[mc] Documentation

[simgrid.git] / doc / doxygen / community_giveback.doc

/*! @page community_giveback Giving back to SimGrid

@tableofcontents

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.

@section contributing_spread Spread the word

There is many ways to help the SimGrid project. The first and most
natural one is to <b>use it for your research, and say so</b>. Cite
the SimGrid framework in your paper and discuss of its advantages with
your colleagues to spread the word. When we ask for new fundings to
sustain the project, the amount of publications enabled by SimGrid is
always the first question that we get. The more you use the framework,
the better for us. 

We use several systems to detect the scientific publications citing
our work, so actually citing correctly the framework (see 
<a href="http://simgrid.gforge.inria.fr/Publications.html">here</a>)
should be enough to get us adding your paper to our list. But if you
remark that your paper is missing on 
<a href="http://simgrid.gforge.inria.fr/Usages.html">our 
page listing all papers using SimGrid</a>, please say so. Likewise, if
your paper is badly cited, please drop us an email so that we can fix
it.

Also, <b>help us constituting an active and welcoming user
community</b>. Get subscribed to the mailing lists, and answer the
questions that newcommers have if you can. Point them (gentely ;) to
the relevant part of the documentation on need, and help them becoming
part of our community too. 

Another easy way to help the project is to add a link to the
<a href="http://simgrid.gforge.inria.fr">SimGrid homepage</a>
on your homepage to <b>improve SimGrid's ranking in the search
engines</b>. Internet visibility is a battle where you can make the
difference. This can be done by adding the following html snipet to
your page, that simply renders as <a href="http://simgrid.gforge.inria.fr/">Simgrid</a>.

@verbatim
<a href="http://simgrid.gforge.inria.fr/" 
   title="Versatile simulation of distributed systems: Grid Simulator, P2P simulator, MPI simulator, Cloud Simulator or HPC Simulator">
   Simgrid</a>
@endverbatim

Or you can prefer this version with a little logo (if you want to
contribute another logo for that, please don't hesitate ;)
<img src="http://simgrid.gforge.inria.fr/logos/simgrid_logo.png"/>

@verbatim
<a href="http://simgrid.gforge.inria.fr/" 
   title="Versatile simulation of distributed systems: Grid Simulator, P2P simulator, MPI simulator, Cloud Simulator or HPC Simulator">
   <img src="http://simgrid.gforge.inria.fr/logos/simgrid_logo.png"
        alt="Versatile simulation of distributed systems: Grid Simulator, P2P simulator, MPI simulator, Cloud Simulator or HPC Simulator" align="center"> 
</a>
@endverbatim

Finally, if you organize a scientific event where you expect many
potential users, <b>invite us to give a tutorial on SimGrid</b>. We
found that 45 minutes to one hour is sufficient. It allows to explain
the main motivations and outcomes of the project in order to motivate
the attendees get more information on SimGrid, and eventually improve
their scientific habits by using a sound simulation framework. 
<a href="http://webloria.loria.fr/~quinson/blog/2012/1120/Simgrid_at_Louvain/">Here</a>
is an example of such a presentation.

@section contributing_bugs Reporting (and fixing) any issue you find

Because of its size and complexity, SimGrid is not 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, <b>report any issue you
find</b>, 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 a bug on our 
<a href="https://gforge.inria.fr/tracker/?atid=165&group_id=12&func=browse">Bug
Tracking System</a> so that we don't forget about it (we have other
obligations too and are sometimes unable to fix bugs right away, no
matter how easy the fix it). The worst way to report such issue 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, and often ensures that it will get fixed very soon.

Of course, a very good way to give back to the SimGrid community is to
<b>triage and fix the bugs in the BTS</b>. If you
can come up with a patch fixing them, we will be more than happy to
apply your changes so that the entier community enjoys them.

@section contributing_contrib Contributing features and associated tools

If you deeply miss a feature in the framework, you should consider
implementing it yourself. That's 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.

If you develop an independent tool that is somehow associated to
SimGrid, we'd love helping you gaining visibility by listing it on our 
<a href="http://simgrid.gforge.inria.fr/contrib.html">Contrib
section</a>. 

@section contributing_todo Enchancements

If you want to want to start working on the SimGrid codebase, here are a few
ideas of things that could be done to improved the current code (not all of them
are easy, though):

@subsection contributing_todo_cxxification Migration to C++

The code is being migrated to C++ but a large part is still C (or C++ with
C idioms). It would be valuable to replace C idioms with C++ ones:

 - replace XBT structures and C dynamic arrays with C++ containers;

 - replace `char*` strings with `std::string`;

 - use exception-safe RAII (`std::unique_ptr`, etc.) instead of explicit
   `malloc/free` or `new/delete`;

 - use `std::function` (or template functionoid arguments) instead of function
   pointers;

@subsubsection contributing_todo_exceptions Exceptions

SimGrid used to implement exceptions in C. This has been replaced with C++
exceptions but some bits of the C exceptions are still remaining:

 - `xbt_ex` was the type of C exceptions. It is now a standard C++ exception.
    We might want to remove this exception and use a more idiomatic C++
    solution with dedicated exception classes for different errors.
    `std::system_error` might be used as well by replacing some `xbt_errcat_t`
    with custom subclasses of `std::error_category`.

