/*! \page faq Frequently Asked Questions
+\htmlinclude .FAQ.doc.toc
+
+\section faq_simgrid I'm new to SimGrid. I have some questions. Where should I start?
+
+You are at the right place... Having a look to these
+<a href="http://graal.ens-lyon.fr/~alegrand/articles/slides_g5k_simul.pdf">slides</a>
+(or to these
+<a href="http://graal.ens-lyon.fr/~alegrand/articles/Simgrid-Introduction.pdf">"obsolete" slides</a>)
+may give you some insights on what SimGrid can help you to do and what
+are its limitations. Then you definitely should read the \ref
+MSG_examples. There is also a mailing list: <simgrid-user@lists.gforge.inria.fr>.
+
+\subsection faq_interfaces What is the difference between MSG, SimDag, and GRAS? Do they serve the same purpose?
+
+It depend on how you define "purpose", I guess ;)
+
+They all allow you to build a prototype of application which you can run
+within the simulator afterward. They all share the same simulation kernel,
+which is the core of the SimGrid project. They differ by the way you express
+your application.
+
+With SimDag, you express your code as a collection of interdependent
+parallel tasks. So, in this model, applications can be seen as a DAG of
+tasks.
+
+With both GRAS and MSG, your application is seen as a set of communicating
+processes, exchanging data by the way of messages and performing computation
+on their own.
+
+The difference between both is that MSG is somehow easier to use, but GRAS
+is not limitated to the simulator. Once you're done writing your GRAS code,
+you can run your code both in the simulator or on a real platform. For this,
+there is two implementations of the GRAS interface, one for simulation, one
+for real execution. So, you just have to relink your code to chose one of
+both world.
+
+\subsection faq_generic First steps with SimGrid
+
+If you decide to go for the MSG interface, please read carefully the
+\ref MSG_examples. You'll find in \ref MSG_ex_master_slave a very
+simple consisting of a master (that owns a bunch of tasks and
+distributes them) , some slaves (that process tasks whenever they
+receive one) and some forwarder agents (that simply pass the tasks
+they receive to some slaves).
+
+If you decide to go for the GRAS interface, you should definitively
+read the \ref GRAS_tut. The first section constitutes an introduction
+to the tool and presents the model we use. The second section
+constitutes a complete step-by-step tutorial building a distributed
+application from the begining and exemplifying most of the GRAS
+features in the process. The last section groups some HOWTOS
+highlighting a given feature of the framework in a more concise way.
+
+If you decide to go for another interface, I'm afraid your only sources
+of information will be the source code and the mailing lists...
+
+\subsection faq_visualization Visualizing and analyzing the results
+
+It is sometime convenient to "see" how the agents are behaving. If you
+like colors, you can use <tt>tools/MSG_visualization/colorize.pl </tt>
+as a filter to your MSG outputs. It works directly with INFO. Beware,
+INFO() prints on stderr. Do not forget to redirect if you want to
+filter (e.g. with bash):
+\verbatim
+./msg_test small_platform.xml small_deployment.xml 2>&1 | ../../tools/MSG_visualization/colorize.pl
+\endverbatim
+
+We also have a more graphical output. Have a look at MSG_paje_output(). It
+generates an input to <a href="http://www-id.imag.fr/Logiciels/paje/">Paje</a>.
+<center>
+\htmlonly
+ <a href="Paje_MSG_screenshot.jpg"><img src="Paje_MSG_screenshot_thn.jpg"></a>
+\endhtmlonly
+</center>
+
+Vizualization with Paje can be seen as a kind of postmortem
+analysis. However, as soon as you start playing with big simulations,
+you'll realize that processing such output is kind of tricky. There is
+so much generic informations that it is hard to find the information
+you are looking for.
+
+As a matter of fact, loging really depends on simulations (e.g. what
+kind of events is important...). That is why we do not propose a big
+dump of your whole simulation (it would slow everything down) but give
+you neat tools to structure you logs. Have a look at \ref XBT_log. In
+fact, rather than a post-mortem analysis, you may want to do it on the
+fly. The process you are running can do whatever you want. Have you
+thought about adding a global structure where you directly compute the
+informations that are really important rather than writing everything
+down and then processing huge files?
+
+\subsection faq_C Argh! Do I really have to code in C?
+
+Up until now, there is no binding for other languages. If you use C++,
+you should be able to use the SimGrid library as a standard C library
+and everything should work fine (simply <i>link</i> against this
+library; recompiling SimGrid with a C++ compiler won't work and it
+wouldn't help if you could).
+
+In fact, we are currently working on Java bindings of MSG to allow
+all the undergrad students of the world to use this tool. This is a
+little more tricky than I would have expected, but the work is moving
+fast forward [2006/05/13]. More languages are evaluated, but for now,
+we do not feel a real demand for any other language. Please speak up!
+
\section faq_installation Installing the SimGrid library
Many people have been asking me questions on how to use SimGrid. Quite
instructions and still have some troubles, drop an e-mail to
<simgrid-user@lists.gforge.inria.fr>.
-\subsection faq_compiling Compiling SimGrid
+\subsection faq_compiling Compiling SimGrid from an archive
+First of all, you need to download the latest version of SimGrid from
+<a href="http://gforge.inria.fr/frs/?group_id=12">here</a>.
Suppose you have uncompressed SimGrid in some temporary location of
-your home directory (say <tt>/home/joe/tmp/simgrid-2.18.2 </tt>). The
+your home directory (say <tt>/home/joe/tmp/simgrid-3.0.1 </tt>). The
simplest way to use SimGrid is to install it in your home
directory. Change your directory to
-<tt>/home/joe/tmp/simgrid-2.18.2</tt> and type
+<tt>/home/joe/tmp/simgrid-3.0.1</tt> and type
\verbatim./configure --prefix=$HOME
make
make install
\endverbatim
-If at some point, something fails, you can report me this problem but,
-please, avoid sending a laconic mail like "There is a problem. Is it
-normal ?". Send me the config.log file which is automatically
-generated by configure. Try to capture both the standard output and
-the error output of the <tt>make</tt> command. There is no way for me
-to help you if you do not give me a little bit of information.
+If at some point, something fails, check the section "\ref
+faq_trouble_compil". If it does not help, you can report this problem to the
+list but, please, avoid sending a laconic mail like "There is a problem. Is it
+okay?". Send the config.log file which is automatically generated by
+configure. Try to capture both the standard output and the error output of the
+<tt>make</tt> command with <tt>script</tt>. There is no way for us to help you
+without the relevant bits of information.
Now, the following directory should have been created :
version are available. Here is what you can find if you try a <tt>ls
/home/joe/lib</tt>:
-\verbatim libsimgrid.a libsimgrid.la libsimgrid.so libsimgrid.so.0 libsimgrid.so.0.0.1
+\verbatim libsimgrid.a libsimgrid.la libsimgrid.so libsimgrid.so.0 libsimgrid.so.0.0.1
\endverbatim
Thus, there is two ways to link your program with SimGrid:
\verbatim export LD_LIBRARY_PATH=$HOME/lib/:$LD_LIBRARY_PATH
\endverbatim
-\subsection faq_setting Setting up your own code
+
+\subsection faq_compiling_cvs Compiling SimGrid from the CVS
+
+The project development takes place in the cvs, where all changes are
+commited when they happen. Then every once in a while, we make sure that the
+code quality meets our standard and release an archive from the code in the
+CVS. We afterward go back to the development in the CVS. So, if you need a
+recently added feature and can afford some little problem with the stability
+of the lastest features, you may want to use the CVS version instead of a
+released one.
+
+For that, you first need to get the "simgrid" module from
+<a href="http://gforge.inria.fr/scm/?group_id=12">here</a>.
+
+You won't find any <tt>configure</tt> and a few other things
+(<tt>Makefile.in</tt>'s, documentation, ...) will be missing as well. The
+reason for that is that all these files have to be regenerated using the
+latest versions of <tt>autoconf</tt>, <tt>libtool</tt>, <tt>automake</tt>
+(>1.9) and <tt>doxygen</tt> (>1.4). To generate the <tt>configure</tt> and
+the <tt>Makefile.in</tt>'s, you just have to launch the <tt>bootstrap</tt>
+command that resides in the top of the source tree. Then just follow the
+instructions of Section \ref faq_compiling.
+
+We insist on the fact that you really need the latest versions of
+autoconf, automake and libtool. Doing this step on exotic architectures/systems
+(i.e. anything different from a recent linux distribution) may be
+... uncertain. If you need to compile the CVS version on a machine where all these
+dependencies are not met, the easiest is to do <tt>make dist</tt> in the CVS
+dir of another machine where all dependencies are met. It will create an
+archive you may deploy on other sites just as a regular stable release.
+
+In summary, the following commands will checkout the CVS, regenerate the
+configure script and friends, configure SimGrid and build it.
+
+\verbatim cvs -d :pserver:anonymous@scm.gforge.inria.fr:/cvsroot/simgrid login
+cvs -d :pserver:anonymous@scm.gforge.inria.fr:/cvsroot/simgrid checkout simgrid
+cd simgrid
+./bootstrap
+./configure --enable-maintainer-mode --prefix=<where to install SimGrid>
+make \endverbatim
+
+Then, if you want to install SimGrid on the current box, just do:
+\verbatim make install \endverbatim
+
+If you want to build an snapshot of the CVS to deploy it on another box (for
+example because the other machine don't have the autotools), do:
+\verbatim make dist \endverbatim
+
+Moreover, you should never call the autotools manually since you must run
+them in a specific order with specific arguments. Most of the times, the
+makefiles will automatically call the tools for you. When it's not possible
+(such as the first time you checkout the CVS), use the ./bootstrap command
+to call them explicitely.
+
+
+\subsection faq_setting_MSG Setting up your own MSG code
Do not build your simulator by modifying the SimGrid examples. Go
outside the SimGrid source tree and create your own working directory
MSG_launch_application()) and the call to
MSG_main()).
-To compile such a program, I suggest to use the following Makefile. It
-is a generic Makefile that I generally use with my students when I
-teach the C language.
+To compile such a program, we suggest to use the following
+Makefile. It is a generic Makefile that we have used many times with
+our students when we teach the C language.
\verbatim
all: masterslave
previous example should be enough for a first try but you may want to
perform some more complex compilations...
-\section faq_simgrid I'm new to SimGrid. I have some questions. Where should I start ?
+\subsection faq_setting_GRAS Setting up your own GRAS code
+
+If you use the GRAS interface instead of the MSG one, then previous section
+is not the better source of information. Instead, you should check the GRAS
+tutorial in general, and the \ref GRAS_tut_tour_setup in particular.
+
+\subsection faq_crosscompile Cross-compiling a Windows DLL of SimGrid from linux
+
+At the moment, we do not distribute Windows pre-compiled version of SimGrid
+because the support for this platform is still experimental. We know that
+some parts of the GRAS environment do not work, and we think that the others
+environments (MSG and SD) have good chances to work, but we didn't test
+ourselves. This section explains how we generate the SimGrid DLL so that you
+can build it for yourself. First of all, you need to have a version more
+recent than 3.1 (ie, a CVS version as time of writting).
+
+In order to cross-compile the package to windows from linux, you need to
+install mingw32 (minimalist gnu win32). On Debian, you can do so by
+installing the packages mingw32 (compiler), mingw32-binutils (linker and
+so), mingw32-runtime.
+
+You can use the VPATH support of configure to compile at the same time for
+linux and windows without dupplicating the source nor cleaning the tree
+between each. Just run bootstrap (if you use the CVS) to run the autotools.
+Then, create a linux and a win directories. Then, type:
+\verbatim cd linux; ../configure --srcdir=.. <usual configure flags>; make; cd ..
+cd win; ../configure --srcdir=.. --host=i586-mingw32msvc <flags>; make; cd ..
+\endverbatim
+The trick to VPATH builds is to call configure from another directory,
+passing it an extra --srcdir argument to tell it where all the sources are.
+It will understand you want to use VPATH. Then, the trick to cross-compile
+is simply to add a --host argument specifying the target you want to build
+for. The i586-mingw32msvc string is what you have to pass to use the mingw32
+environment as distributed in Debian.
+
+After that, you can run all make targets from both directories, and test
+easily that what you change for one arch does not break the other one.
+
+It is possible that this VPATH build thing breaks from time to time in the
+CVS since it's quite fragile, but it's granted to work in any released
+version. If you experience problems, drop us a mail.
+
+Another possible source of issue is that at the moment, building the
+examples request to use the gras_stub_generator tool, which is a compiled
+program, not a script. In cross-compilation, you need to cross-execute with
+wine for example, which is not really pleasant. We are working on this, but
+in the meanwhile, simply don't build the examples in cross-compilation
+(<tt>cd src</tt> before running make).
+
+Program (cross-)compiled with mingw32 do request an extra DLL at run-time to be
+usable. For example, if you want to test your build with wine, you should do
+the following to put this library where wine looks for DLLs.
+\verbatim
+cp /usr/share/doc/mingw32-runtime/mingwm10.dll.gz ~/.wine/c/windows/system/
+gunzip ~/.wine/c/windows/system/mingwm10.dll.gz
+\endverbatim
-You are at the right place... Having a look to these
-<a href="http://graal.ens-lyon.fr/~alegrand/articles/Simgrid-Introduction.pdf">slides</a>
-may give you some insights on what SimGrid can help you to do and what
-are its limitations. Then you definitely should read the \ref
-MSG_examples. There is also a mailing list: <simgrid-user@lists.gforge.inria.fr>.
+The DLL is builded in src/.libs, and installed in the <i>prefix</i>/bin directory
+when you run make install.
+
+If you want to use it in a native project on windows, you need to use
+simgrid.dll and mingwm10.dll. For each DLL, you need to build .def file
+under linux (listing the defined symbols), and convert it into a .lib file
+under windows (specifying this in a way that windows compilers like). To
+generate the def files, run (under linux):
+\verbatim echo "LIBRARY libsimgrid-0.dll" > simgrid.def
+echo EXPORTS >> simgrid.def
+nm libsimgrid-0.dll | grep ' T _' | sed 's/.* T _//' >> simgrid.def
+nm libsimgrid-0.dll | grep ' D _' | sed 's/.* D _//' | sed 's/$/ DATA/' >> simgrid.def
+
+echo "LIBRARY mingwm10.dll" > mingwm10.def
+echo EXPORTS >> mingwm10.def
+nm mingwm10.dll | grep ' T _' | sed 's/.* T _//' >> mingwm10.def
+nm mingwm10.dll | grep ' D _' | sed 's/.* D _//' | sed 's/$/ DATA/' >> mingwm10.def
+\endverbatim
-\subsection faq_generic Building a generic simulator
+To create the import .lib files, use the <tt>lib</tt> windows tool (from
+MSVC) the following way to produce simgrid.lib and mingwm10.lib
+\verbatim lib /def:simgrid.def
+lib /def:mingwm10.def
+\endverbatim
-Please read carefully the \ref MSG_examples. You'll find in \ref
-MSG_ex_master_slave a very simple consisting of a master (that owns a bunch of
-tasks and distributes them) , some slaves (that process tasks whenever
-they receive one) and some forwarder agents (that simply pass the
-tasks they receive to some slaves).
+If you happen to use Borland C Builder, the right command line is the
+following (note that you don't need any file.def to get this working).
+\verbatim implib simgrid.lib libsimgrid-0.dll
+implib mingwm10.lib mingwm10.dll
+\endverbatim
-\subsection faq_examples I want some more complex examples !
+Then, set the following parameters in Visual C++ 2005:
+Linker -> Input -> Additional dependencies = simgrid.lib mingwm10.lib
+
+Just in case you wonder how to generate a DLL from libtool in another
+project, we added -no-undefined to any lib*_la_LDFLAGS variables so that
+libtool accepts to generate a dynamic library under windows. Then, to make
+it true, we pass any dependencies (such as -lws2 under windows or -lpthread
+on need) on the linking line. Passing such deps is a good idea anyway so
+that they get noted in the library itself, avoiding the users to know about
+our dependencies and put them manually on their compilation line. Then we
+added the AC_LIBTOOL_WIN32_DLL macro just before AC_PROG_LIBTOOL in the
+configure.ac. It means that we exported any symbols which need to be.
+Nowadays, functions get automatically exported, so we don't need to load our
+header files with tons of __declspec(dllexport) cruft. We only need to do so
+for data, but there is no public data in SimGrid so we are good.
+
+\section faq_howto Feature related questions
+
+\subsection faq_MIA "Could you please add (your favorite feature here) to SimGrid?"
+
+Here is the deal. The whole SimGrid project (MSG, SURF, GRAS, ...) is
+meant to be kept as simple and generic as possible. We cannot add
+functions for everybody's need when these functions can easily be
+built from the ones already in the API. Most of the time, it is
+possible and when it was not possible we always have upgraded the API
+accordingly. When somebody asks us a question like "How to do that?
+Is there a function in the API to simply do this?", we're always glad
+to answer and help. However if we don't need this code for our own
+need, there is no chance we're going to write it... it's your job! :)
+The counterpart to our answers is that once you come up with a neat
+implementation of this feature (task duplication, RPC, thread
+synchronization, ...), you should send it to us and we will be glad to
+add it to the distribution. Thus, other people will take advantage of
+it (and we don't have to answer this question again and again ;).
+
+You'll find in this section a few "Missing In Action" features. Many
+people have asked about it and we have given hints on how to simply do
+it with MSG. Feel free to contribute...
+
+\subsection faq_MIA_MSG MSG features
+
+\subsubsection faq_MIA_examples I want some more complex MSG examples!
Many people have come to ask me a more complex example and each time,
they have realized afterward that the basics were in the previous three
communications or computations). But the examples are sufficient to
start.
-I know I should add some more examples, but not some more complex
-ones... I should add some examples that illustrate some other
+We know. We should add some more examples, but not really some more
+complex ones... We should add some examples that illustrate some other
functionalities (like how to simply encode asynchronous
communications, RPC, process migrations, thread synchronization, ...)
-and I will do it when I will have a little bit more time. I have tried
-to document the examples so that they are understandable. I know it is
-not really satisfying but it is the best I have managed to do yet.
+and we will do it when we will have a little bit more time. We have
+tried to document the examples so that they are understandable. Tell
+us if something is not clear and once again feel free to participate!
+:)
+
+\subsubsection faq_MIA_taskdup Missing in action: MSG Task duplication/replication
+
+There is no task duplication in MSG. When you create a task, you can
+process it or send it somewhere else. As soon as a process has sent
+this task, he doesn't have this task anymore. It's gone. The receiver
+process has got the task. However, you could decide upon receiving to
+create a "copy" of a task but you have to handle by yourself the
+semantic associated to this "duplication".
+
+As we already told, we prefer keeping the API as simple as
+possible. This kind of feature is rather easy to implement by users
+and the semantic you associate really depends on people. Having a
+*generic* task duplication mechanism is not that trivial (in
+particular because of the data field). That is why I would recommand
+that you write it by yourself even if I can give you advice on how to
+do it.
+
+You have the following functions to get informations about a task:
+MSG_task_get_name(), MSG_task_get_compute_duration(),
+MSG_task_get_remaining_computation(), MSG_task_get_data_size(),
+and MSG_task_get_data().
+
+You could use a dictionnary (#xbt_dict_t) of dynars (#xbt_dynar_t). If
+you still don't see how to do it, please come back to us...
