+++ /dev/null
-.. _intro_concepts:
-
-Main Concepts
-=============
-
-Typical Study based on SimGrid
-------------------------------
-
-.. raw:: html
-
- <object data="graphical-toc.svg" width="100%" type="image/svg+xml"></object>
-
-
-Any SimGrid study entails the following components:
-
- - The studied **Application**. This can be either a distributed
- algorithm described in our simple APIs, or a full featured real
- parallel application using for example the MPI interface
- :ref:`(more info) <application>`.
-
- - The **Simulated Platform**. This is a description of a given
- distributed system (machines, links, disks, clusters, etc). Most of
- the platform files are written in XML althrough a Lua interface is
- under development. SimGrid makes it easy to augment the Simulated
- Platform with a Dynamic Scenario where for example the links are
- slowed down (because of external usage) or the machines fail. You
- have even support to specify the applicative workload that you want
- to feed to your application
- :ref:`(more info) <platform>`.
-
- - The application's **Deployment Description**. In SimGrid
- terminology, the application is an inert set of source files and
- binaries. To make it run, you have to describe how your application
- should be deployed on the simulated platform. You need to specify
- which process is mapped on which machine, along with their parameters
- :ref:`(more info) <scenario>`.
-
- - The **Platform Models**. They describe how the simulated platform
- reacts to the actions of the application. For example, they compute
- the time taken by a given communication on the simulated platform.
- These models are already included in SimGrid, and you only need to
- pick one and maybe tweak its configuration to get your results
- :ref:`(more info) <models>`.
-
-These components are put together to run a **simulation**, that is an
-experiment or a probe. The result of one or many simulation provides
-an **outcome** (logs, visualization, or statistical analysis) that help
-answering the **question** targeted by this study.
-
-Here are some questions on which SimGrid is particularly relevant:
-
- - **Compare an Application to another**. This is the classical use
- case for scientists, who use SimGrid to test how the solution that
- they contribute to compares to the existing solutions from the
- literature.
-
- - **Design the best [Simulated] Platform for a given Application.**
- Tweaking the platform file is much easier than building a new real
- platform for testing purpose. SimGrid also allows for the co-design
- of the platform and the application by modifying both of them.
-
- - **Debug Real Applications**. With real systems, is sometimes
- difficult to reproduce the exact run leading to the bug that you
- are tracking. With SimGrid, you are *clairvoyant* about your
- *reproducible experiments*: you can explore every part of the
- system, and your probe will not change the simulated state. It also
- makes it easy to mock some parts of the real system that are not
- under study.
-
-Depending on the context, you may see some parts of this process as
-less important, but you should pay close attention if you want to be
-confident in the results coming out of your simulations. In
-particular, you should not blindly trust your results but always
-strive to double-check them. Likewise, you should question the realism
-of your input configuration, and we even encourage you to doubt (and
-check) the provided performance models.
-
-To ease such questioning, you really should logically separate these
-parts in your experimental setup. It is seen as a very bad practice to
-merge the application, the platform, and the deployment all together.
-SimGrid is versatile and your mileage may vary, but you should start
-with your Application specified as a C++ or Java program, using one of
-the provided XML platform file, and with your deployment in a separate
-XML file.
-
-SimGrid Execution Modes
------------------------
-
-Depending on the intended study, SimGrid can be run in several execution modes.
-
-**Simulation Mode**. This is the most common execution mode, where you want
-to study how your application behaves on the simulated platform under
-the experimental scenario.
-
-In this mode, SimGrid can provide information about the time taken by
-your application, the amount of energy dissipated by the platform to
-run your application, and the detailed usage of each resource.
-
-**Model-Checking Mode**. This can be seen as a sort of exhaustive
-testing mode, where every possible outcome of your application is
-explored. In some sense, this mode tests your application for all
-possible platforms that you could imagine (and more).
-
-You just provide the application and its deployment (amount of
-processes and parameters), and the model-checker will literally
-explore all possible outcomes by testing all possible message
-interleavings: if at some point a given process can either receive the
-message A first or the message B depending on the platform
-characteristics, the model-checker will explore the scenario where A
-arrives first, and then rewind to the same point to explore the
-scenario where B arrives first.
-
-This is a very powerful mode, where you can evaluate the correction of
-your application. It can verify either **safety properties** (asserts)
-or **liveless properties** stating for example that if a given event
-occurs, then another given event will occur in a finite amount of
-steps. This mode is not only usable with the abstract algorithms
-developed on top of the SimGrid APIs, but also with real MPI
-applications (to some extent).
-
-The main limit of Model Checking lays in the huge amount of scenarios
-to explore. SimGrid tries to explore only non-redundant scenarios
-thanks to classical reduction techniques (such as DPOR and stateful
-exploration) but the exploration may well never finish if you don't
-carefully adapt your application to this mode.
-
-A classical trap is that the Model Checker can only verify whether
-your application fits the provided properties, which is useless if you
-have a bug in your property. Remember also that one way for your
-application to never violate a given assert is to not start at all
-because of a stupid bug.
