make doxygen happy

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

/*! \page FAQ Frequently Asked Questions

@tableofcontents

\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://www.loria.fr/~quinson/blog/2010/06/28/Tutorial_at_HPCS/">the slides of the HPCS'10 tutorial</a>
(or to these <a href="http://graal.ens-lyon.fr/~alegrand/articles/slides_g5k_simul.pdf">ancient
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. 

If you are stuck at any point and if this FAQ cannot help you, please drop us a
mail to the user mailing list: <simgrid-user@lists.gforge.inria.fr>.

\subsection faq_interfaces What is the difference between MSG and SimDag? 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. This is the interface of choice for people wanting to port old
code designed for SimGrid v1 or v2 to the framework current version.

With MSG, your application is seen as a set of communicating
processes, exchanging data by the way of messages and performing
computation on their own.

\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 section \ref options_tracing.

\subsection faq_C Argh! Do I really have to code in C?

Currently bindings on top of MSG are supported for Java, Ruby and Lua. You can find a few
documentation about them on the doc page. Note that bindings are released separately from the main dist
and so have their own version numbers.

Moreover 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).

For now,
we do not feel a real demand for any other language. But if you think there is one,
 please speak up!

\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, ...) is
meant to be kept as simple and generic as possible. We cannot add
functions for everybody's needs 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
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.

We know. We should add some more examples, but not really some more
complex ones... We should add some examples that illustrate some other
functionalists (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!
:)

\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 recommend
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 information 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 dictionary (#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

In the past (version <= 3.4), there was no function to perform asynchronous communications.
It could easily be implemented by creating new process when needed though. Since version 3.5,
we have introduced the following functions:
 - MSG_task_isend()
 - MSG_task_irecv()
 - MSG_comm_test()
 - MSG_comm_wait()
 - MSG_comm_waitall()
 - MSG_comm_waitany()
 - MSG_comm_destroy()

We refer you to the description of these functions for more details on their usage as well
as to the example section on \ref MSG_ex_asynchronous_communications.

\subsubsection faq_MIA_thread_synchronization I need to synchronize my MSG processes

You obviously cannot use pthread_mutexes of pthread_conds since we handle every
scheduling related decision within SimGrid.

In the past (version <=3.3.4) you could do it by playing with
MSG_process_suspend() and MSG_process_resume() or with fake communications (using MSG_task_get(),
MSG_task_put() and MSG_task_Iprobe()).

Since version 3.4, you can use classical synchronization structures. See page \ref XBT_synchro or simply check in
include/xbt/synchro_core.h.

\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.

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?

First of all, it's near to impossible to predict the load beforehand in the
simulator since it depends on too much parameters (background load
variation, bandwidth sharing algorithmic complexity) some of them even being
not known beforehand (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.

\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.

\subsubsection faq_MIA_communication_time How can I get the *real* communication time?

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.

\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);
  XBT_INFO("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);
  XBT_INFO("Communication time :  \"%f\" ", time2-time1);
  free(task->data);
  MSG_task_destroy(task);
  return 0;
}
\endverbatim

\subsection faq_MIA_SimDag SimDag related questions

\subsubsection faq_SG_comm Implementing communication delays between tasks.

A classic question of SimDag newcomers is about how to express a
communication delay between tasks. The thing is that in SimDag, both
computation and communication are seen as tasks.  So, if you want to
model a data dependency between two DAG tasks t1 and t2, you have to
create 3 SD_tasks: t1, t2 and c and add dependencies in the following
way:

\verbatim
SD_task_dependency_add(NULL, NULL, t1, c);
SD_task_dependency_add(NULL, NULL, c, t2);
\endverbatim

This way task t2 cannot start before the termination of communication c
which in turn cannot start before t1 ends.

When creating task c, you have to associate an amount of data (in bytes)
corresponding to what has to be sent by t1 to t2.

Finally to schedule the communication task c, you have to build a list
comprising the workstations on which t1 and t2 are scheduled (w1 and w2
for example) and build a communication matrix that should look like
[0;amount ; 0; 0].

\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. 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, 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 contexts. 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 contexts
   (<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 contexts 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 contexts 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 contexts 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, i.e. 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/mutual_exclusion/simple_token/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 pf_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 hairy, the default value is a bit overestimated so that
     user doesn't get into trouble about this. You want to tune this
     size to increase the number of processes. This is the
     <tt>STACK_SIZE</tt> define in
     <tt>src/xbt/xbt_context_sysv.c</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.
   - You may tweak the logs to reduce the stack size further.  When
     logging something, we try to build the string to display in a
     char array on the stack. The size of this array is constant (and
     equal to XBT_LOG_BUFF_SIZE, defined in include/xbt/log/h). If the
     string is too large to fit this buffer, we move to a dynamically
     sized buffer. In which case, we have to traverse one time the log
     event arguments to compute the size we need for the buffer,
     malloc it, and traverse the argument list again to do the actual
     job.\n
     The idea here is to move XBT_LOG_BUFF_SIZE to 1, forcing the logs
     to use a dynamic array each time. This allows us to lower further
     the stack size at the price of some performance loss...\n
     This allowed me to run the reduce the stack size to ... 4k. Ie,
     on my 1Gb laptop, I can run more than 250,000 processes!