 - The C API currently throws exceptions. Throwing exceptions out of a C API is
   not very friendly. C code does not expect them, cannot catch them and cannot
   handle resource management properly in face of exceptions. We should clearly
   separate the C++ API and the C API and catch all exceptions before they get
   ouf of C APIs.

@subsubsection contributing_todo_time Time and duration

Some support for C++11-style time/duration is implemented (see `chrono.hpp`)
but only available in some (S4U) APIs. It would be nice to add support for
them in the rest of the C++ code.

@subsubsection contributing_todo_futures Futures

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

@subsection contributing_todo_smpi SMPI

@subsubsection contributing_smpi_split_process Process-based privatization

Currently, all the simulated processes live in the same process as the SimGrid
simulator. The benefit is that we don't have to do context switches and IPC
between the simulator and the processes.

The fact that they share the same address space means that one memory corruption
in one simulated process can propagate to the other ones and the SimGrid
simulator itself.

Moreover, the current design for SMPI applications is to compile the MPI code
normally and execute it once per simulated process in the same system process:
This means that all the existing simulated MPI processes share the same virtual
address space and share by default sthe same global variables. This is not
correct as each MPI process is expected to use its own address space and have
its own global variables. In order to fix, this problem we have an optional
SMPI privatization feature which creates a instanciation of the executable
data segment per MPI process and map the correct one (using `mmap`) at each
context switch.

This approach has many problems:

 1. It is not completely safe. We only handle SMPI privatization for the global
    variables in the execute data segment. Shared objects are ignored but some
    may contain global variables which may need to be privatized:

    - libsimgrid for example must not be privatized because it contains
      shared state for the simulator;

    - libc must not be privatized for the same reason (but some global variables
      in the libc may not be privatized);

    - if we use global variables of some shared object in the executable, this
      global variable will be instanciated in the executable (because of copy
      relocation) and will be privatized even if it shoud not.

 2. We cannot execute the MPI processes in parallel. Only one can execute at
    the same time because only one privatization segment can be mapped at a
    given time.

In order to fix this, the standard solution is to move each MPI process in its
system process and use IPC to communicate with the simulator. One concern would
be the impact on performance and memory consumption:

 - It would introduce a lot of context switches and IPC communications between
   the MPI processes and the SimGrid simulator. However, currently every context
   switch needs a `mmap` for SMPI privatization which is costly as well
   (TLB flush).

 - Instanciating a lot of processes might consume more memory which might be a
   problem if we want to simulate a lot of MPI processes. Compiling MPI programs
   as static executables with a lightweight libc might help and we might want to
   support that. The SMPI processes should probably not embed all the SimGrid
   simulator and its dependencies, the C++ runtime, etc.

We would need to modify the model-checker as well which currently can only
manage on model-checked process. For the model-checker we can expect some
benefits from this approach: if a process did not execute, we know its state
did not change and we don't need to take its snapshot and compare its state.

Other solutions for this might include:

 - Mapping each MPI process in the process of the simulator but in a different
   symbol namespace (see `dlmopen`). Each process would have its own separate
   instanciation and would not share libraries.

 - Instanciate each MPI process in a separate lightweight VM (for example based
   on WebAssembly) in the simualtor process.

@subsection contributing_todo_mc Model-checker

@subsubsection contributing_todo_mc_state_compare 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 understanda and debug.
It is in need or an overhaul:

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

  - The state comparison code works by infering 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 might ignore
    the corresponding elements when comparing the states and infering 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 infering
    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.

@subsubsection contributing_todo_mc_separation Separate the model-checker code from libsimgrid

@subsubsection contributing_todo_mc_mced_interface Interface with the model-checked processes

The model-checker reads many informations about the model-checked process
by `process_vm_readv()`-ing brutally the data structure of the model-checked
process leading to some horrible code such as walking a swag from another
process. It prevents us as well from replacing some XBT data structures with
standard C++ ones. We need a sane way to expose the relevant informations to
the model-checker.

@subsubsection contributing_todo_mc_generic_simcalls Generic simcalls

We have introduced some generic simcalls which can be used to execute a
callback in 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.

@subsubsection contributing_todo_mc_api Definig 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 muste
be exposed in a a language which is more adequate to this task.

Tasks:

  1. Design and implement a clean API for expression 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
     (eg. Lua).

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

*/