X-Git-Url: http://info.iut-bm.univ-fcomte.fr/pub/gitweb/simgrid.git/blobdiff_plain/3822f6694a3dff704094df6a86ece9c253b271e2..d5b29830df70e823202e384a7655e4371193ecd7:/doc/doxygen/FAQ.doc?ds=sidebyside

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@@ -4,14 +4,15 @@
 
 \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. 
+You are at the right place... To understand what you can do or
+cannot do with SimGrid, you should read the
+<a href="http://simgrid.gforge.inria.fr/tutorials.php">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 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>.
@@ -140,19 +141,44 @@ MSG_task_isend() and MSG_task_irecv();
 MSG_comm_test(), MSG_comm_wait(), MSG_comm_waitall() and MSG_comm_waitany();
 MSG_comm_destroy(). 
 
-There is even a specific 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.
+There is even a specific example section on \ref msg_ex_async .
+
+\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_host_load Where is the get_host_load function hidden in MSG?