\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 SVN so that other can
keep working on it. You may find inspiring 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 exist 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_example Where can I find SimGrid platform files?

There are several little examples in the archive, in the examples/msg
directory. From time to time, we are asked for other files, but we
don't have much at hand right now.

You should refer to the Platform Description Archive
(http://pda.gforge.inria.fr) project to see the other platform file we
have available, as well as the Simulacrum simulator, meant to generate
SimGrid platforms using all classical generation algorithms.

\subsubsection faq_platform_alnem How can I automatically map an existing platform?

We are working on a project called ALNeM (Application-Level Network
Mapper) which goal is to automatically discover the topology of an
existing network. Its output will be a platform description file
following the SimGrid syntax, so everybody will get the ability to map
their own lab network (and contribute them to the catalog project).
This tool is not ready yet, but it move quite fast forward. Just stay
tuned.

\subsubsection faq_platform_synthetic Generating synthetic but realistic platforms

The third possibility to get a platform file (after manual or
automatic mapping of real platforms) is to generate synthetic
platforms. Getting a realistic result is not a trivial task, and
moreover, nobody is really able to define what "realistic" means when
speaking of topology files. You can find some more thoughts on this
topic in these
<a href="http://graal.ens-lyon.fr/~alegrand/articles/Simgrid-Introduction.pdf">slides</a>.

If you are looking for an actual tool, there we have a little tool to
annotate Tiers-generated topologies. This perl-script is in
<tt>tools/platform_generation/</tt> directory of the SVN. Dinda et Al.
released a very comparable tool, and called it GridG.


The specified computing power will be available to up to 6 sequential
tasks without sharing. If more tasks are placed on this host, the
resource will be shared accordingly. For example, if you schedule 12
tasks on the host, each will get half of the computing power. Please
note that although sound, this model were never scientifically
assessed. Please keep this fact in mind when using it.




\section faq_troubleshooting Troubleshooting

\subsection faq_trouble_changelog The feature X stopped to work after my last update 

I guess that you want to read the ChangeLog file, that always contains
all the information that could be important to the users during the
upgrade. Actually, you may want to read it (alongside with the NEWS
file that highlights the most important changes) even before you
upgrade your copy of SimGrid, too.

Backward compatibility is very important to us, as we want to provide
a scientific tool allowing to evaluate the code you write in several
years, too. That being said, we sometimes change the interface to make
them more usable to the users. When we do so, we always keep the old
interface as DEPRECATED. If you still want to use them, you want to
define the SIMGRID_DEPRECATED preprocessor symbol before loading the
SimGrid files:

@verbatim
#define SIMGRID_DEPRECATED
#include <msg/msg.h>
@endverbatim

\subsection faq_trouble_lib_compil SimGrid compilation and installation problems

\subsubsection faq_trouble_lib_config cmake fails!

We know only one reason for the configure to fail:

 - <b>You are using a broken build environment</b>\n
   If symptom is that the configury magic 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! "ctest" fails on my machine!

Don't assume we never run this target, because we do. Check
http://cdash.inria.fr/CDash/index.php?project=Simgrid (click on
previous if there is no result for today: results are produced only by
11am, French time) and
https://buildd.debian.org/status/logs.php?pkg=simgrid if you don't believe us.

If it's failing on your machine in a way not experienced by the
autobuilders above, 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_compil User code compilation problems

\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_trouble_pthreadstatic "gcc: undefined reference to pthread_key_create"

This indicates that one of the library SimGrid depends on (libpthread
here) was missing on the linking command line. Dependencies of
libsimgrid are expressed directly in the dynamic library, so it's
quite impossible that you see this message when doing dynamic linking.

If you compile your code statically (and if you use a pthread version
of SimGrid -- see \ref faq_more_processes), you must absolutely
specify <tt>-lpthread</tt> on the linker command line. As usual, this should
come after <tt>-lsimgrid</tt> on this command line.

\subsubsection faq_trouble_lib_msg_deprecated "gcc: undefined reference to MSG_*"

Since version 3.7 all the m_channel_t mecanism is deprecated. So functions
about this mecanism may get removed in future releases.

