.. _community:

The SimGrid Community
=====================

SimGrid is a free software, written by a community of people. It
started as a little software to help ourselves in our own research,
and as more people put their input into the pot, it turned into
something that we hope to be valuable to many people. So yes. We hope
that SimGrid is helping you doing what you want, and that you will
join our community of happy simgriders.

Contacting the community
------------------------

There are several locations where you can connect and discuss about
SimGrid. If you have a question, please have a look at the
documentation and examples first, but if some remain don't hesitate to
ask the community for help. If you do not have a question, just come
to us and say hello! We love earing about how people use SimGrid.

 - For questions or remarks, drop us an email on the `user mailing
   list <mailto:simgrid-community@inria.fr>`_ (to subscribe,
   visit the `web interface
   <https://sympa.inria.fr/sympa/info/simgrid-community>`__).
   We prefer you to **not use private emails**. SimGrid is an open
   framework, and you never know who have the time and knowledge to
   answer your question, so please keep messages on the public mailing
   list.

 - If you want to chat with the community, join us on `Mattermost
   <https://framateam.org/simgrid/channels/town-square>`_. Be warned
   that even if many people are connected to the channel, they may not
   be staring at their chat windows. So don't be surprised if you
   don't get an answer in the second, and please be patient.

   If you prefer, you can reach us on IRC on \#simgrid at
   ``irc.debian.org`` (the `logs are available
   <http://colabti.org/irclogger/irclogger_logs/simgrid>`_). When no
   non-french speaker are connected, we usually chat in french on
   these channel, but we do switch back to english when we have a
   guest.

 - Asking your question on
   `StackOverflow <http://stackoverflow.com/questions/tagged/simgrid>`_
   is also a good idea, as this
   site is very well indexed. We answer questions there too (don't
   forget to use the SimGrid tag in your question so that we can see
   it), and they remain usable for the next users.

Giving back to SimGrid
----------------------

We are sometimes asked by users how to give back to the project. Here
are some ideas, but if you have new ones, feel free to share them with us.

Provide fresh-eyes feedback
^^^^^^^^^^^^^^^^^^^^^^^^^^^

We are working on the project since years. We take for granted things that are hard to infer at first, and traps we don't even see anymore.
Likewise, it's hard for us to maintain the documentation uptodate with the current situation, because we don't rely on the doc when navigating the project.
This is why your first impression as a newcomer to the project is utterly precious for us.
Please make sure to write a `discovery report <https://diff.wikimedia.org/2014/03/25/seeing-through-the-eyes-of-new-technical-contributors/>`_ to enlight us.
You can send it either as a bug report, as a mail on the list or simply post it as is to the Mattermost channel.

Spread the word
^^^^^^^^^^^^^^^

A simple way to help the SimGrid project is to **use SimGrid for your research, and say so**.
`Cite the SimGrid framework <https://simgrid.org/publications.html>`_ in your papers and speak of it with your colleagues to spread the word.
The number of publications enabled by SimGrid is really important when asking for further fundings to sustain the project:
The more you use the framework, the better for us.

Add a link to the `SimGrid homepage <https://simgrid.org>`_ on your site to improve our ranking in search engines.

You can also **help us constituting an active and welcoming user community**. Answer to the question sent to the mailing lists if you can, gently pointing to
the relevant part of the documentation on need, and help newscomers becoming part of our community too.

Finally, you can invite us for a talk on SimGrid to events you organize.
We have various format, ranging from a focused `20 minute talks <http://people.irisa.fr/Martin.Quinson/blog/2020/1124/SimGrid_presentations>`_
to a `45mn dense tutorial <http://people.irisa.fr/Martin.Quinson/blog/2012/1120/Simgrid_at_Louvain/>`_,
to a `2 hours seminar <http://people.irisa.fr/Martin.Quinson/blog/2016/0524/Experimental_methodology_for_distributed_systems>`_, or
even to `multi-days events <https://simgrid.org/tutorials.html>`_.
Note that even if most of these examples are from the same individual, several people in the team can present the project.
It's just that I wrote this paragraph so took the examples I knew, from my own experience :)

Report (and fix) issues
^^^^^^^^^^^^^^^^^^^^^^^

Because of its size and complexity, SimGrid far from perfect and
contains a large amount of glitches and issues. When you find one,
don't assume that it's here because we don't care. It survived only
because nobody told us. We unfortunately cannot endlessly review our
large code and documentation base. So please, **report any issue you
find**, be it a typo in the documentation, a paragraph that needs to
be reworded, a bug in the code, or any other problem. The best way to
do so is to open an issue on our
`Bug Tracker <https://framagit.org/simgrid/simgrid/issues>`_ so
that we don't forget about it.

The worst way to report such issues is to go through private emails.
These are unreliable, and we are trying to develop SimGrid openly, so
private discussions are to be avoided if possible.

If you can provide a patch fixing the issue you report, that's even
better. If you cannot, then you need to give us a minimal working
example (MWE), that is a ready to use solution that reproduces the
problem you face. Your bug will take much more time
for us to reproduce and fix if you don't give us the MWE, so you want
to help us helping you to get things efficient.

Of course, a very good way to give back to the SimGrid community is to
triage and fix the bugs in the Bug Tracking Systems. If the bug report
has no MWE, we'd love you to contribute one. If you can come up with a
patch, we will be more than happy to apply your changes so that the
whole community enjoys them.

Extending SimGrid and its Ecosystem
-----------------------------------

Contributing Code
^^^^^^^^^^^^^^^^^

If you deeply miss a feature in the framework, you should consider
implementing it yourself. SimGrid is free software, meaning that you are
free to help yourself. Of course, we'll do our best to assist you in
this task, so don't hesitate to contact us with your idea.

You could write a new plugin extending SimGrid in some way, or a
routing model for another kind of network. But even if you write your own
platform file, this is probably interesting to other users too, and
could be included to SimGrid. Modeling accurately a given platform is
a difficult work, which outcome is very precious to us.

Or maybe you developed an independent tool on top of SimGrid. We'd
love helping you gaining visibility by listing it in our
`Contrib <https://simgrid.org/contrib.html>`_.

Possible Enhancements
^^^^^^^^^^^^^^^^^^^^^

If you want to start working on the SimGrid codebase, here are a few
ideas of things that could be done to improve the current code (not all of them
are difficult, do trust yourself ;)

Time and duration
"""""""""""""""""

We should avoir using "-1" to mean "forever" at least in S4U and in
the internal code.  We should probably always use separate functions
(`wait` vs `wait_for`).

Futures and Promises
""""""""""""""""""""

 - Some features are missing in the Maestro future implementation
   (`simgrid::kernel::Future`, `simgrid::kernel::Promise`)
   could be extended to support additional features:
   `when_any`, `shared_future`, etc.

 - The corresponding feature might then be implemented in the user process
   futures (`simgrid::simix::Future`).

 - Currently `.then()` is not available for user futures. We would need to add
   a basic user event loop in order to queue the pending continuations.

 - We might need to provide an option to cancel a pending operation. This
   might be achieved by defining some `Action` or `Operation` class with an
   API compatible with `Future` (and convertible to it) but with an
   additional `.cancel()` method.

MC: Overhaul the state comparison code
""""""""""""""""""""""""""""""""""""""

The state comparison code is quite complicated. It has very long functions and
is programmed mostly using C idioms and is difficult to understand and debug.
It is in need of an overhaul:

  - cleanup, refactoring, usage of C++ features.

  - The state comparison code works by inferring types of blocks allocated on the
    heap by following pointers from known roots (global variables, local
    variables). Usually the first type found for a given block is used even if
    a better one could be found later. By using a first pass of type inference,
    on each snapshot before comparing the states, we might use a better type
    information on the different blocks.

  - We might benefit from adding logic for handling some known types. For
    example, both `std::string` and `std::vector` have a capacity which might
    be larger than the current size of the container. We should ignore
    the corresponding elements when comparing the states and inferring the types.

  - Another difficulty in the state comparison code is the detection of
    dangling pointers. We cannot easily know if a pointer is dangling and
    dangling pointers might lead us to choose the wrong type when inferring
    heap blocks. We might mitigate this problem by delaying the reallocation of
    a freed block until there is no blocks pointing to it anymore using some
    sort of basic garbage-collector.

MC: Hashing the states
""""""""""""""""""""""

In order to speed up the state comparison an idea was to create a hash of the
state. Only states with the same hash would need to be compared using the
state comparison algorithm. Some information should not be included in the
hash in order to avoid considering different states which would otherwise
would have been considered equal.

The states could be indexed by their hash. Currently they are indexed
by the number of processes and the amount of heap currently allocated
(see `DerefAndCompareByNbProcessesAndUsedHeap`).

Good candidate information for the state hashing:

 - number of processes;

 - their backtraces (instruction addresses);

 - their current simcall numbers;

 - some simcall arguments (eg. number of elements in a waitany);

 - number of pending communications;

 - etc.

Some basic infrastructure for this is already in the code (see `mc_hash.cpp`)
but it is currently disabled.

Interface with the model-checked processes
""""""""""""""""""""""""""""""""""""""""""

The model checker reads many information about the model-checked process by
`process_vm_readv()`-ing brutally the data structure of the model-checked
process leading to some inefficient code such as maintaining copies of complex
C++ structures in XBT dynars. We need a sane way to expose the relevant
information to the model checker.

Generic simcalls
""""""""""""""""

We have introduced some generic simcalls which can be used to execute a
callback in a SimGrid Maestro context. It makes it a lot easier to interface
the simulated process with the maestro. However, the callbacks for the
model checker which cannot decide how it should handle them. We would need a
solution for this if we want to be able to replace the simcalls the
model checker cares about by generic simcalls.

Defining an API for writing Model-Checking algorithms
"""""""""""""""""""""""""""""""""""""""""""""""""""""

Currently, writing a new model-checking algorithms in SimGridMC is quite
difficult: the logic of the model-checking algorithm is mixed with a lot of
low-level concerns about the way the model checker is implemented. This makes it
difficult to write new algorithms and difficult to understand, debug, and modify
the existing ones. We need a clean API to express the model-checking algorithms
in a form which is closer to the text-book/paper description. This API must
be exposed in a language which is more adequate to this task.

Tasks:

  1. Design and implement a clean API to express model-checking algorithms.
     A `Session` class currently exists for this but is not feature complete
     and should probably be rewritten. It should be easy to create bindings
     for different languages on top of this API.

  2. Create a binding to some better suited, dynamic, scripting language
     (e.g., Lua).

  3. Rewrite the existing model-checking algorithms in this language using the
     new API.
