If you want to push the scalability limits of your code, you really
want to reduce the \b contexts/stack_size item. Its default value
-is 8 (in MiB), while our Chord simulation works with stacks as small
+is 8192 (in KiB), while our Chord simulation works with stacks as small
as 16 KiB, for example. For the thread factory, the default value
is the one of the system, if it is too large/small, it has to be set
with this parameter.
Simulation time: 1e3 seconds.
\endverbatim
-\subsection options_model_smpi_detached Simulating MPI detached send
+\subsection options_smpi_global Automatic privatization of global variables
+
+MPI executables are meant to be executed in separated processes, but SMPI is
+executed in only one process. Global variables from executables will be placed
+in the same memory zone and shared between processes, causing hard to find bugs.
+To avoid this, several options are possible :
+ - Manual edition of the code, for example to add __thread keyword before data
+ declaration, which allows the resulting code to work with SMPI, but only
+ if the thread factory (see \ref options_virt_factory) is used, as global
+ variables are then placed in the TLS (thread local storage) segment.
+ - Source-to-source transformation, to add a level of indirection
+ to the global variables. SMPI does this for F77 codes compiled with smpiff,
+ and used to provide coccinelle scripts for C codes, which are not functional anymore.
+ - Compilation pass, to have the compiler automatically put the data in
+ an adapted zone.
+ - Runtime automatic switching of the data segments. SMPI stores a copy of
+ each global data segment for each process, and at each context switch replaces
+ the actual data with its copy from the right process. This mechanism uses mmap,
+ and is for now limited to systems supporting this functionnality (all Linux
+ and some BSD should be compatible).
+ Another limitation is that SMPI only accounts for global variables defined in
+ the executable. If the processes use external global variables from dynamic
+ libraries, they won't be switched correctly. To avoid this, using static
+ linking is advised (but not with the simgrid library, to avoid replicating
+ its own global variables).
+
+ To use this runtime automatic switching, the variable \b smpi/privatize_global_variables
+ should be set to yes
-(this configuration item is experimental and may change or disapear)
+
+
+\subsection options_model_smpi_detached Simulating MPI detached send
This threshold specifies the size in bytes under which the send will return
immediately. This is different from the threshold detailed in \ref options_model_network_asyncsend
uses naive version of collective operations). Each collective operation can be manually selected with a
\b smpi/collective_name:algo_name. Available algorithms are listed in \ref SMPI_collective_algorithms .
+\subsection options_model_smpi_computation_simulation Benchmarking/simulating application computation
+
+By default, SMPI benchmarks computational phases of the simulated application (i.e., CPU bursts in
+between MPI calls) so that these phases can be simulated. In some cases, however, one may wish to
+disable simulation of application computation. This is the case when SMPI is used not to simulate
+an MPI applications, but instead an MPI code that performs "live replay" of another MPI app (e.g.,
+ScalaTrace's replay tool, various on-line simulators that run an app at scale). In this case the
+computation of the replay/simulation logic should not be simulated by SMPI. Instead, the replay
+tool or on-line simulator will issue "computation events", which correspond to the actual MPI simulation
+being replayed/simulated. At the moment, these computation events can be simulated using SMPI by
+calling internal smpi_execute*() functions.
+
+To disable the benchmarking/simulation of computation in the simulated application via this runtime automatic
+switching, the variable \b smpi/privatize_global_variables should be set to no
+
\section options_generic Configuring other aspects of SimGrid
\subsection options_generic_path XML file inclusion path
