``ns3::RngSeedManager::SetRun``.
+
+If left blank, no seed is set in ns-3. If the value 'time' is
+provided, the current amount of seconds since epoch is used as a seed.
+Otherwise, the provided value must be a number to use as a seed.
+
 Configuring the Storage model
 .............................
 
 .. _cfg=storage/max_file_descriptors:
 
-File Descriptor Cound per Host
+File Descriptor Count per Host
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 **Option** ``storage/max_file_descriptors`` **Default:** 1024
@@ -447,7 +468,7 @@ application requires it or to reduce it to save memory space.
 Activating Plugins
 ------------------
 
-SimGrid plugins allow to extend the framework without changing its
+SimGrid plugins allow one to extend the framework without changing its
 source code directly. Read the source code of the existing plugins to
 learn how to do so (in ``src/plugins``), and ask your questions to the
 usual channels (Stack Overflow, Mailing list, IRC). The basic idea is
@@ -461,21 +482,19 @@ modification to your simulation code. For example, you can activate
 the host energy plugin by adding ``--cfg=plugin:host_energy`` to your
 command line.
 
-Here is the full list of plugins that can be activated this way:
+Here is a partial list of plugins that can be activated this way. You can get
+the full list by passing ``--cfg=plugin:help`` to your simulator.
 
- - **host_energy:** keeps track of the energy dissipated by
-   computations. More details in @ref plugin_energy.
- - **link_energy:** keeps track of the energy dissipated by
-   communications. More details in @ref SURF_plugin_energy.
- - **host_load:** keeps track of the computational load.
-   More details in @ref plugin_load.
+  - :ref:`Host Energy <plugin_host_energy>`: models the energy dissipation of the compute units.
+  - :ref:`Link Energy <plugin_link_energy>`: models the energy dissipation of the network.
+  - :ref:`Host Load <plugin_host_load>`: monitors the load of the compute units.
 
 .. _options_modelchecking:
 
 Configuring the Model-Checking
 ------------------------------
 
-To enable the SimGrid model-checking support the program should
+To enable SimGrid's model-checking support, the program should
 be executed using the simgrid-mc wrapper:
 
 .. code-block:: shell
@@ -485,6 +504,32 @@ be executed using the simgrid-mc wrapper:
 Safety properties are expressed as assertions using the function
 :cpp:func:`void MC_assert(int prop)`.
 
+.. _cfg=smpi/buffering:
+
+Specifying the MPI buffering behavior
+.....................................
+
+**Option** ``smpi/buffering`` **Default:** infty
+
+Buffering in MPI has a huge impact on the communication semantic. For example,
+standard blocking sends are synchronous calls when the system buffers are full
+while these calls can complete immediately without even requiring a matching
+receive call for small messages sent when the system buffers are empty.
+
+In SMPI, this depends on the message size, that is compared against two thresholds:
+
+- if (size < :ref:`smpi/async-small-thresh <cfg=smpi/async-small-thresh>`) then
+  MPI_Send returns immediately, even if the corresponding receive has not be issued yet.
+- if (:ref:`smpi/async-small-thresh <cfg=smpi/async-small-thresh>` < size < :ref:`smpi/send-is-detached-thresh <cfg=smpi/send-is-detached-thresh>`) then
+  MPI_Send returns as soon as the corresponding receive has been issued. This is known as the eager mode.
+- if (:ref:`smpi/send-is-detached-thresh <cfg=smpi/send-is-detached-thresh>` < size) then
+  MPI_Send returns only when the message has actually been sent over the network. This is known as the rendez-vous mode.
+
+The ``smpi/buffering`` (only valid with MC) option gives an easier interface to choose between these semantics. It can take two values:
+
+- **zero:** means that buffering should be disabled. All communications are actually blocking.
+- **infty:** means that buffering should be made infinite. All communications are non-blocking.
+
 .. _cfg=model-check/property:
 
 Specifying a liveness property
@@ -494,8 +539,8 @@ Specifying a liveness property
 
 If you want to specify liveness properties, you have to pass them on
 the command line, specifying the name of the file containing the
-property, as formatted by the ltl2ba program.
-
+property, as formatted by the `ltl2ba <https://github.com/utwente-fmt/ltl2ba>`_ program.
+Note that ltl2ba is not part of SimGrid and must be installed separately.
 
 .. code-block:: shell
 
@@ -507,7 +552,7 @@ Going for Stateful Verification
 ...............................
 
 By default, the system is backtracked to its initial state to explore
-another path instead of backtracking to the exact step before the fork
+another path, instead of backtracking to the exact step before the fork
 that we want to explore (this is called stateless verification). This
 is done this way because saving intermediate states can rapidly
 exhaust the available memory. If you want, you can change the value of
@@ -523,42 +568,51 @@ Specifying the kind of reduction
 ................................
 
 The main issue when using the model-checking is the state space
-explosion. To counter that problem, you can chose a exploration
-reduction techniques with
+explosion. You can activate some reduction technique with
 ``--cfg=model-check/reduction:<technique>``. For now, this
 configuration variable can take 2 values:
 
- - **none:** Do not apply any kind of reduction (mandatory for now for
-   liveness properties)
+ - **none:** Do not apply any kind of reduction (mandatory for
+   liveness properties, as our current DPOR algorithm breaks cycles)
  - **dpor:** Apply Dynamic Partial Ordering Reduction. Only valid if
    you verify local safety properties (default value for safety
    checks).
 
