^^^^^^^^^^^^^^^^^^^^^^^^^^
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
+The values provided by default were computed a long time ago 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 if 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
+We found out that many networking effects can be realistically accounted for with the 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
+This option specifies a multiplier to apply to the *physical* latency (i.e., the one described in the platform) of the set of links involved in a communication. The factor can either be a constant to apply to any
+communication, or it may 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
+possible values depending on the interval in which the message size falls. 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 whose 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.
+This multiplier is applied to the latency computed from the platform, that is the sum of all link *physical* latencies over the :ref:`network path <platform_routing>` used by the considered communication, to derive the *effective* end-to-end latency.
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
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
+model only provide some default settings, 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;
**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
+Setting this option automatically adjusts the *effective* bandwidth (i.e., the one perceived by the application) 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``).
**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
+Value used to account for RTT-unfairness when sharing a bottleneck (network connections with a large RTT are generally penalized against those with a small one). 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.
Configuring loopback link
^^^^^^^^^^^^^^^^^^^^^^^^^
-Several network model provide an implicit loopback link to account for local
+Several network models 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.
+items. Note that this loopback is conveniently modeled with a :ref:`single FATPIPE link <pf_loopback>`
+for the whole platform. If modeling contention inside nodes is important then you should
+rather add such loopback links (one for each host) yourself.
.. _cfg=smpi/IB-penalty-factors:
