/*! \page faq Frequently Asked Questions
-\htmlinclude FAQ.toc
+\htmlinclude .FAQ.doc.toc
\section faq_installation Installing the SimGrid library
\subsection faq_compiling Compiling SimGrid
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_compil_trouble". 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:
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 ?
+\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/Simgrid-Introduction.pdf">slides</a>
they receive one) and some forwarder agents (that simply pass the
tasks they receive to some slaves).
-\subsection faq_examples I want some more complex 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
-examples.
-
-Of course they have often been needing more complex functions like
-MSG_process_suspend(), MSG_process_resume() and
-MSG_process_isSuspended() (to perform synchronization), or
-MSG_task_Iprobe() and MSG_process_sleep() (to avoid blocking
-receptions), or even MSG_process_create() (to design asynchronous
-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
-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.
-
-\subsection faq_platform Building a realistic platform
-
-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).
-
\subsection faq_visualization Visualizing the schedule
It is sometime convenient to "see" how the agents are behaving. If you
\endhtmlonly
</center>
-\subsection faq_context_1000 I want thousands of simulated processes
+\subsection faq_postmortem_analysis Online/postmortem analysis
+
+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, the bindings needed to allow one to use SimGrid from Perl,
+Python, Java, etc. are double-layered. The first layer would allow
+you to call for example the MSG_task_get_name(task) function while
+what you really want is a proper object wrapping allowing you to call
+task->name(). That's the purpose of the second layer. The first one
+is granted with C++ but can be done with tools like
+<a href="www.swig.org/">swig</a> for other languages like Perl, Ruby,
+Python, CAML. None of us really need the second one (which is a bit
+more demanding and cannot be automatically generated) yet and there is
+no real point in doing the first one without the second. :)
+
+As usual, you're welcome to participate.
+
+\section faq_MIA How to ....? Is there a function in the API to simply ....?
+
+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_examples I want some more complex examples!
-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 contextes allow me to raise the limit to 25,000
-simulated processes on my laptop.
-
-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.
-
-We experienced some issues with contextes on some rare systems (solaris 8
-and lower 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 ;)
-
-\subsection faq_context_10000 I want hundred thousands of simulated processes
+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
+examples.
-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.
+Of course they have often been needing more complex functions like
+MSG_process_suspend(), MSG_process_resume() and
+MSG_process_isSuspended() (to perform synchronization), or
+MSG_task_Iprobe() and MSG_process_sleep() (to avoid blocking
+receptions), or even MSG_process_create() (to design asynchronous
+communications or computations). But the examples are sufficient to
+start.
-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).
+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 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!
+:)
+
+\subsection faq_MIA_taskdup Missing in action: 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_dict_t). If
+you still don't see how to do it, please come back to us...
+
+\subsection faq_MIA_asynchronous I want to do asynchronous communications.
+
+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.
+
+\subsection faq_MIA_thread_synchronization I need to synchronize my 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()).
+
+\subsection 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.
- - 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 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?
- - 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.
+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.
- - The deployment file became quite big, so I had to do what is in the FAQ
- entry \ref faq_flexml_limit
+\verbatim
+double get_host_load() {
+ m_task_t task = MSG_task_create("test", 0.001, 0, NULL);
+ double date = MSG_get_clock();
- - 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.
+ MSG_task_execute(task);
+ date = MSG_get_clock() - date;
+ MSG_task_destroy(task);
+ return (0.001/date);
+}
+\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.
-\section faq_SG Where has SG disappeared ?!?
+\subsection 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.
+
+\subsection 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.
+
+\section 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
+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:
\verbatim
to gforge so people that are interested by helping on this part will
have the possibility to do it.
-\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 ?
+\subsection 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.
+wide variety of conditions. 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
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).
+\subsection faq_SG_future Will SG come back in the maintained branch one day?