+
+\subsubsection faq_MIA_asynchronous I want to do asynchronous communications in MSG
+
+Up until now, there is no asynchronous communications in MSG. However,
+you can create as many process as you want so you should be able to do
+whatever you want... I've written a queue module to help implementing
+some asynchronous communications at low cost (creating thousands of
+process only to handle communications may be problematic in term of
+performance at some point). I'll add it in the distribution asap.
+
+\subsubsection faq_MIA_thread_synchronization I need to synchronize my MSG processes
+
+You obviously cannot use pthread_mutexes of pthread_conds. The best
+thing would be to propose similar structures. Unfortunately, we
+haven't found time to do it yet. However you can try to play with
+MSG_process_suspend() and MSG_process_resume(). You can even do some
+synchronization with fake communications (using MSG_task_get(),
+MSG_task_put() and MSG_task_Iprobe()).
+
+\subsubsection faq_MIA_host_load Where is the get_host_load function hidden in MSG?
+
+There is no such thing because its semantic wouldn't be really
+clear. Of course, it is something about the amount of host throughput,
+but there is as many definition of "host load" as people asking for
+this function. First, you have to remember that resource availability
+may vary over time, which make any load notion harder to define.
-\subsection faq_platform Building a realistic platform
+It may be instantaneous value or an average one. Moreover it may be only the
+power of the computer, or may take the background load into account, or may
+even take the currently running tasks into account. In some SURF models,
+communications have an influence on computational power. Should it be taken
+into account too?
-I can speak more than an hour on this subject and I still do not have
-the right answer, just some ideas. You can read the following
-<a href="http://graal.ens-lyon.fr/~alegrand/articles/Simgrid-Introduction.pdf">slides</a>.
-It may give you some hints. You can also have a look at the
-<tt>tools/platform_generation/</tt> directory. There is a perl-script
-I use to annotate a Tiers generated platform (may not be up-to-date
-though).
+First of all, it's near to impossible to predict the load beforehands in the
+simulator since it depends on too much parameters (background load
+variation, bandwidth sharing algorithmic complexity) some of them even being
+not known beforehands (other task starting at the same time). So, getting
+this information is really hard (just like in real life). It's not just that
+we want MSG to be as painful as real life. But as it is in some way
+realistic, we face some of the same problems as we would face in real life.
+
+How would you do it for real? The most common option is to use something
+like NWS that performs active probes. The best solution is probably to do
+the same within MSG, as in next code snippet. It is very close from what you
+would have to do out of the simulator, and thus gives you information that
+you could also get in real settings to not hinder the realism of your
+simulation.
-\subsection faq_visualization Visualizing the schedule
+\verbatim
+double get_host_load() {
+ m_task_t task = MSG_task_create("test", 0.001, 0, NULL);
+ double date = MSG_get_clock();
-It is sometime convenient to "see" how the agents are behaving. If you
-like colors, you can use <tt>tools/MSG_visualization/colorize.pl </tt>
-as a filter to your MSG outputs. It works directly with INFO. Beware,
-INFO() prints on stderr. Do not forget to redirect if you want to
-filter (e.g. with bash):
-\verbatim
-./msg_test small_platform.xml small_deployment.xml 2>&1 | ../../tools/MSG_visualization/colorize.pl
+ MSG_task_execute(task);
+ date = MSG_get_clock() - date;
+ MSG_task_destroy(task);
+ return (0.001/date);
+}
\endverbatim
-We also have a more graphical output. Have a look at MSG_paje_output(). It
-generates an input to <a href="http://www-id.imag.fr/Logiciels/paje/">Paje</a>.
-<center>
-\htmlonly
- <a href="Paje_MSG_screenshot.jpg"><img src="Paje_MSG_screenshot_thn.jpg"></a>
-\endhtmlonly
-</center>
+Of course, it may not match your personal definition of "host load". In this
+case, please detail what you mean on the mailing list, and we will extend
+this FAQ section to fit your taste if possible.
+
+\subsubsection faq_MIA_communication_time How can I get the *real* communication time?
-\subsection faq_context I have tons of process and it is limiting my simulation.
+Communications are synchronous and thus if you simply get the time
+before and after a communication, you'll only get the transmission
+time and the time spent to really communicate (it will also take into
+account the time spent waiting for the other party to be
+ready). However, getting the *real* communication time is not really
+hard either. The following solution is a good starting point.
-MSG can use either pthreads or the GNU context library. On most
-systems, the number of pthreads is limited (especially with the
-current linux pthreads) and then your simulation may be limited for a
-stupid reason. If you enable the context option
-(<tt>--enable-context</tt> in the <tt>./configure</tt> phase), you
-will not use the pthread anymore and the context switching will be
-done manually, which enables us to have as much agents as your memory
-can hold and should be much faster... So think about it if your
-simulation is getting really big.
+\verbatim
+int sender()
+{
+ m_task_t task = MSG_task_create("Task", task_comp_size, task_comm_size,
+ calloc(1,sizeof(double)));
+ *((double*) task->data) = MSG_get_clock();
+ MSG_task_put(task, slaves[i % slaves_count], PORT_22);
+ INFO0("Send completed");
+ return 0;
+}
+int receiver()
+{
+ m_task_t task = NULL;
+ double time1,time2;
+
+ time1 = MSG_get_clock();
+ a = MSG_task_get(&(task), PORT_22);
+ time2 = MSG_get_clock();
+ if(time1<*((double *)task->data))
+ time1 = *((double *) task->data);
+ INFO1("Communication time : \"%f\" ", time2-time1);
+ free(task->data);
+ MSG_task_destroy(task);
+ return 0;
+}
+\endverbatim
-Nevertheless, be aware that this code does not work on some system. It
-is not very clean. As usual, as soon as I will have a little bit more
-time, I will recode it in a cleaner way.
+\subsection faq_MIA_SimDag SimDag related questions
-\section faq_SG Where has SG disappeared ?!?
+\subsubsection faq_SG Where has SG disappeared?
OK, it's time to explain what's happening to the SimGrid project. Let's
start with a little bit of history.
* Historically, SimGrid was a low-level toolkit for scheduling with
-classical models such as DAGs. That was SimGrid v.1.* aka SG, written by
-Henri Casanova. I had been using it in its earliest versions during an
-internship at UCSD.
+classical models such as DAGs. That was SimGrid v.1.* aka SG, written
+by Henri Casanova. I (Arnaud) had been using it in its earliest
+versions during an internship at UCSD.
Then we have realized that encoding distributed algorithm in SG was a
real pain.
Those two things are working, but we want to make everything as clean as
possible before releasing SimGrid v.3.