-There is unfortunately no silver bullet here, and the most efficient
-reduction techniques cannot be applied to any properties. In
-particular, the DPOR method cannot be applied on liveness properties
-since our implementation of DPOR may break some cycles, while cycles
-are very important to the soundness of the exploration for liveness
-properties.
+Another way to mitigate the state space explosion is to search for
+cycles in the exploration with the :ref:`cfg=model-check/visited`
+configuration. Note that DPOR and state-equality reduction may not
+play well together. You should choose between them.
+
+Our current DPOR implementation could be improved in may ways. We are
+currently improving its efficiency (both in term of reduction ability
+and computational speed), and future work could make it compatible
+with liveness properties.
 
 .. _cfg=model-check/visited:
 
-Size of Cycle Detection Set
-...........................
+Size of Cycle Detection Set (state equality reduction)
+......................................................
+
+Mc SimGrid can be asked to search for cycles during the exploration,
+i.e. situations where a new explored state is in fact the same state
+than a previous one.. This can prove useful to mitigate the state
+space explosion with safety properties, and this is the crux when
+searching for counter-examples to the liveness properties.
 
-In order to detect cycles, the model-checker needs to check if a new
-explored state is in fact the same state than a previous one. For
-that, the model-checker can take a snapshot of each visited state:
-this snapshot is then used to compare it with subsequent states in the
-exploration graph.
+Note that this feature may break the current implementation of the
+DPOR reduction technique.
 
-The ``model-check/visited`` item is the maximum number of states which
+The ``model-check/visited`` item is the maximum number of states, which
 are stored in memory. If the maximum number of snapshotted state is
 reached, some states will be removed from the memory and some cycles
 might be missed. Small values can lead to incorrect verifications, but
-large value can exhaust your memory, so choose carefully.
+large values can exhaust your memory and be CPU intensive as each new
+state must be compared to that amount of older saved states.
 
-By default, no state is snapshotted and cycles cannot be detected.
+The default settings depend on the kind of exploration. With safety
+checking, no state is snapshotted and cycles cannot be detected. With
+liveness checking, all states are snapshotted because missing a cycle
+could hinder the exploration soundness.
 
 .. _cfg=model-check/termination:
 
@@ -567,7 +621,7 @@ Non-Termination Detection
 
 The ``model-check/termination`` configuration item can be used to
 report if a non-termination execution path has been found. This is a
-path with a cycle which means that the program might never terminate.
+path with a cycle, which means that the program might never terminate.
 
 This only works in safety mode, not in liveness mode.
 
@@ -579,10 +633,10 @@ Dot Output
 ..........
 
 If set, the ``model-check/dot-output`` configuration item is the name
-of a file in which to write a dot file of the path leading the found
-property (safety or liveness violation) as well as the cycle for
-liveness properties. This dot file can then fed to the graphviz dot
-tool to generate an corresponding graphical representation.
+of a file in which to write a dot file of the path leading to the
+property violation discovered (safety or liveness violation), as well
+as the cycle for liveness properties. This dot file can then be fed to the
+graphviz dot tool to generate a corresponding graphical representation.
 
 .. _cfg=model-check/max-depth:
 
@@ -590,19 +644,19 @@ Exploration Depth Limit
 .......................
 
 The ``model-checker/max-depth`` can set the maximum depth of the
-exploration graph of the model-checker. If this limit is reached, a
+exploration graph of the model checker. If this limit is reached, a
 logging message is sent and the results might not be exact.
 
-By default, there is not depth limit.
+By default, there is no depth limit.
 
 .. _cfg=model-check/timeout:
 
 Handling of Timeouts
 ....................
 
-By default, the model-checker does not handle timeout conditions: the `wait`
+By default, the model checker does not handle timeout conditions: the `wait`
 operations never time out. With the ``model-check/timeout`` configuration item
-set to **yes**, the model-checker will explore timeouts of `wait` operations.
+set to **yes**, the model checker will explore timeouts of `wait` operations.
 
 .. _cfg=model-check/communications-determinism:
 .. _cfg=model-check/send-determinism:
@@ -612,57 +666,43 @@ Communication Determinism
 
 The ``model-check/communications-determinism`` and
 ``model-check/send-determinism`` items can be used to select the
-communication determinism mode of the model-checker which checks
+communication determinism mode of the model checker, which checks
 determinism properties of the communications of an application.
 
+.. _options_mc_perf:
+
 Verification Performance Considerations
 .......................................
 
 The size of the stacks can have a huge impact on the memory
 consumption when using model-checking. By default, each snapshot will
-save a copy of the whole stacks and not only of the part which is
+save a copy of the whole stacks and not only of the part that is
 really meaningful: you should expect the contribution of the memory
-consumption of the snapshots to be @f$ @mbox{number of processes}
-@times @mbox{stack size} @times @mbox{number of states} @f$.
+consumption of the snapshots to be:
+:math:`\text{number of processes} \times \text{stack size} \times \text{number of states}`.
 
 When compiled against the model checker, the stacks are not
 protected with guards: if the stack size is too small for your
-application, the stack will silently overflow on other parts of the
+application, the stack will silently overflow into other parts of the
 memory (see :ref:`contexts/guard-size <cfg=contexts/guard-size>`).
 