List of functions:

\li XBT_PUBLIC(int) MSG_get_host_number(void);

\li XBT_PUBLIC(m_host_t *) MSG_get_host_table(void);

\li XBT_PUBLIC(MSG_error_t) MSG_get_errno(void);

\li XBT_PUBLIC(MSG_error_t) MSG_task_get(m_task_t * task, m_channel_t channel);

\li XBT_PUBLIC(MSG_error_t) MSG_task_get_with_timeout(m_task_t * task, m_channel_t channel, double max_duration);

\li XBT_PUBLIC(MSG_error_t) MSG_task_get_from_host(m_task_t * task, int channel, m_host_t host);

\li XBT_PUBLIC(MSG_error_t) MSG_task_get_ext(m_task_t * task, int channel, double max_duration, m_host_t host);

\li XBT_PUBLIC(MSG_error_t) MSG_task_put(m_task_t task, m_host_t dest, m_channel_t channel);

\li XBT_PUBLIC(MSG_error_t) MSG_task_put_bounded(m_task_t task, m_host_t dest, m_channel_t channel, double max_rate);

\li XBT_PUBLIC(MSG_error_t) MSG_task_put_with_timeout(m_task_t task, m_host_t dest, m_channel_t channel, double max_duration);

\li XBT_PUBLIC(int) MSG_task_Iprobe(m_channel_t channel);

\li XBT_PUBLIC(int) MSG_task_probe_from(m_channel_t channel);

\li XBT_PUBLIC(int) MSG_task_probe_from_host(int channel, m_host_t host);

\li XBT_PUBLIC(MSG_error_t) MSG_set_channel_number(int number);

\li XBT_PUBLIC(int) MSG_get_channel_number(void);

If you want them you have to compile Simgrid v3.7 with option "-Denable_msg_deprecated=ON".
Using them should print warning to inform what new function you have to use.

\subsection faq_trouble_errors Runtime error messages

\subsubsection faq_flexml_limit "surf_parse_lex: Assertion `next limit' failed."

This is because your platform file is too big for the parser.

Actually, the message comes directly from FleXML, the technology on top of
which the parser is built. FleXML has the bad idea of fetching the whole
document in memory before parsing it. And moreover, the memory buffer size
must be determined at compilation time.

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.

\verbatim
#define FLEXML_BUFFERSTACKSIZE 1000000000
\endverbatim

Then recompile and everything should be fine, provided that your version of
Flex is recent enough (>= 2.5.31). If not the compilation process should
warn you.

A while ago, we worked on FleXML to reduce a bit its memory consumption, but
these issues remain. There is two things we should do:

  - use a dynamic buffer instead of a static one so that the only limit
    becomes your memory, not a stupid constant fixed at compilation time
    (maybe not so difficult).
  - change the parser so that it does not need to get the whole file in
    memory before parsing
    (seems quite difficult, but I'm a complete newbe wrt flex stuff).

These are changes to FleXML itself, not SimGrid. But since we kinda hijacked
the development of FleXML, I can grant you that any patches would be really
welcome and quickly integrated.

<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 pf_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 repair the bypassing
functionality to use the latest FleXML version and fix the memory usage in
SimGrid.

\subsubsection faq_trouble_errors_big_fat_warning I'm told that my XML files are too old.

The format of the XML platform description files is sometimes
improved. For example, we decided to change the units used in SimGrid
from MBytes, MFlops and seconds to Bytes, Flops and seconds to ease
people exchanging small messages. We also reworked the route
descriptions to allow more compact descriptions.

That is why the XML files are versionned using the 'version' attribute
of the root tag. Currently, it should read:
\verbatim
  <platform version="2">
\endverbatim

If your files are too old, you can use the simgrid_update_xml.pl
script which can be found in the tools directory of the archive.

\subsection faq_trouble_valgrind Valgrind-related and other debugger issues

If you don't, you really should use valgrind to debug your code, it's
almost magic.

\subsubsection faq_trouble_vg_longjmp longjmp madness in valgrind

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

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

\subsubsection faq_trouble_backtraces Truncated backtraces

When debugging SimGrid, it's easier to pass the
--disable-compiler-optimization flag to the configure if valgrind or
gdb get fooled by the optimization done by the compiler. But you
should remove these flag when everything works before going in
production (before launching your 1252135 experiments), or everything
will run only one half of the true SimGrid potential.

\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

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 Da SimGrid team <simgrid-devel@lists.gforge.inria.fr>

*/

******************************************************************
*              OLD CRUFT NOT USED ANYMORE                        *
******************************************************************


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 SVN 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 SVN) 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
SVN 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

The DLL is built 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

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

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

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.