-So what about those nice DAGs we used to have in SimGrid v.1. ? They're not
-anymore in SimGrid v.3. Let me recall you the way SimGrid 3 is organized:
+So what about those nice DAGs we used to have in SimGrid v.1.? They're
+not anymore in SimGrid v.3. At least not in their original form... Let
+me recall you the way SimGrid 3 is organized:
\verbatim
________________
----------------
\endverbatim
-XBT is our tool box and now, you should have an idea of what the other ones
-are. As you can see, the primitive SG is not here anymore. However it could
-still be brought back if people really need it. Here is how it would fit.
+XBT is our tool box and now, you should have an idea of what the other
+ones are. As you can see, the primitive SG is not here
+anymore. However we have written a brand new and cleaner API for this
+purpose: \ref SD_API. It is built directly on top of SURF and provides
+an API rather close to the old SG:
\verbatim
______________________
| User code |
|____________________|
-| | MSG | GRAS | SG |
+| | MSG | GRAS | SD |
| -------------------|
| | SURF |
| -------------------|
----------------------
\endverbatim
-Re-implementing SG on top of SURF is really straightforward (it only
-requires a little bit of time that I really don't have right now)
-since the only thing that lacks to SURF is the DAG part. But adding it
-to SURF would slow it down and therefore slow MSG and GRAS which is
-not a good thing. However it is really not on the top of our TODO
-list because we have to work on GRAS, and its MPI counterpart, and a
-parallel task model, and ... Anyway, we finally have migrated our CVS
-to gforge so people that are interested by helping on this part will
-have the possibility to do it.
+The nice thing is that, as it is writen on top of SURF, it seamlessly
+support DAG of parallel tasks as well as complex communications
+patterns. Some old codes using SG are currently under rewrite using
+\ref SD_API to check that all needful functions are provided.
-\subsection faq_SG_DAG But I wanted to implement a distributed dynamic scheduler of DAGs... How can I do that it SG is not available anymore in the next versions ?
+\subsubsection faq_SG_DAG How to implement a distributed dynamic scheduler of DAGs.
Distributed is somehow "contagious". If you start making distributed
-decisions, there is no way to handle DAGs directly anymore (unless I am
-missing something). You have to encode your DAGs in term of communicating
-process to make the whole scheduling process distributed. Believe me, it is
-worth the effort since you'll then be able to try your algorithms in a very
-wide variety of conditions.
+decisions, there is no way to handle DAGs directly anymore (unless I
+am missing something). You have to encode your DAGs in term of
+communicating process to make the whole scheduling process
+distributed. Here is an example of how you could do that. Assume T1
+has to be done before T2.
+\verbatim
+ int your_agent(int argc, char *argv[] {
+ ...
+ T1 = MSG_task_create(...);
+ T2 = MSG_task_create(...);
+ ...
+ while(1) {
+ ...
+ if(cond) MSG_task_execute(T1);
+ ...
+ if((MSG_task_get_remaining_computation(T1)=0.0) && (you_re_in_a_good_mood))
+ MSG_task_execute(T2)
+ else {
+ /* do something else */
+ }
+ }
+ }
+\endverbatim
+
If you decide that the distributed part is not that much important and that
-DAG is really the level of abstraction you want to work with (but it
-prevents you from having "realistic" platform modeling), then you should
-keep using the 2.18.5 versions until somebody has ported SG on top of SURF.
-Note however that SURF will be slower than the old SG to handle traces with
-a lots of variations (there is no trace integration anymore).
-
+DAG is really the level of abstraction you want to work with, then you should
+give a try to \ref SD_API.
+
+\subsection faq_MIA_generic Generic features
+
+\subsubsection faq_more_processes Increasing the amount of simulated processes
+
+Here are a few tricks you can apply if you want to increase the amount
+of processes in your simulations.
+
+ - <b>A few thousands of simulated processes</b> (soft tricks)\n
+ SimGrid can use either pthreads library or the UNIX98 contextes. On
+ most systems, the number of pthreads is limited and then your
+ simulation may be limited for a stupid reason. This is especially
+ true with the current linux pthreads, and I cannot get more than
+ 2000 simulated processes with pthreads on my box. The UNIX98
+ contexts allow me to raise the limit to 25,000 simulated processes
+ on my laptop.\n\n
+ The <tt>--with-context</tt> option of the <tt>./configure</tt>
+ script allows you to choose between UNIX98 contextes
+ (<tt>--with-context=ucontext</tt>) and the pthread version
+ (<tt>--with-context=pthread</tt>). The default value is ucontext
+ when the script detect a working UNIX98 context implementation. On
+ Windows boxes, the provided value is discarded and an adapted
+ version is picked up.\n\n
+ We experienced some issues with contextes on some rare systems
+ (solaris 8 and lower or old alpha linuxes comes to mind). The main
+ problem is that the configure script detect the contextes as being
+ functional when it's not true. If you happen to use such a system,
+ switch manually to the pthread version, and provide us with a good
+ patch for the configure script so that it is done automatically ;)
+
+ - <b>Hundred thousands of simulated processes</b> (hard-core tricks)\n
+ As explained above, SimGrid can use UNIX98 contextes to represent
+ and handle the simulated processes. Thanks to this, the main
+ limitation to the number of simulated processes becomes the
+ available memory.\n\n
+ Here are some tricks I had to use in order to run a token ring
+ between 25,000 processes on my laptop (1Gb memory, 1.5Gb swap).\n
+ - First of all, make sure your code runs for a few hundreds
+ processes before trying to push the limit. Make sure it's
+ valgrind-clean, ie that valgrind does not report neither memory
+ error nor memory leaks. Indeed, numerous simulated processes
+ result in *fat* simulation hindering debugging.
+ - It was really boring to write 25,000 entries in the deployment
+ file, so I wrote a little script
+ <tt>examples/gras/tokenS/make_deployment.pl</tt>, which you may
+ want to adapt to your case. You could also think about hijacking
+ the SURFXML parser (have look at \ref faq_flexml_bypassing).
+ - The deployment file became quite big, so I had to do what is in
+ the FAQ entry \ref faq_flexml_limit
+ - Each UNIX98 context has its own stack entry. As debugging this is
+ quite hairly, the default value is a bit overestimated so that
+ user don't get into trouble about this. You want to tune this
+ size to increse the number of processes. This is the
+ <tt>STACK_SIZE</tt> define in
+ <tt>src/xbt/context_private.h</tt>, which is 128kb by default.
+ Reduce this as much as you can, but be warned that if this value
+ is too low, you'll get a segfault. The token ring example, which
+ is quite simple, runs with 40kb stacks.
+
+\subsubsection faq_MIA_batch_scheduler Is there a native support for batch schedulers in SimGrid?