-.. _cfg=model-checker/hash:
-
-State Hashing
-.............
-
-Usually most of the time of the model-checker is spent comparing states. This
-process is complicated and consumes a lot of bandwidth and cache.
-In order to speedup the state comparison, the experimental ``model-checker/hash``
-configuration item enables the computation of a hash summarizing as much
-information of the state as possible into a single value. This hash can be used
-to avoid most of the comparisons: the costly comparison is then only used when
-the hashes are identical.
-
-Currently most of the state is not included in the hash because the
-implementation was found to be buggy and this options is not as useful as
-it could be. For this reason, it is currently disabled by default.
-
 .. _cfg=model-check/replay:
 
-Replaying buggy execution paths out of the model-checker
-........................................................
+Replaying buggy execution paths from the model checker
+......................................................
 
-Debugging the problems reported by the model-checker is challenging: First, the
-application under verification cannot be debugged with gdb because the
-model-checker already traces it. Then, the model-checker may explore several
-execution paths before encountering the issue, making it very difficult to
-understand the outputs. Fortunately, SimGrid provides the execution path leading
-to any reported issue so that you can replay this path out of the model checker,
-enabling the usage of classical debugging tools.
+Debugging the problems reported by the model checker is challenging:
+First, the application under verification cannot be debugged with gdb
+because the model checker already traces it. Then, the model checker may
+explore several execution paths before encountering the issue, making it
+very difficult to understand the output. Fortunately, SimGrid provides
+the execution path leading to any reported issue so that you can replay
+this path reported by the model checker, enabling the usage of classical
+debugging tools.
 
-When the model-checker finds an interesting path in the application
+When the model checker finds an interesting path in the application
 execution graph (where a safety or liveness property is violated), it
-generates an identifier for this path. Here is an example of output:
+generates an identifier for this path. Here is an example of the output:
 
 .. code-block:: shell
 
@@ -679,10 +719,20 @@ generates an identifier for this path. Here is an example of output:
    [  0.000000] (0:@) Executed transitions = 46
 
 The interesting line is ``Path = 1/3;1/4``, which means that you should use
-`--cfg=model-check/replay:1/3;1/4`` to replay your application on the buggy
-execution path. The other options should be the same (but the model-checker
-should be disabled). Note that format and meaning of the path may change between
-different releases.
+``--cfg=model-check/replay:1/3;1/4`` to replay your application on the buggy
+execution path. All options (but the model checker related ones) must
+remain the same. In particular, if you ran your application with
+``smpirun -wrapper simgrid-mc``, then do it again. Remove all
+MC-related options, keep non-MC-related ones and add
+``--cfg=model-check/replay:???``.
+
+Currently, if the path is of the form ``X;Y;Z``, each number denotes
+the actor's pid that is selected at each indecision point. If it's of
+the form ``X/a;Y/b``, the X and Y are the selected pids while the a
+and b are the return values of their simcalls. In the previous
+example, ``1/3;1/4``, you can see from the full output that the actor
+1 is doing MC_RANDOM simcalls, so the 3 and 4 simply denote the values
+that these simcall return.
 
 Configuring the User Code Virtualization
 ----------------------------------------
@@ -728,9 +778,9 @@ the slowest to the most efficient:
    of our own, directly implemented in assembly (only available for x86
    and amd64 platforms for now) and without any unneeded system call.
 
-The main reason to change this setting is when the debugging tools get
+The main reason to change this setting is when the debugging tools become
 fooled by the optimized context factories. Threads are the most
-debugging-friendly contextes, as they allow to set breakpoints
+debugging-friendly contexts, as they allow one to set breakpoints
 anywhere with gdb and visualize backtraces for all processes, in order
 to debug concurrency issues. Valgrind is also more comfortable with
 threads, but it should be usable with all factories (Exception: the
@@ -753,10 +803,16 @@ stacks), leading to segfaults with corrupted stack traces.
 If you want to push the scalability limits of your code, you might
 want to reduce the ``contexts/stack-size`` item. Its default value is
 8192 (in KiB), while our Chord simulation works with stacks as small
-as 16 KiB, for example. This *setting is ignored* when using the
-thread factory. Instead, you should compile SimGrid and your
-application with ``-fsplit-stack``. Note that this compilation flag is
-not compatible with the model-checker right now.
+as 16 KiB, for example. You can ensure that some actors have a specific
+size by simply changing the value of this configuration item before
+creating these actors. The :cpp:func:`simgrid::s4u::Engine::set_config` 
+functions are handy for that.
+
+This *setting is ignored* when using the thread factory (because there
+is no way to modify the stack size with C++ system threads). Instead,
+you should compile SimGrid and your application with
+``-fsplit-stack``. Note that this compilation flag is not compatible
+with the model checker right now.
 
 The operating system should only allocate memory for the pages of the
 stack which are actually used and you might not need to use this in
@@ -785,7 +841,6 @@ on other parts of the memory if their size is too small for the
 application.
 
 .. _cfg=contexts/nthreads:
-.. _cfg=contexts/parallel-threshold:
 .. _cfg=contexts/synchro:
 
 Running User Code in Parallel
@@ -798,21 +853,10 @@ simulations may well fail in parallel mode. It is described in
 
 If you are using the **ucontext** or **raw** context factories, you can
 request to execute the user code in parallel. Several threads are
-launched, each of them handling as much user contexts at each run. To
-actiave this, set the ``contexts/nthreads`` item to the amount of
-cores that you have in your computer (or lower than 1 to have
-the amount of cores auto-detected).
-
-Even if you asked several worker threads using the previous option,
-you can request to start the parallel execution (and pay the
-associated synchronization costs) only if the potential parallelism is
-large enough. For that, set the ``contexts/parallel-threshold``
-item to the minimal amount of user contexts needed to start the
-parallel execution. In any given simulation round, if that amount is
-not reached, the contexts will be run sequentially directly by the
-main thread (thus saving the synchronization costs). Note that this
-option is mainly useful when the grain of the user code is very fine,
-because our synchronization is now very efficient.
+launched, each of them handling the same number of user contexts at each
+run. To activate this, set the ``contexts/nthreads`` item to the amount
+of cores that you have in your computer (or lower than 1 to have the
+amount of cores auto-detected).
 
 When parallel execution is activated, you can choose the
 synchronization schema used with the ``contexts/synchro`` item,
@@ -833,10 +877,10 @@ Configuring the Tracing
 -----------------------
 
 The :ref:`tracing subsystem <outcomes_vizu>` can be configured in
-several different ways depending on the nature of the simulator (MSG,
-SimDag, SMPI) and the kind of traces that need to be obtained. See the
+several different ways depending on the used interface (S4U, SMPI, SimDag)
+and the kind of traces that needs to be obtained. See the
 :ref:`Tracing Configuration Options subsection
-<tracing_tracing_options>` to get a detailed description of each
+<tracing_tracing_options>` for a full description of each
 configuration option.
 
 We detail here a simple way to get the traces working for you, even if
@@ -847,24 +891,21 @@ you never used the tracing API.
 
   .. code-block:: shell
 
-     --cfg=tracing:yes --cfg=tracing/uncategorized:yes --cfg=triva/uncategorized:uncat.plist
+     --cfg=tracing:yes --cfg=tracing/uncategorized:yes
 
-  The first parameter activates the tracing subsystem, the second
+  The first parameter activates the tracing subsystem, and the second
   tells it to trace host and link utilization (without any
-  categorization) and the third creates a graph configuration file to
-  configure Triva when analysing the resulting trace file.
+  categorization).
 
 - MSG or SimDag-based simulator and categorized traces (you need to
   declare categories and classify your tasks according to them) 
 
   .. code-block:: shell
 
-     --cfg=tracing:yes --cfg=tracing/categorized:yes --cfg=triva/categorized:cat.plist
+     --cfg=tracing:yes --cfg=tracing/categorized:yes
 
-  The first parameter activates the tracing subsystem, the second
-  tells it to trace host and link categorized utilization and the
-  third creates a graph configuration file to configure Triva when
-  analysing the resulting trace file.
+  The first parameter activates the tracing subsystem, and the second
+  tells it to trace host and link categorized utilization.
 
 - SMPI simulator and traces for a space/time view:
 
@@ -915,7 +956,7 @@ Configuring SMPI
 ----------------
 
 The SMPI interface provides several specific configuration items.
-These are uneasy to see since the code is usually launched through the
+These are not easy to see, since the code is usually launched through the
 ``smiprun`` script directly.
 
 .. _cfg=smpi/host-speed:
@@ -932,28 +973,34 @@ a ``MPI_Send()``, SMPI will automatically benchmark the duration of
 this code, and create an execution task within the simulator to take
 this into account. For that, the actual duration is measured on the
 host machine and then scaled to the power of the corresponding
-simulated machine. The variable ``smpi/host-speed`` allows to specify
-the computational speed of the host machine (in flop/s) to use when
-scaling the execution times. It defaults to 20000, but you really want
-to update it to get accurate simulation results.
-
-When the code is constituted of numerous consecutive MPI calls, the
+simulated machine. The variable ``smpi/host-speed`` allows one to
+specify the computational speed of the host machine (in flop/s by
+default) to use when scaling the execution times.
+
+The default value is ``smpi/host-speed=20kf`` (= 20,000 flop/s). This
+is probably underestimated for most machines, leading SimGrid to
+overestimate the amount of flops in the execution blocks that are
+automatically injected in the simulator. As a result, the execution
+time of the whole application will probably be overestimated until you
+use a realistic value.
+
+When the code consists of numerous consecutive MPI calls, the
 previous mechanism feeds the simulation kernel with numerous tiny
 computations. The ``smpi/cpu-threshold`` item becomes handy when this
-impacts badly the simulation performance. It specifies a threshold (in
+impacts badly on the simulation performance. It specifies a threshold (in
 seconds) below which the execution chunks are not reported to the
 simulation kernel (default value: 1e-6).
 
 .. note:: The option ``smpi/cpu-threshold`` ignores any computation
    time spent below this threshold. SMPI does not consider the
-   `amount` of these computations; there is no offset for this. Hence,
-   a value that is too small, may lead to unreliable simulation
-   results.
+   `amount of time` of these computations; there is no offset for
+   this. Hence, a value that is too small, may lead to unreliable
+   simulation results.
 
 In some cases, however, one may wish to disable simulation of
-application computation. This is the case when SMPI is used not to
-simulate an MPI applications, but instead an MPI code that performs
-"live replay" of another MPI app (e.g., ScalaTrace's replay tool,
+the computation of an application. This is the case when SMPI is used not to
+simulate an MPI application, but instead an MPI code that performs
+"live replay" of another MPI app (e.g., ScalaTrace's replay tool, or
 various on-line simulators that run an app at scale). In this case the
 computation of the replay/simulation logic should not be simulated by
 SMPI. Instead, the replay tool or on-line simulator will issue
@@ -961,12 +1008,12 @@ SMPI. Instead, the replay tool or on-line simulator will issue
 being replayed/simulated. At the moment, these computation events can
 be simulated using SMPI by calling internal smpi_execute*() functions.
 
-To disable the benchmarking/simulation of computation in the simulated
+To disable the benchmarking/simulation of a computation in the simulated
 application, the variable ``smpi/simulate-computation`` should be set
-to no.  This option just ignores the timings in your simulation; it
+to **no**.  This option just ignores the timings in your simulation; it
 still executes the computations itself. If you want to stop SMPI from
 doing that, you should check the SMPI_SAMPLE macros, documented in
-Section :ref:`SMPI_adapting_speed`.
+Section :ref:`SMPI_use_faster`.
 
 +------------------------------------+-------------------------+-----------------------------+
 |  Solution                          | Computations executed?  | Computations simulated?     |
@@ -998,7 +1045,7 @@ The first line is the header - you must include it.  The following
 line means that the code between two consecutive MPI calls on line 30
 in exchange_1.f and line 130 in exchange_1.f should receive a speedup
 of 1.18244559422142. The value for the second column is therefore a
-speedup, if it is larger than 1 and a slow-down if it is smaller
+speedup, if it is larger than 1 and a slowdown if it is smaller
 than 1. Nothing will be changed if it is equal to 1.
 
 Of course, you can set any arbitrary filenames you want (so the start
@@ -1021,9 +1068,9 @@ The possible throughput of network links is often dependent on the
 message sizes, as protocols may adapt to different message sizes. With
 this option, a series of message sizes and factors are given, helping
 the simulation to be more realistic. For instance, the current default
-value means that messages with size 65472 and more will get a total of
+value means that messages with size 65472 bytes and more will get a total of
 MAX_BANDWIDTH*0.940694, messages of size 15424 to 65471 will get
-MAX_BANDWIDTH*0.697866 and so on (where MAX_BANDWIDTH denotes the
+MAX_BANDWIDTH*0.697866, and so on (where MAX_BANDWIDTH denotes the
 bandwidth of the link).
 
 An experimental script to compute these factors is available online. See
@@ -1053,9 +1100,9 @@ Keeping temporary files after simulation
 **Option** ``smpi/keep-temps`` **default:** 0 (false)
 
 SMPI usually generates a lot of temporary files that are cleaned after
-use. This option request to preserve them, for example to debug or
+use. This option requests to preserve them, for example to debug or
 profile your code. Indeed, the binary files are removed very early
-under the dlopen privatization schema, which tend to fool the
+under the dlopen privatization schema, which tends to fool the
 debuggers.
 
 .. _cfg=smpi/lat-factor:
@@ -1080,7 +1127,7 @@ Trace hardware counters with PAPI
 
 **Option** ``smpi/papi-events`` **default:** unset
 
-When the PAPI support was compiled in SimGrid, this option takes the
+When the PAPI support is compiled into SimGrid, this option takes the
 names of PAPI counters and adds their respective values to the trace
 files (See Section :ref:`tracing_tracing_options`).
 
@@ -1108,14 +1155,14 @@ Automatic Privatization of Global Variables
 
 **Option** ``smpi/privatization`` **default:** "dlopen" (when using smpirun)
 
-MPI executables are usually meant to be executed in separated
+MPI executables are usually meant to be executed in separate
 processes, but SMPI is executed in only one process. Global variables
-from executables will be placed in the same memory zone and shared
+from executables will be placed in the same memory region and shared
 between processes, causing intricate bugs.  Several options are
 possible to avoid this, as described in the main `SMPI publication
 <https://hal.inria.fr/hal-01415484>`_ and in the :ref:`SMPI
 documentation <SMPI_what_globals>`. SimGrid provides two ways of
-automatically privatizing the globals, and this option allows to
+automatically privatizing the globals, and this option allows one to
 choose between them.
 
   - **no** (default when not using smpirun): Do not automatically
@@ -1146,7 +1193,7 @@ Multiple libraries can be given, semicolon separated.
 
 This configuration option can only use either full paths to libraries,
 or full names.  Check with ldd the name of the library you want to
-use.  Example:
+use.  For example:
 
 .. code-block:: shell
 
@@ -1168,9 +1215,9 @@ Simulating MPI detached send
 
 This threshold specifies the size in bytes under which the send will
 return immediately. This is different from the threshold detailed in
-:ref:`options_model_network_asyncsend` because the message is not
-effectively sent when the send is posted. SMPI still waits for the
-correspondant receive to be posted to perform the communication
+:ref:`cfg=smpi/async-small-thresh` because the message is not
+really sent when the send is posted. SMPI still waits for the
+corresponding receive to be posted, in order to perform the communication
 operation.
 
 .. _cfg=smpi/coll-selector:
@@ -1183,8 +1230,8 @@ Simulating MPI collective algorithms
 SMPI implements more than 100 different algorithms for MPI collective
 communication, to accurately simulate the behavior of most of the
 existing MPI libraries. The ``smpi/coll-selector`` item can be used to
-use the decision logic of either OpenMPI or MPICH libraries (by
-default SMPI uses naive version of collective operations).
+select the decision logic either of the OpenMPI or the MPICH libraries. (By
+default SMPI uses naive version of collective operations.)
 
