prepare the release of 3.22.4

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

/*! @page FAQ MSG Frequently Asked Questions

@tableofcontents

This document is the FAQ of the MSG interface. Some entries are a bit aging and it should be refreshed at some point.

@section faq_simgrid I'm new to SimGrid. I have some questions. Where should I start?

You are at the right place... To understand what you can do or
cannot do with SimGrid, you should read the
<a href="https://simgrid.org/tutorials.html">tutorial
slides</a> from the SimGrid's website. You may find more uptodate
material on the
<a href="http://people.irisa.fr/Martin.Quinson/blog/SimGrid/">blog of
Martin Quinson</a>. 

Another great source of inspiration can be found in the @ref s4u_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?

We provide Java bindings of the MSG interface, which is the main
SimGrid user API.

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_thread_synchronization How to synchronize my user processes?

It depends on why you want to synchronize them.  If you just want to
have a shared state between your processes, then you probably don't
need to do anything. User processes never get forcefully interrupted
in SimGrid (unless you explicitly request the parallel execution of
user processes -- see @ref options_virt_parallel).

Even if several processes are executed at the exact same time within
the simulation, they are linearized in reality by default: one process
always run in an exclusive manner, atomic, uninterrupted until it does
a simcall (until it ask a service from the simulation kernel). This is
surprising at first, but things are much easier this way, both for the
user (who don't have to protect her shared data) and for the kernel
(that avoid many synchronization issues too). Processes are executed
concurrently in the simulated realm, but you don't need to bother with
this in the real realm.

If you really need to synchronize your processes (because it's what
you are studying or to create an atomic section that spans over
several simcalls), you obviously cannot use regular synchronization
mechanisms (pthread_mutexes in C or the synchronized keyword in Java).
This is because the SimGrid kernel locks all processes and unlock them
one after the other when they are supposed to run, until they give the
control back in their simcall. If one of them gets locked by the OS 
before returning the control to the kernel, that's definitively a
deadlock.

Instead, you should use the synchronization mechanism provided by the
simulation kernel. This could with a SimGrid mutex, a SimGrid
condition variables or a SimGrid semaphore, as described in @ref
msg_synchro (in Java, only semaphores are available). But actually,
many synchronization patterns can be encoded with communication on
mailboxes. Typically, if you need one process to notify another one,
you could use a condition variable or a semphore, but sending a
message to a specific mailbox does the trick in most cases.

@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.

@code
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;
}
@endcode

@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:

@code
SD_task_dependency_add(t1, c);
SD_task_dependency_add(c, t2);
@endcode

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.

@code
 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 */
     }
   }
 }
@endcode

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_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/platforms
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_synthetic Generating synthetic but realistic platforms

Another possibility to get a platform file 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.

We do our best to maintain the backward compatibility, but we
sometimes have to fix the things that are too broken. If we happen to
kill a feature that you were using, we are sorry. We think that you
should update to the new way of doing things, but if you can't afford
it, that's ok. Just stick to the last version that were working for
you, and have a pleasant day.

@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 mechanism, 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), 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.

@subsection faq_trouble_errors Runtime error messages

@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="4">
@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_debug Debugging SMPI applications

In order to debug SMPI programs, you can use the following options:

- <b>-wrapper 'gdb --args'</b>: this option is used to use a wrapper
  in order to call the SMPI process. Good candidates for this options
  are "gdb --args", "valgrind", "rr record", "strace", etc;

- <b>-foreground</b>: this options gives the debugger access to the terminal
  which is needed in order to use an interactive debugger.

Both options are needed in order to run the SMPI process under GDB.

@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_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 information 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).

*/