-
-Another limit of this mode is that it does not use the performance
-models of the simulation mode. Time becomes discrete: You can say for
-example that the application took 42 steps to run, but there is no way
-to know how much time it took or the amount of watts that were dissipated.
-
-Finally, the model checker only explores the interleavings of
-computations and communications. Other factors such as thread
-execution interleaving are not considered by the SimGrid model
-checker.
-
-The model checker may well miss existing issues, as it computes the
-possible outcomes *from a given initial situation*. There is no way to
-prove the correction of your application in all generality with this
-tool.
-
-**Benchmark Recording Mode**. During debug sessions, continuous
-integration testing, and other similar use cases, you are often only
-interested in the control flow. If your application apply filters to
-huge images split in small blocks, the filtered image is probably not
-what you are interested in. You are probably looking for a way to run
-each computation kernel only once, save on disk the time it takes and
-some other metadata. This code block can then be skipped in simulation
-and replaced by a synthetic block using the cached information. The
-simulated platform will take this block into account without requesting
-the real hosting machine to benchmark it.
-
-SimGrid Limits
---------------
-
-This framework is by no means the perfect holly grail able to solve
-every problem on earth.
-
-**SimGrid scope is limited to distributed systems.** Real-time
-multi-threaded systems are out of scope. You could probably tweak
-SimGrid for such studies (or the framework could possibly be extended
-in this direction), but another framework specifically targeting such a
-use case would probably be more suited.
-
-**There is currently no support for wireless networks**.
-The framework could certainly be improved in this direction, but this
-still has to be done.
-
-**There is no perfect model, only models adapted to your study.**
-The SimGrid models target fast and large studies yet requesting
-realistic results. In particular, our models abstract away parameters
-and phenomena that are often irrelevant to the realism in our
-context.
-
-SimGrid is simply not intended to any study that would mandate the
-abstracted phenomenon. Here are some **studies that you should not do
-with SimGrid**:
-
- - Studying the effect of L3 vs. L2 cache effects on your application
- - Comparing kernel schedulers and policies
- - Comparing variants of TCP
- - Exploring pathological cases where TCP breaks down, resulting in
- abnormal executions.
- - Studying security aspects of your application, in presence of
- malicious agents.
-
-SimGrid Success Stories
------------------------
-
-SimGrid was cited in over 1,500 scientific papers (according to Google
-Scholar). Among them
-`over 200 publications <https://simgrid.org/Usages.html>`_
-(written by about 300 individuals) use SimGrid as a scientific
-instrument to conduct their experimental evaluation. These
-numbers do not include the articles contributing to SimGrid.
-This instrument was used in many research communities, such as
-`High-Performance Computing <https://hal.inria.fr/inria-00580599/>`_,
-`Cloud Computing <http://dx.doi.org/10.1109/CLOUD.2015.125>`_,
-`Workflow Scheduling <http://dl.acm.org/citation.cfm?id=2310096.2310195>`_,
-`Big Data <https://hal.inria.fr/hal-01199200/>`_ and
-`MapReduce <http://dx.doi.org/10.1109/WSCAD-SSC.2012.18>`_,
-`Data Grid <http://ieeexplore.ieee.org/document/7515695/>`_,
-`Volunteer Computing <http://www.sciencedirect.com/science/article/pii/S1569190X17301028>`_,
-`Peer-to-Peer Computing <https://hal.archives-ouvertes.fr/hal-01152469/>`_,
-`Network Architecture <http://dx.doi.org/10.1109/TPDS.2016.2613043>`_,
-`Fog Computing <http://ieeexplore.ieee.org/document/7946412/>`_, or
-`Batch Scheduling <https://hal.archives-ouvertes.fr/hal-01333471>`_
-`(more info) <https://simgrid.org/Usages.html>`_.
-
-If your platform description is accurate enough (see
-`here <http://hal.inria.fr/hal-00907887>`_ or
-`there <https://hal.inria.fr/hal-01523608>`_),
-SimGrid can provide high-quality performance predictions. For example,
-we determined the speedup achieved by the Tibidabo ARM-based
-cluster before its construction
-(`paper <http://hal.inria.fr/hal-00919507>`_). In this case,
-some differences between the prediction and the real timings were due to
-misconfiguration or other problems with the real platform. To some extent,
-SimGrid could even be used to debug the real platform :)
-
-SimGrid is also used to debug, improve, and tune several large
-applications.
-`BigDFT <http://bigdft.org>`_ (a massively parallel code
-computing the electronic structure of chemical elements developped by
-the CEA), `StarPU <http://starpu.gforge.inria.fr/>`_ (a
-Unified Runtime System for Heterogeneous Multicore Architectures
-developped by Inria Bordeaux) and
-`TomP2P <https://tomp2p.net/dev/simgrid/>`_ (a high performance
-key-value pair storage library developed at University of Zurich).
-Some of these applications enjoy large user communities themselves.
-
-.. LocalWords: SimGrid
-