 Each collective operation can be manually selected with a
 ``smpi/collective_name:algo_name``. Available algorithms are listed in
@@ -1212,10 +1259,10 @@ Reduce speed for iprobe calls
 **Option** ``smpi/iprobe-cpu-usage`` **default:** 1 (no change)
 
 MPI_Iprobe calls can be heavily used in applications. To account
-correctly for the energy cores spend probing, it is necessary to
+correctly for the energy that cores spend probing, it is necessary to
 reduce the load that these calls cause inside SimGrid.
 
-For instance, we measured a max power consumption of 220 W for a
+For instance, we measured a maximum power consumption of 220 W for a
 particular application but only 180 W while this application was
 probing. Hence, the correct factor that should be passed to this
 option would be 180/220 = 0.81.
@@ -1260,7 +1307,7 @@ values, for example ``1:3:2;10:5:1``.  The sections are divided by ";"
 so this example contains two sections.  Furthermore, each section
 consists of three values.
 
-1. The first value denotes the minimum size for this section to take effect;
+1. The first value denotes the minimum size in bytes for this section to take effect;
    read it as "if message size is greater than this value (and other section has a larger
    first value that is also smaller than the message size), use this".
    In the first section above, this value is "1".
@@ -1274,7 +1321,7 @@ consists of three values.
    and hence accounts also for larger messages. In the first
    section of the example above, this value is "2".
 
-Now, SMPI always checks which section it should take for a given
+Now, SMPI always checks which section it should use for a given
 message; that is, if a message of size 11 is sent with the
 configuration of the example above, only the second section will be
 used, not the first, as the first value of the second section is
@@ -1305,7 +1352,7 @@ Inject constant times for MPI_Test
 By setting this option, you can control the amount of time a process
 sleeps when MPI_Test() is called; this is important, because SimGrid
 normally only advances the time while communication is happening and
-thus, MPI_Test will not add to the time, resulting in a deadlock if
+thus, MPI_Test will not add to the time, resulting in deadlock if it is
 used as a break-condition as in the following example:
 
 .. code-block:: cpp
@@ -1315,8 +1362,8 @@ used as a break-condition as in the following example:
        ...
    }
 
-To speed up execution, we use a counter to keep track on how often we
-already checked if the handle is now valid or not. Hence, we actually
+To speed up execution, we use a counter to keep track of how often we
+checked if the handle is now valid or not. Hence, we actually
 use counter*SLEEP_TIME, that is, the time MPI_Test() causes the
 process to sleep increases linearly with the number of previously
 failed tests. This behavior can be disabled by setting
@@ -1333,36 +1380,36 @@ Factorize malloc()s
 
 If your simulation consumes too much memory, you may want to modify
 your code so that the working areas are shared by all MPI ranks. For
-example, in a bloc-cyclic matrix multiplication, you will only
-allocate one set of blocs, and every processes will share them.
+example, in a block-cyclic matrix multiplication, you will only
+allocate one set of blocks, and all processes will share them.
 Naturally, this will lead to very wrong results, but this will save a
-lot of memory so this is still desirable for some studies. For more on
+lot of memory. So this is still desirable for some studies. For more on
 the motivation for that feature, please refer to the `relevant section
 <https://simgrid.github.io/SMPI_CourseWare/topic_understanding_performance/matrixmultiplication>`_
 of the SMPI CourseWare (see Activity #2.2 of the pointed
-assignment). In practice, change the call to malloc() and free() into
+assignment). In practice, change the calls for malloc() and free() into
 SMPI_SHARED_MALLOC() and SMPI_SHARED_FREE().
 
 SMPI provides two algorithms for this feature. The first one, called 
-``local``, allocates one bloc per call to SMPI_SHARED_MALLOC() in your
-code (each call location gets its own bloc) and this bloc is shared
-amongst all MPI ranks.  This is implemented with the shm_* functions
+``local``, allocates one block per call to SMPI_SHARED_MALLOC()
+(each call site gets its own block) ,and this block is shared
+among all MPI ranks.  This is implemented with the shm_* functions
 to create a new POSIX shared memory object (kept in RAM, in /dev/shm)
-for each shared bloc.
+for each shared block.
 
 With the ``global`` algorithm, each call to SMPI_SHARED_MALLOC()
-returns a new adress, but it only points to a shadow bloc: its memory
-area is mapped on a 1MiB file on disk. If the returned bloc is of size
-N MiB, then the same file is mapped N times to cover the whole bloc.
-At the end, no matter how many SMPI_SHARED_MALLOC you do, this will
+returns a new address, but it only points to a shadow block: its memory
+area is mapped on a 1 MiB file on disk. If the returned block is of size
+N MiB, then the same file is mapped N times to cover the whole block.
+At the end, no matter how many times you call SMPI_SHARED_MALLOC, this will
 only consume 1 MiB in memory.
 
 You can disable this behavior and come back to regular mallocs (for
-example for debugging purposes) using @c "no" as a value.
+example for debugging purposes) using ``no`` as a value.
 
 If you want to keep private some parts of the buffer, for instance if these
 parts are used by the application logic and should not be corrupted, you
-can use SMPI_PARTIAL_SHARED_MALLOC(size, offsets, offsets_count). Example:
+can use SMPI_PARTIAL_SHARED_MALLOC(size, offsets, offsets_count). For example:
 
 .. code-block:: cpp
 
@@ -1375,8 +1422,8 @@ Then, it can be deallocated by calling SMPI_SHARED_FREE(mem).
 
 When smpi/shared-malloc:global is used, the memory consumption problem
 is solved, but it may induce too much load on the kernel's pages table.
-In this case, you should use huge pages so that we create only one
-entry per Mb of malloced data instead of one entry per 4k.
+In this case, you should use huge pages so that the kernel creates only one
+entry per MB of malloced data instead of one entry per 4 kB.
 To activate this, you must mount a hugetlbfs on your system and allocate
 at least one huge page:
 
@@ -1400,7 +1447,7 @@ Inject constant times for MPI_Wtime, gettimeofday and clock_gettime
 This option controls the amount of (simulated) time spent in calls to
 MPI_Wtime(), gettimeofday() and clock_gettime(). If you set this value
 to 0, the simulated clock is not advanced in these calls, which leads
-to issue if your application contains such a loop:
+to issues if your application contains such a loop:
 
 .. code-block:: cpp
 
@@ -1409,15 +1456,15 @@ to issue if your application contains such a loop:
    }
 
 When the option smpi/wtime is set to 0, the time advances only on
-communications and computations, so the previous code results in an
+communications and computations. So the previous code results in an
 infinite loop: the current [simulated] time will never reach
 ``some_time_bound``.  This infinite loop is avoided when that option
-is set to a small amount, as it is by default since SimGrid v3.21.
+is set to a small value, as it is by default since SimGrid v3.21.
 
 Note that if your application does not contain any loop depending on
 the current time only, then setting this option to a non-zero value
 will slow down your simulations by a tiny bit: the simulation loop has
-to be broken and reset each time your code ask for the current time.
+to be broken out of and reset each time your code asks for the current time.
 If the simulation speed really matters to you, you can avoid this
 extra delay by setting smpi/wtime to 0.
 
@@ -1432,7 +1479,7 @@ Cleanup at Termination
 **Option** ``debug/clean-atexit`` **default:** on
 
 If your code is segfaulting during its finalization, it may help to
-disable this option to request SimGrid to not attempt any cleanups at
+disable this option to request that SimGrid not attempt any cleanups at
 the end of the simulation. Since the Unix process is ending anyway,
 the operating system will wipe it all.
 
@@ -1443,7 +1490,7 @@ Search Path
 
 **Option** ``path`` **default:** . (current dir)
 
-It is possible to specify a list of directories to search into for the
+It is possible to specify a list of directories to search in for the
 trace files (see :ref:`pf_trace`) by using this configuration
 item. To add several directory to the path, set the configuration
 item several times, as in ``--cfg=path:toto --cfg=path:tutu``
@@ -1476,7 +1523,7 @@ Behavior on Ctrl-C
 
 By default, when Ctrl-C is pressed, the status of all existing actors
 is displayed before exiting the simulation. This is very useful to
-debug your code, but it can reveal troublesome if you have many
+debug your code, but it can become troublesome if you have many
 actors. Set this configuration item to **off** to disable this
 feature.
 
@@ -1489,14 +1536,126 @@ Truncate local path from exception backtrace
 
 This configuration option is used to remove the path from the
 backtrace shown when an exception is thrown. This is mainly useful for
-the tests: the full file path makes the tests not reproducible because
-the path of source files depend of the build settings. That would
-break most of our tests as we keep comparing output.
+the tests: the full file path would makes the tests non-reproducible because
+the paths of source files depend of the build settings. That would
+break most of the tests since their output is continually compared.
+
+.. _logging_config:
 
