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+.. _platform:
+
+.. raw:: html
+
+
+
+
+
+
+Modeling Hints
+##############
+
+There is no perfect model, but only models that are adapted to the
+specific study that you want to do. SimGrid provide several advanced
+mechanisms that you can adapt to model the situation that you are
+interested into, and it is often uneasy to see where to start with.
+This page collects several hints and tricks on modeling situations.
+Even if you are looking for a very advanced, specific use case, these
+examples may help you to design the solution you need.
+
+Doing Science with SimGrid
+**************************
+
+Many users are using SimGrid as a scientific instrument for their
+research. This tool was indeed invented to that extend, and we strive
+to streamline this kind of usage. But SimGrid is no magical tool, and
+it is of your responsability that the tool actually provides sensible
+results. Fortunately, there is a vast literature on how to avoid
+Modeling & Simulations pitfalls. We review here two particular works.
+
+In `An Integrated Approach to Evaluating Simulation Credibility
+`_, the authors
+provide a methodology enabling the users to increase their confidence
+in the simulation tools they use. First of all, you must know what you
+actually expect to discover whether the tool actually covers your
+needs. Then, as they say, "a fool with a tool is still a fool", so you
+need to think about your methodology before you submit your articles.
+You really should read this article, that contains many other
+interesting advices.
+
+`Seven Pitfalls in Modeling and Simulation Research
+`_ is even more
+specific. Here are the listed pitfalls: (1) Don't know whether it's
+modeling or simulation, (2) No separation of concerns, (3) No clear
+scientific question, (4) Implementing everything from scratch, (5)
+Unsupported claims, (6) Toy duck approach, and (7) The tunnel view. As
+you can see, this article is a must read. It's a pitty that it's not
+freely available, though.
+
+Modeling Churn in P2P
+*********************
+
+One of the biggest challenges in P2P settings is to cope with the
+churn, meaning that resources keep appearing and disappearing. In
+SimGrid, you can always change the state of each host manually, with
+eg :cpp:func:`simgrid::s4u::Host::turn_on`. To reduce the burden when
+the churn is high, you can also attach a **state profile** to the host
+directly.
+
+This is not possible from S4U yet (TODO), and you should use the
+``state_file`` attribute of :ref:`pf_tag_host`, :ref:`pf_tag_cluster`
+or :ref:`pf_tag_link`.
+
+Every lines (but the last) of such files describe timed events with
+the form "date value". Example:
+
+.. code-block:: python
+
+ 1 0
+ 2 1
+ LOOPAFTER 8
+
+- At time t=1, the host is turned off (value 0 means OFF)
+- At time t=2, it is turned back on (other values means ON)
+- At time t=10, the history is reset (because that's 8 seconds after
+ the last event). So the host will be turned off again at t=11.
+
+If your trace does not contain a LOOPAFTER line, then your profile is
+only executed once and not repetitively.
+
+
+Modeling Multicore Machines
+***************************
+
+Default Model
+=============
+
+Multicore machines are very complex, and there is many way to model
+them. The default models of SimGrid are coarse grain and capture some
+elements of this reality. Here is how to declare simple multicore hosts:
+
+.. code-block:: xml
+
+
+
+It declares a 4-cores host called "mymachine", each core computing 8
+GFlops per second. If you put one activity of 8 GFlop on this host, it
+will be computed in 1 second (by default, activities are
+single-threaded and cannot leverage the computing power of more than
+one core). If you put two of them together, they will still be
+computed in one second, and so on up to 4 tasks. If you put 5 tasks,
+they will share the total computing resource, and all tasks will be
+computed at 5/4 = 1.25 second. That's a very simple model, but that's
+all what you will get by default from SimGrid.
+
+Pinning tasks to cores
+======================
+
+The default model does not account for task pinning, where you
+manually select on which core each of the existing activity should
+execute. The best solution to model this is probably to model your
+4-core processor as 4 separte hosts, and assigning the activities to
+cores by migrating them to the declared hosts. In some sense, this
+takes the whole Network-On-Chip idea really seriously.
+
+Some extra complications may arise here. If you have more tasks than
+cores, you'll have to `schedule your tasks
+`_
+yourself on the cores (so you'd better avoid this complexity). Since
+you cannot have more than one network model in a given SimGrid
+simulation, you will end up with a TCP connexion between your cores. A
+possible work around is to never start any simulated communication
+between the cores and have the same routes from each core to the
+rest of the external network.
+
+Modeling a multicore CPU as a set of SimGrid hosts may seem strange
+and unconvincing, but some users achieved very realistic simulations
+of multi-core and GPU machines this way.
+
+Modeling machine bootup and shutdown periods
+********************************************
+
+When a physical host boots up, a lot of things happen. It takes time
+during which the machine is not usable but dissipates energy, and
+programs actually die and restart during a reboot. Since there is many
+ways to model it, SimGrid does not do any modeling choice for you but
+the most obvious ones.
+
+Any actor (or process in MSG) running on an host that is shut down
+will be killed and all its activities (tasks in MSG) will be
+automatically canceled. If killed the actor was marked as
+auto-restartable (with
+:cpp:func:`simgrid::s4u::Actor::set_auto_restart` or with
+:cpp:func:`MSG_process_auto_restart_set`), it will start anew with the
+same parameters when the host boots back up.
+
+By default, shutdowns and bootups are instantaneous. If you want to
+add an extra delay, you have to do that yourself, for example from an
+`controler` actor that runs on another host. The best way to do so is
+to declare a fictionous pstate where the CPU delivers 0 flop per
+second (so every activity on that host will be frozen when the host is
+in this pstate). When you want to switch the host off, your controler
+switches the host to that specific pstate (with
+:cpp:func:`simgrid::s4u::Host::set_pstate`), waits for the amount of
+time that you decided necessary for your host to shut down, and turns
+the host off (with :cpp:func:`simgrid::s4u::Host::turn_off`). To boot
+up, switch the host on, go into the specific pstate, wait a while and
+go to a more regular pstate.
+
+To model the energy dissipation, you need to put the right energy
+consumption in your startup/shutdown specific pstate. Remember that
+the energy consumed is equal to the instantaneous consumption
+multiplied by the time in which the host keeps in that state. Do the
+maths, and set the right instantaneous consumption to your pstate, and
+you'll get the whole boot period to consume the amount of energy that
+you want. You may want to have one fictionous pstate for the bootup
+period and another one for the shutdown period.
+
+Of course, this is only one possible way to model these things. YMMV ;)
+