+
+Sure. In fact, we already have thought about a new and cleaner API:
+\verbatim
+void* SG_link_get_data(SG_link_t link);
+void SG_link_set_data(SG_link_t link, void *data);
+const char* SG_link_get_name(SG_link_t link);
+double SG_link_get_capacity(SG_link_t link);
+double SG_link_get_current_bandwidth(SG_link_t link);
+double SG_link_get_current_latency(SG_link_t link);
+
+SG_workstation_t SG_workstation_get_by_name(const char *name);
+SG_workstation_t* SG_workstation_get_list(void);
+int SG_workstation_get_number(void);
+void SG_workstation_set_data(SG_workstation_t workstation, void *data);
+void * SG_workstation_get_data(SG_workstation_t workstation);
+const char* SG_workstation_get_name(SG_workstation_t workstation);
+SG_link_t* SG_workstation_route_get_list(SG_workstation_t src, SG_workstation_t dst);
+int SG_workstation_route_get_size(SG_workstation_t src, SG_workstation_t dst);
+double SG_workstation_get_power(SG_workstation_t workstation);
+double SG_workstation_get_available_power(SG_workstation_t workstation);
+
+SG_task_t SG_task_create(const char *name, void *data, double amount);
+int SG_task_schedule(SG_task_t task, int workstation_nb,
+ SG_workstation_t **workstation_list, double *computation_amount,
+ double *communication_amount, double rate);
+
+void* SG_task_get_data(SG_task_t task);
+void SG_task_set_data(SG_task_t task, void *data);
+const char* SG_task_get_name(SG_task_t task);
+double SG_task_get_amount(SG_task_t task);
+double SG_task_get_remaining_amount(SG_task_t task);
+void SG_task_dependency_add(const char *name, void *data, SG_task_t src, SG_task_t dst);
+void SG_task_dependency_remove(SG_task_t src, SG_task_t dst);
+e_SG_task_state_t SG_task_state_get(SG_task_t task); /* e_SG_task_state_t can be either SG_SCHEDULED, SG_RUNNING, SG_DONE, or SG_FAILED */
+void SG_task_watch(SG_task_t task, e_SG_task_state_t state); /* SG_simulate will stop as soon as the state of this task is the one given in argument.
+ Watch-point is then automatically removed */
+void SG_task_unwatch(SG_task_t task, e_SG_task_state_t state);
+
+void SG_task_unschedule(SG_task_t task); /* change state and rerun.. */
+
+SG_task_t *SG_simulate(double how_long); /* returns a NULL-terminated array of SG_task_t whose state has changed */
+\endverbatim
+
+We're just looking for somebody to implement it... :)
+
+\section faq_dynamic Dynamic resources and platform building
-\subsection faq_SURF_dynami How can I have variable resource availability?
+\subsection 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_dynamic How can I have variable 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
bandwidth_file and latency_file do not express fraction of available
power but are expressed directly in Mb/s and seconds.
-\section faq_flexml_bypassing How could I have some C functions do what the platform and deployment files do?
+\subsection faq_flexml_bypassing How can I have some C functions do what the platform file does?
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_compil_trouble ./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.
+
+\subsection faq_distcheck_fails Dude! "make check" fails on my machine!
+
+Don't assume we never run this target, because we do. Really. Promise!
+
+There is several reasons which may cause the make check to fail on your
+machine:
+
+ - <b>You are using a borken compiler</b>.\n
+ The symptom may be that the "make check" fails within testsuite/gras
+ directory.\n
+ For example, the breezy release of Ubuntu comes with a prerelease of the
+ 4.0 gcc compiler. This version happens to be completely unusable, and you
+ should install a gcc-3.4 compiler and change the /usr/bin/gcc link to let
+ it point on /usr/bin/gcc-3.4.
+ - <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.\n
+ On some x86_64, the pointer to function is stored into a integer, but int
+ are 32bits only on this arch while pointers are 64bits. Our detection
+ mecanism also fails to detect the problem, which leads to segfaults.\n
+ In both cases, there is not much we can do to fix the bug. We are working
+ on a workaround for x86_64 machines, but in the meanwhile, you can
+ 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.
+ - <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.
+
+\subsection faq_context_1000 I want thousands of simulated processes
+
+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.
+
+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.
+
+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 ;)
+
+\subsection faq_context_10000 I want hundred thousands of simulated processes
+
+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.
+
+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).
+
+ - 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.
+
+ - 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.
+
+\subsection faq_longjmp longjmp madness
+
+This is when valgrind starts complaining about longjmp things, just like:
+
+\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
+
+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??
+
+\subsection faq_flexml_limit I get the message "surf_parse_lex: Assertion `next<limit' failed."
This is because your platform file is too big for the parser.
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?
-
-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_deadlock There is a deadlock !!!
-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?
-
-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.
-
-\verbatim
-double get_host_load() {
- m_task_t task = MSG_task_create("test", 0.001, 0, NULL);
- double date = MSG_get_clock();
-
- MSG_task_execute(task);
- date = MSG_get_clock() - date;
- MSG_task_destroy(task);
- return (0.001/date);
-}
-\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.
+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! ;)
-\author Arnaud Legrand (arnaud.legran::imag.fr)
+\author Arnaud Legrand (arnaud.legrand::imag.fr)
\author Martin Quinson (martin.quinson::loria.fr)
