- **smpi/barrier-finalization:** :ref:`cfg=smpi/barrier-finalization`
- **smpi/barrier-collectives:** :ref:`cfg=smpi/barrier-collectives`
- **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`
- **smpi/cpu-threshold:** :ref:`cfg=smpi/cpu-threshold`
- **smpi/iprobe-cpu-usage:** :ref:`cfg=smpi/iprobe-cpu-usage`
- **smpi/init:** :ref:`cfg=smpi/init`
- **smpi/keep-temps:** :ref:`cfg=smpi/keep-temps`
-- **smpi/lat-factor:** :ref:`cfg=smpi/lat-factor`
- **smpi/ois:** :ref:`cfg=smpi/ois`
- **smpi/or:** :ref:`cfg=smpi/or`
- **smpi/os:** :ref:`cfg=smpi/os`
.. _cfg=network/latency-factor:
.. _cfg=network/weight-S:
-Correcting Important Network Parameters
-^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-
-SimGrid can take network irregularities such as a slow startup or
-changing behavior depending on the message size into account. You
-should not change these values unless you really know what you're
-doing. The corresponding values were computed through data fitting
-one the timings of packet-level simulators, as described in `Accuracy
-Study and Improvement of Network Simulation in the SimGrid Framework
-<http://mescal.imag.fr/membres/arnaud.legrand/articles/simutools09.pdf>`_.
-
-- **network/latency-factor**: apply a multiplier to latency.
- Models the TCP slow-start mechanism.
-- **network/bandwidth-factor**: actual bandwidth perceived by the
- user.
-- **network/weight-S**: bottleneck sharing constant parameter. Used
- to calculate RTT.
-
-These parameters are the same for all communications in your simulation,
-independently of message size or source/destination hosts. A more flexible
-mechanism based on callbacks was introduced in SimGrid. It provides the user
-a callback that will be called for each communication, allowing the user
-to set different latency and bandwidth factors, based on the message size, links used
-or zones traversed. To more details of how to use it, please look at the
-`examples/cpp/network-factors/s4u-network-factors.cpp <https://framagit.org/simgrid/simgrid/tree/master/examples/cpp/network-factors/s4u-network-factors.cpp>`_.
-
-
-If you are using the SMPI model, these correction coefficients are
-themselves corrected by constant values depending on the size of the
-exchange. By default SMPI uses factors computed on the Stampede
-Supercomputer at TACC, with optimal deployment of processes on
-nodes. Again, only hardcore experts should bother about this fact.
-For more details, see SMPI sections about :ref:`cfg=smpi/bw-factor` and :ref:`cfg=smpi/lat-factor`.
+Manual calibration factors
+^^^^^^^^^^^^^^^^^^^^^^^^^^
+SimGrid can take network irregularities such as a slow startup or changing behavior depending on the message size into account.
+The values provided by default were computed through data fitting one the timings of either packet-level simulators or direct
+experiments on real platforms. These default values should be OK for most users, but simulation realism is really important to
+you, you probably want to recalibrate the models (i.e., devise sensible values for your specific settings). This section only
+describes how to pass new values to the models while the calibration process involved in the computation of these values is
+described :ref:`in the relevant chapter <models_calibration>`.
+
+We found out that many networking effects can be realistically modeled with three following correction factors. They were shown
+to be enough to capture slow-start effects, the different transmission modes of MPI systems (eager vs. rendez-vous mode), or the
+non linear effects of wifi sharing.
+
+**Option** ``network/latency-factor`` **Default:** 1.0, but overridden by most models
+
+This option specifies a multiplier to apply to the platform latency. The factor can either be a constant to apply to any
+communication, or it can depend on the message size. The ``CM02`` model does not use any correction factor, so the
+latency-factor remains to 1. The ``LV08`` model sets it to 13.01 to model slow-start, while the ``SMPI`` model has several
+values, each for an interval of sizes. The default SMPI setting given below specifies for example that a message smaller than
+257 bytes will get a latency multiplier of 2.01467 while a message which size is in [15424, 65472] will get a latency multiplier
+of 3.48845. The ``wifi`` model goes further and uses a callback in the program to compute the factor that must be non-linear in
+this case.
+
+This multiplier is applied to the latency computed from the platform, that is the sum of all link latencies over the :ref:`network
+path <platform_routing>` used by the considered communication.
+
+Constant factors are easy to express, but the interval-based syntax used in SMPI is somewhat complex. It expects a set of
+factors separated by semicolons, each of the form ``boundary:factor``. For example if your specification is
+``0:1;1000:2;5000:3``, it means that on [0, 1000) the factor is 1. On [1000,5000), the factor is 2 while the factor is 3 for
+5000 and beyond. If your first interval does include size=0, then the default value of 1 is used before. Changing the factor
+callback is not possible from the command line and must be done from your code, as shown in `this example
+<https://framagit.org/simgrid/simgrid/tree/master/examples/cpp/network-factors/s4u-network-factors.cpp>`_. Note that the chosen
+model only provide some default setting, not more. You can pick a ``LV08`` to get some of the settings, and override the latency
+with an interval-based value.