+
+No, there is no native support for batch schedulers and none is
+planned because this is a very specific need (and doing it in a
+generic way is thus very hard). However some people have implemented
+their own batch schedulers. Vincent Garonne wrote one during his PhD
+and put his code in the contrib directory of our CVS so that other can
+keep working on it. You may find inspinring ideas in it.
+
+\subsubsection faq_MIA_checkpointing I need a checkpointing thing
+
+Actually, it depends on whether you want to checkpoint the simulation, or to
+simulate checkpoints.
+
+The first one could help if your simulation is a long standing process you
+want to keep running even on hardware issues. It could also help to
+<i>rewind</i> the simulation by jumping sometimes on an old checkpoint to
+cancel recent calculations.\n
+Unfortunately, such thing will probably never exist in SG. One would have to
+duplicate all data structures because doing a rewind at the simulator level
+is very very hard (not talking about the malloc free operations that might
+have been done in between). Instead, you may be interested in the Libckpt
+library (http://www.cs.utk.edu/~plank/plank/www/libckpt.html). This is the
+checkpointing solution used in the condor project, for example. It makes it
+easy to create checkpoints (at the OS level, creating something like core
+files), and rerunning them on need.
+
+If you want to simulate checkpoints instead, it means that you want the
+state of an executing task (in particular, the progress made towards
+completion) to be saved somewhere. So if a host (and the task executing on
+it) fails (cf. #MSG_HOST_FAILURE), then the task can be restarted
+from the last checkpoint.\n
+
+Actually, such a thing does not exists in SimGrid either, but it's just
+because we don't think it is fundamental and it may be done in the user code
+at relatively low cost. You could for example use a watcher that
+periodically get the remaining amount of things to do (using
+MSG_task_get_remaining_computation()), or fragment the task in smaller
+subtasks.
+
+\subsection faq_platform Platform building and Dynamic resources
+
+\subsubsection faq_platform Building a realistic platform
+
+We can speak more than an hour on this subject and we still do not have
+the right answer, just some ideas. You can read the following
+<a href="http://graal.ens-lyon.fr/~alegrand/articles/Simgrid-Introduction.pdf">slides</a>.
+It may give you some hints. You can also have a look at the
+<tt>tools/platform_generation/</tt> directory. There is a perl-script
+we use to annotate a Tiers generated platform.
-\subsection faq_SURF_dynami How can I have variable resource availability?
+\subsubsection faq_SURF_dynamic Building a dynamic platform, with resource availability
A nice feature of SimGrid is that it enables you to seamlessly have
resources whose availability change over time. When you build a
20.0 0.9
\endverbatim
-At time 0, our CPU will deliver 100 Mflop/s. At time 11.0, it will
-deliver only 50 Mflop/s until time 20.0 where it will will start
-delivering 90 Mflop/s. Last at time 21.0 (20.0 plus the periodicity
-1.0), we'll be back to the beginning and it will deliver 100Mflop/s.
+At time 0, our CPU will deliver 100 flop/s. At time 11.0, it will
+deliver only 50 flop/s until time 20.0 where it will will start
+delivering 90 flop/s. Last at time 21.0 (20.0 plus the periodicity
+1.0), we'll be back to the beginning and it will deliver 100 flop/s.
Now let's look at the state file:
\verbatim
You have at your disposal the following options: bandwidth_file,
latency_file and state_file. The only difference with CPUs is that
bandwidth_file and latency_file do not express fraction of available
-power but are expressed directly in Mb/s and seconds.
+power but are expressed directly in bytes per seconds and seconds.
-\section faq_flexml_bypassing How could I have some C functions do what the platform and deployment files do?
+\subsubsection faq_flexml_bypassing Bypassing the XML parser with your own C functions
So you want to bypass the XML files parser, uh? Maybe doin some parameter
sweep experiments on your simulations or so? This is possible, but it's not
An example of this trick is distributed in the file examples/msg/msg_test_surfxml_bypassed.c
-\section faq_flexml_limit I get the message "surf_parse_lex: Assertion `next<limit' failed."
+\section faq_troubleshooting Troubleshooting
+
+\subsection faq_trouble_compil Compilation and installation problems
+
+\subsubsection faq_trouble_config ./configure fails!
+
+We now only one reason for the configure to fail:
+
+ - <b>You are using a borken build environment</b>\n
+ If symptom is that configure complains about gcc not being able to build
+ executables, you are probably missing the libc6-dev package. Damn Ubuntu.
+
+If you experience other kind of issue, please get in touch with us. We are
+always interested in improving our portability to new systems.
+
+\subsubsection faq_trouble_distcheck Dude! "make check" fails on my machine!
+
+Don't assume we never run this target, because we do. Check
+http://bob.loria.fr:8010 if you don't believe us.
+
+There is several reasons which may cause the make check to fail on your
+machine:
+
+ - <b>You are using a borken libc (probably concerning the contextes)</b>.\n
+ The symptom is that the "make check" fails within the examples/msg directory.\n
+ By default, SimGrid uses something called ucontexts. This is part of the
+ libc, but it's quite undertested. For example, some (old) versions of the
+ glibc on alpha do not implement these functions, but provide the stubs
+ (which return ENOSYS: not implemented). It fools our detection mecanism
+ and leads to segfaults. There is not much we can do to fix the bug.
+ A workaround is to compile with --with-context=pthread to avoid
+ ucontext completely. You'll be a bit more limitated in the number
+ of simulated processes you can start concurently, but 5000
+ processes is still enough for most purposes, isn't it?\n
+ This limitation is the reason why we insist on using this piece of ...
+ software even if it's so troublesome.\n
+ <b>=> use --with-pthread on AMD64 architecture that do not have an
+ ultra-recent libc.</b>
+
+ - <b>There is a bug in SimGrid we aren't aware of</b>.\n
+ If none of the above apply, please drop us a mail on the mailing list so
+ that we can check it out. Make sure to read \ref faq_bugrepport
+ before you do so.
+
+\subsection faq_trouble_errors Understanding error messages
+
+\subsubsection faq_trouble_err_logcat "gcc: _simgrid_this_log_category_does_not_exist__??? undeclared (first use in this function)"
+
+This is because you are using the log mecanism, but you didn't created
+any default category in this file. You should refer to \ref XBT_log
+for all the details, but you simply forgot to call one of
+XBT_LOG_NEW_DEFAULT_CATEGORY() or XBT_LOG_NEW_DEFAULT_SUBCATEGORY().
+
+\subsubsection faq_flexml_limit "surf_parse_lex: Assertion `next limit' failed."
This is because your platform file is too big for the parser.
document in memory before parsing it. And moreover, the memory buffer size
must be determinded at compilation time.