-Logging Configuration
+Logging configuration
 ---------------------
 
-It can be done by using XBT. Go to :ref:`XBT_log` for more details.
+As introduced in :ref:`outcome_logs`, the SimGrid logging mechanism allows to configure at runtime the messages that should be displayed and those that should be omitted. Each
+message produced in the code is given a category (denoting its topic) and a priority. Then at runtime, each category is given a threshold (only messages of priority higher than
+that threshold are displayed), a layout (deciding how the messages in this category are formatted), and an appender (deciding what to do with the message: either print on stderr or
+to a file).
+
+This section explains how to configure this logging features. You can also refer to the documentation of the :ref:`programmer's interface <logging_prog>`, that allows to produce
+messages from your code.
+
+Most of the time, the logging mechanism is configured at runtime using the ``--log`` command-line argument, even if you can also use :c:func:`xbt_log_control_set()` to control it from
+your program. To pass configure more than one setting, you can either pass several ``--log`` arguments, or separate your settings with spaces, that must be quoted accordingly. In
+practice, the following is equivalent to the above settings: ``--log=root.thresh:error --log=s4u_host.thresh:debug``.
+
+If you want to specify more than one setting, you can either pass several ``--log`` argument to your program as above, or separate them with spaces. In this case, you want to quote
+your settings, as in ``--log="root.thresh:error s4u_host.thresh:debug"``. The parameters are interpreted in order, from left to right.
+
+
+Threshold configuration
+.......................
+
+The keyword ``threshold`` controls which logging event will get displayed in a given category. For example, ``--log=root.threshold:debug`` displays *every* message produced in the
+``root`` category and its subcategories (i.e., every message produced -- this is *extremely* verbose), while ``--log=root.thres:critical`` turns almost everything off. As you can
+see, ``threshold`` can be abbreviated here.
+
+Existing thresholds:
+
+ - ``trace`` some functions display a message at this level when entering or returning
+ - ``debug`` output that is mostly useful when debugging the corresponding module.
+ - ``verbose`` verbose output that is only mildly interesting and can easily be ignored
+ - ``info`` usual output (this is the default threshold of all categories)
+ - ``warning`` minor issue encountered
+ - ``error`` issue encountered
+ - ``critical`` major issue encountered, such as assertions failures
+
+.. _log/fmt:
+
+Format configuration
+....................
+
+The keyword ``fmt`` controls the layout (the format) of a logging category. For example, ``--log=root.fmt:%m`` reduces the output to the user-message only, removing any decoration such
+as the date, or the actor ID, everything. Existing format directives:
+
+ - %%: the % char
+ - %n: line separator (LOG4J compatible)
+ - %e: plain old space (SimGrid extension)
+
+ - %m: user-provided message
+
+ - %c: Category name (LOG4J compatible)
+ - %p: Priority name (LOG4J compatible)
+
+ - %h: Hostname (SimGrid extension)
+ - %a: Actor name (SimGrid extension -- note that with SMPI this is the integer value of the process rank)
+ - %i: Actor PID (SimGrid extension -- this is a 'i' as in 'i'dea)
+ - %t: Thread "name" (LOG4J compatible -- actually the address of the thread in memory)
+
+ - %F: file name where the log event was raised (LOG4J compatible)
+ - %l: location where the log event was raised (LOG4J compatible, like '%%F:%%L' -- this is a l as in 'l'etter)
+ - %L: line number where the log event was raised (LOG4J compatible)
+ - %M: function name (LOG4J compatible -- called method name here of course).
+
+ - %d: date (UNIX-like epoch)
+ - %r: application age (time elapsed since the beginning of the application)
+
+
+``--log=root.fmt:'[%h:%a:(%i) %r] %l: %m%n'`` gives you the default layout used for info messages while ``--log=root.fmt:'[%h:%a:(%i) %r] %l: [%c/%p] %m%n'`` gives you the default
+layout for the other priorities (it adds the source code location). Also, the actor identification is omitted by the default layout for the messages coming directly from the
+SimGrid kernel, so info messages are formatted with ``[%r] [%c/%p] %m%n`` in this case. When specifying the layout manually, such distinctions are currently impossible, and the
+provided layout is used for every messages.
+
+As with printf, you can specify the precision and width of the fields. For example, ``%.4r`` limits the date precision to four digits while ``%15h`` limits the host name to at most
+15 chars.
+
+
+If you want to have spaces in your log format, you should protect it. Otherwise, SimGrid will consider that this is a space-separated list of several parameters. But you should
+also protect it from the shell that also splits command line arguments on spaces. At the end, you should use something such as ``--log="'root.fmt:%l: [%p/%c]: %m%n'"``. 
+Another option is to use the ``%e`` directive for spaces, as in ``--log=root.fmt:%l:%e[%p/%c]:%e%m%n``. 
+
+Category appender
+.................
+
+The keyword ``app`` controls the appended of a logging category. For example ``--log=root.app:file:mylogfile`` redirects every output to the file ``mylogfile``.
+
+With the ``splitfile`` appender, a new file is created when the size of the output reaches the specified size. The format is ``--log=root.app:splitfile:<size>:<file name>``. For
+example, ``--log=root.app:splitfile:500:mylog_%`` creates log files of at most 500 bytes, using the names ``mylog_0``, ``mylog_1``, ``mylog_2``, etc.
+
+The ``rollfile`` appender uses one file only, but the file is emptied and recreated when its size reaches the specified maximum. For example, ``--log=root.app:rollfile:500:mylog``
+ensures that the log file ``mylog`` will never overpass 500 bytes in size.
+
+Any appender setup this way have its own layout format, that you may change afterward. When specifying a new appender, its additivity is set to false to prevent log event displayed
+by this appender to "leak" to any other appender higher in the hierarchy. You can naturally change that if you want your messages to be displayed twice.
+
+Category additivity
+...................
+
+The keyword ``add`` controls the additivity of a logging category. By default, the messages are only passed one appender only: the more specific, i.e. the first one found when
+climbing the tree from the category in which they were produced. In Log4J parlance, it is said that the default additivity of appenders is false. If you change this setting to
+``on`` (or ``yes`` or ``1``), the produced messages will also be passed to the upper appender.
+
+Let's consider a more complex example: ``--log="root.app:file:all.log s4u.app:file:iface.log xbt.app:file:xbt.log xbt.add:yes``. Here, the logging of s4u will be sent to the
+``iface.log`` file; the logging of the xbt toolbox will be sent to both the ``xbt.log`` file and the ``all.log`` file (because xbt additivity was enabled); and every other loggings
+will only be sent to ``all.log``.
+
+Other options
+.............
+
+``--help-logs`` displays a complete help message about logging in SimGrid.
+
+``--help-log-categories`` displays the actual hierarchy of log categories for this binary.
+
+``--log=no_loc`` hides the source locations (file names and line numbers) from the messages. This is useful to make tests reproducible.
+
 
 .. |br| raw:: html