+
+SMPI default value: 65472:11.6436; 15424:3.48845; 9376:2.59299; 5776:2.18796; 3484:1.88101; 1426:1.61075; 732:1.9503;
+257:1.95341;0:2.01467 (interval boundaries are sorted automatically). These values were computed by data fitting on the Stampede
+Supercomputer at TACC, with optimal deployment of processes on nodes. To accurately model your settings, you should redo the
+:ref:`calibration <models_calibration>`.
+
+**Option** ``network/bandwidth-factor`` **Default:** 1.0, but overridden by most models
+
+Setting this option automatically adjusts the bandwidth used by any given communication. As with latency-factor above, the value
+can be a constant (``CM02`` uses 1 -- no correction -- while ``LV08`` uses 0.97 to discount TCP headers while computing the
+payload bandwidth), interval-based (as the default provided by the ``SMPI``), or using in-program callbacks (as with ``wifi``).
+
+SMPI default value: 65472:0.940694;15424:0.697866;9376:0.58729;5776:1.08739;3484:0.77493;1426:0.608902;732:0.341987;257:0.338112;0:0.812084
+This was also computed on the Stampede Supercomputer.
+
+**Option** ``network/weight-S`` **Default:** depends on the model
+
+Value of the bottleneck sharing, that is used to calculate RTT. Described in `Accuracy Study and Improvement of Network
+Simulation in the SimGrid Framework <http://mescal.imag.fr/membres/arnaud.legrand/articles/simutools09.pdf>`_
+
+Default values for ``CM02`` is 0. ``LV08`` sets it to 20537 while both ``SMPI`` and ``IB`` set it to 8775.
+
+.. _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/IB-penalty-factors:
^^^^^^^^^^^^^^^^
InfiniBand network behavior can be modeled through 3 parameters
-``smpi/IB-penalty-factors:"βe;βs;γs"``, as explained in `this PhD
-thesis
+``smpi/IB-penalty-factors:"βe;βs;γs"``, as explained in `the PhD
+thesis of Jean-Marc Vincent
<http://mescal.imag.fr/membres/jean-marc.vincent/index.html/PhD/Vienne.pdf>`_ (in French)
or more concisely in `this paper <https://hal.inria.fr/hal-00953618/document>`_,
even if that paper does only describe models for myrinet and ethernet.
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 Asynchronous Send
Configuring SMPI
----------------
-The SMPI interface provides several specific configuration items.
-These are not easy to see, since the code is usually launched through the
-``smiprun`` script directly.
+The SMPI interface provides several specific configuration items.
+These are not easy to see with ``--help-cfg``, since SMPI binaries are usually launched through the ``smiprun`` script.
.. _cfg=smpi/host-speed:
.. _cfg=smpi/cpu-threshold:
smpicc or smpiff, respectively. This flag activates some internal
macro definitions that help with obtaining the call location.
-.. _cfg=smpi/bw-factor:
-
-Bandwidth Factors
-.................
-
-**Option** ``smpi/bw-factor``
-|br| **Default:** 65472:0.940694;15424:0.697866;9376:0.58729;5776:1.08739;3484:0.77493;1426:0.608902;732:0.341987;257:0.338112;0:0.812084
-
-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 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
-bandwidth of the link).
+Bandwidth and latency factors
+.............................
-An experimental script to compute these factors is available online. See
-https://framagit.org/simgrid/platform-calibration/
-https://simgrid.org/contrib/smpi-saturation-doc.html
+Adapting the bandwidth and latency acurately to the network conditions is of a paramount importance to get realistic results.
+This is done through the :ref:`network/bandwidth-factor <cfg=network/bandwidth-factor>` and :ref:`network/latency-factor
+<cfg=network/latency-factor>` items. You probably also want to read the following section: :ref:`models_calibration`.
.. _cfg=smpi/display-timing:
under the dlopen privatization schema, which tends to fool the
debuggers.
-.. _cfg=smpi/lat-factor:
-
-Latency factors
-...............
-
-**Option** ``smpi/lat-factor`` |br|
-**default:** 65472:11.6436;15424:3.48845;9376:2.59299;5776:2.18796;3484:1.88101;1426:1.61075;732:1.9503;257:1.95341;0:2.01467
-
-The motivation and syntax for this option is identical to the motivation/syntax
-of :ref:`cfg=smpi/bw-factor`.
-
-There is an important difference, though: While smpi/bw-factor `reduces` the
-actual bandwidth (i.e., values between 0 and 1 are valid), latency factors
-increase the latency, i.e., values larger than or equal to 1 are valid here.
-
.. _cfg=smpi/papi-events:
Trace hardware counters with PAPI