-We use a value which seems big enough for our need withour bloating the
+We use a value which seems big enough for our need without bloating the
simulators footprints. But of course your mileage may vary. In this case,
just edit src/surf/surfxml.l modify the definition of
FLEXML_BUFFERSTACKSIZE. E.g.
the development of FleXML, I can grant you that any patches would be really
welcome and quickly integrated.
-\section faq_host_load Where is the get_host_load function hidden in MSG?
+<b>Update:</b> A new version of FleXML (1.7) was released. Most of the work
+was done by William Dowling, who use it in his own work. The good point is
+that it now use a dynamic buffer, and that the memory usage was greatly
+improved. The downside is that William also changed some things internally,
+and it breaks the hack we devised to bypass the parser, as explained in
+\ref faq_flexml_bypassing. Indeed, this is not a classical usage of the
+parser, and Will didn't imagine that we may have used (and even documented)
+such a crude usage of FleXML. So, we now have to repare the bypassing
+functionnality to use the lastest FleXML version and fix the memory usage in
+SimGrid.
+
+\subsubsection faq_trouble_gras_transport GRAS spits networking error messages
+
+Gras, on real platforms, naturally use regular sockets to communicate. They
+are deeply hiden in the gras abstraction, but when things go wrong, you may
+get some weird error messages. Here are some example, with the probable
+reason:
+
+ - <b>Transport endpoint is not connected</b>: several processes try to open
+ a server socket on the same port number of the same machine. This is
+ naturally bad and each process should pick its own port number for this.\n
+ Maybe, you just have some processes remaining from a previous experiment
+ on your machine.\n
+ Killing them may help, but again if you kill -KILL them, you'll have to
+ wait for a while: they didn't close there sockets properly and the system
+ needs a while to notice that this port is free again.
+
+ - <b>Socket closed by remote side</b>: if the remote process is not
+ supposed to close the socket at this point, it may be dead.
+
+ - <b>Connection reset by peer</b>: I found this on internet about this
+ error. I think it's what's happening here, too:\n
+ <i>This basically means that a network error occurred while the client was
+ receiving data from the server. But what is really happening is that the
+ server actually accepts the connection, processes the request, and sends
+ a reply to the client. However, when the server closes the socket, the
+ client believes that the connection has been terminated abnormally
+ because the socket implementation sends a TCP reset segment telling the
+ client to throw away the data and report an error.\n
+ Sometimes, this problem is caused by not properly closing the
+ input/output streams and the socket connection. Make sure you close the
+ input/output streams and socket connection properly. If everything is
+ closed properly, however, and the problem persists, you can work around
+ it by adding a one-second sleep before closing the streams and the
+ socket. This technique, however, is not reliable and may not work on all
+ systems.</i>\n
+ Since GRAS sockets are closed properly (repeat after me: there is no bug
+ in GRAS), it is either that you are closing your sockets on server side
+ before the client get a chance to read them (use gras_os_sleep() to delay
+ the server), or the server died awfully before the client got the data.
+
+\subsubsection faq_trouble_errors_big_fat_warning I'm told that my XML files are too old.
+
+We have decided to change the units in SimGrid. Now we use Bytes, Flops and
+seconds instead of MBytes, MFlops and seconds... Units should be updated
+accordingly and the version of platform_description should be set to a
+valuer greater than 1:
+\verbatim
+ <platform_description version="1">
+\endverbatim
+You should try to use the surfxml_update.pl script that can be found
+<a href="http://gforge.inria.fr/plugins/scmcvs/cvsweb.php/contrib/platform_generation/?cvsroot=cvsroot%2Fsimgrid">here</a>.
-There is no such thing because its semantic wouldn't be really clear. Of
-course, it is something about the amount of host throughput, but there is as
-many definition of "host load" as people asking for this function.
+\subsection faq_trouble_valgrind Valgrind-related issues
-It may be instantaneous value or an average one. Moreover it may be only the
-power of the computer, or may take the background load into account, or may
-even take the currently running tasks into account. In some SURF models,
-communications have an influence on computational power. Should it be taken
-into account too?
+If you don't, you really should use valgrind to debug your code, it's
+almost magic.
-So, we decided not to include such a function into MSG and let people do it
-thereselves so that they get the value matching exactly what they mean. One
-possibility is to run active measurement as in next code snippet. It is very
-close from what you would have to do out of the simulator, and thus gives
-you information that you could also get in real settings to not hinder the
-realism of your simulation.
+\subsubsection faq_trouble_vg_longjmp longjmp madness in valgrind
-\verbatim
-double get_host_load() {
- m_task_t task = MSG_task_create("test", 0.001, 0, NULL);
- double date = MSG_get_clock();
+This is when valgrind starts complaining about longjmp things, just like:
- MSG_task_execute(task);
- date = MSG_get_clock() - date;
- MSG_task_destroy(task);
- return (0.001/date);
-}
+\verbatim ==21434== Conditional jump or move depends on uninitialised value(s)
+==21434== at 0x420DBE5: longjmp (longjmp.c:33)
+==21434==
+==21434== Use of uninitialised value of size 4
+==21434== at 0x420DC3A: __longjmp (__longjmp.S:48)
\endverbatim
-Of course, it may not match your personal definition of "host load". In this
-case, please detail what you mean on the mailing list, and we will extend
-this FAQ section to fit your taste if possible.
+or even when it reports scary things like:
+
+\verbatim ==24023== Warning: client switching stacks? SP change: 0xBE3FF618 --> 0xBE7FF710
+x86->IR: unhandled instruction bytes: 0xF4 0xC7 0x83 0xD0
+==24023== to suppress, use: --max-stackframe=4194552 or greater
+==24023== Your program just tried to execute an instruction that Valgrind
+==24023== did not recognise. There are two possible reasons for this.
+==24023== 1. Your program has a bug and erroneously jumped to a non-code
+==24023== location. If you are running Memcheck and you just saw a
+==24023== warning about a bad jump, it's probably your program's fault.
+==24023== 2. The instruction is legitimate but Valgrind doesn't handle it,
+==24023== i.e. it's Valgrind's fault. If you think this is the case or
+==24023== you are not sure, please let us know.
+==24023== Either way, Valgrind will now raise a SIGILL signal which will
+==24023== probably kill your program.
+==24023==
+==24023== Process terminating with default action of signal 4 (SIGILL)
+==24023== Illegal opcode at address 0x420D234
+==24023== at 0x420D234: abort (abort.c:124)
+\endverbatim
+This is the sign that you didn't used the exception mecanism well. Most
+probably, you have a <tt>return;</tt> somewhere within a <tt>TRY{}</tt>
+block. This is <b>evil</b>, and you must not do this. Did you read the section
+about \ref XBT_ex??
+
+\subsubsection faq_trouble_vg_libc Valgrind spits tons of errors about backtraces!
+
+It may happen that valgrind, the memory debugger beloved by any decent C
+programmer, spits tons of warnings like the following :
+\verbatim ==8414== Conditional jump or move depends on uninitialised value(s)
+==8414== at 0x400882D: (within /lib/ld-2.3.6.so)
+==8414== by 0x414EDE9: (within /lib/tls/i686/cmov/libc-2.3.6.so)
+==8414== by 0x400B105: (within /lib/ld-2.3.6.so)
+==8414== by 0x414F937: _dl_open (in /lib/tls/i686/cmov/libc-2.3.6.so)
+==8414== by 0x4150F4C: (within /lib/tls/i686/cmov/libc-2.3.6.so)
+==8414== by 0x400B105: (within /lib/ld-2.3.6.so)
+==8414== by 0x415102D: __libc_dlopen_mode (in /lib/tls/i686/cmov/libc-2.3.6.so)
+==8414== by 0x412D6B9: backtrace (in /lib/tls/i686/cmov/libc-2.3.6.so)
+==8414== by 0x8076446: xbt_dictelm_get_ext (dict_elm.c:714)
+==8414== by 0x80764C1: xbt_dictelm_get (dict_elm.c:732)
+==8414== by 0x8079010: xbt_cfg_register (config.c:208)
+==8414== by 0x806821B: MSG_config (msg_config.c:42)
+\endverbatim
+
+This problem is somewhere in the libc when using the backtraces and there is
+very few things we can do ourselves to fix it. Instead, here is how to tell
+valgrind to ignore the error. Add the following to your ~/.valgrind.supp (or
+create this file on need). Make sure to change the obj line according to
+your personnal mileage (change 2.3.6 to the actual version you are using,
+which you can retrieve with a simple "ls /lib/ld*.so").
+
+\verbatim {
+ name: Backtrace madness
+ Memcheck:Cond
+ obj:/lib/ld-2.3.6.so
+ fun:dl_open_worker
+ fun:_dl_open
+ fun:do_dlopen
+ fun:dlerror_run
+ fun:__libc_dlopen_mode
+}\endverbatim
+
+Then, you have to specify valgrind to use this suppression file by passing
+the <tt>--suppressions=$HOME/.valgrind.supp</tt> option on the command line.
+You can also add the following to your ~/.bashrc so that it gets passed
+automatically. Actually, it passes a bit more options to valgrind, and this
+happen to be my personnal settings. Check the valgrind documentation for
+more information.
+
+\verbatim export VALGRIND_OPTS="--leak-check=yes --leak-resolution=high --num-callers=40 --tool=memcheck --suppressions=$HOME/.valgrind.supp" \endverbatim
+
+\subsection faq_deadlock There is a deadlock in my code!!!
+
+Unfortunately, we cannot debug every code written in SimGrid. We
+furthermore believe that the framework provides ways enough
+information to debug such informations yourself. If the textual output
+is not enough, Make sure to check the \ref faq_visualization FAQ entry to see
+how to get a graphical one.
+
+Now, if you come up with a really simple example that deadlocks and
+you're absolutely convinced that it should not, you can ask on the
+list. Just be aware that you'll be severely punished if the mistake is
+on your side... We have plenty of FAQ entries to redact and new
+features to implement for the impenitents! ;)
+
+\subsection faq_surf_network_latency I get weird timings when I play with the latencies.
+
+OK, first of all, remember that units should be Bytes, Flops and
+Seconds. If you don't use such units, some SimGrid constants (e.g. the
+SG_TCP_CTE_GAMMA constant used in most network models) won't have the
+right unit and you'll end up with weird results.
+
+Here is what happens with a single transfer of size L on a link
+(bw,lat) when nothing else happens.
+
+\verbatim
+0-----lat--------------------------------------------------t
+|-----|**** real_bw =min(bw,SG_TCP_CTE_GAMMA/(2*lat)) *****|
+\endverbatim
-\author Arnaud Legrand (arnaud.legran::imag.fr)
+In more complex situations, this min is the solution of a complex
+max-min linear system. Have a look
+<a href="http://lists.gforge.inria.fr/pipermail/simgrid-devel/2006-April/thread.html">here</a>
+and read the two threads "Bug in SURF?" and "Surf bug not
+fixed?". You'll have a few other examples of such computations. You
+can also read "A Network Model for Simulation of Grid Application" by
+Henri Casanova and Loris Marchal to have all the details. The fact
+that the real_bw is smaller than bw is easy to understand. The fact
+that real_bw is smaller than SG_TCP_CTE_GAMMA/(2*lat) is due to the
+window-based congestion mechanism of TCP. With TCP, you can't exploit
+your huge network capacity if you don't have a good round-trip-time
+because of the acks...
+
+Anyway, what you get is t=lat + L/min(bw,SG_TCP_CTE_GAMMA/(2*lat)).
+
+ * if I you set (bw,lat)=(100 000 000, 0.00001), you get t = 1.00001 (you fully
+use your link)
+ * if I you set (bw,lat)=(100 000 000, 0.0001), you get t = 1.0001 (you're on the
+limit)
+ * if I you set (bw,lat)=(100 000 000, 0.001), you get t = 10.001 (ouch!)
+
+This bound on the effective bandwidth of a flow is not the only thing
+that may make your result be unexpected. For example, two flows
+competing on a saturated link receive an amount of bandwidth inversely
+proportional to their round trip time.
+
+\subsection faq_bugrepport So I've found a bug in SimGrid. How to report it?
+
+We do our best to make sure to hammer away any bugs of SimGrid, but this is
+still an academic project so please be patient if/when you find bugs in it.
+If you do, the best solution is to drop an email either on the simgrid-user
+or the simgrid-devel mailing list and explain us about the issue. You can
+also decide to open a formal bug report using the
+<a href="https://gforge.inria.fr/tracker/?atid=165&group_id=12&func=browse">relevant
+interface</a>. You need to login on the server to get the ability to submit
+bugs.
+
+We will do our best to solve any problem repported, but you need to help us
+finding the issue. Just telling "it segfault" isn't enough. Telling "It
+segfaults when running the attached simulator" doesn't really help either.
+You may find the following article interesting to see how to repport
+informative bug repports:
+http://www.chiark.greenend.org.uk/~sgtatham/bugs.html (it is not SimGrid
+specific at all, but it's full of good advices).
+
+\author Arnaud Legrand (arnaud.legrand::imag.fr)
\author Martin Quinson (martin.quinson::loria.fr)
