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 128 (in KiB), 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 kernel (default value: 1e-6). Please note that in some
circonstances, this optimization can hinder the simulation accuracy.
+If the "application" is in fact doing a "live replay" of another MPI
+app (e.g., ScalaTrace's replay tool, various on-line simulators that
+run an app at scale), the computation due to the replay logic should
+not be simulated by SMPI. In that case the \b
+smpi/simulation_computation item can be set to 'no' causing all the
+compute bursts between MPI calls to be ignored by SMPI. Then only the
+communications are simulated. This implies to add explicit calls to \c
+smpi_execute() in the "application" to simulate computations.
+
+
\subsection options_smpi_timing Reporting simulation time
Most of the time, you run MPI code through SMPI to compute the time it
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
- \c surf/nthreads: \ref options_model_nthreads
+- \c smpi/simulation_computation: \ref options_smpi_bench
- \c smpi/running_power: \ref options_smpi_bench
- \c smpi/display_timing: \ref options_smpi_timing
- \c smpi/cpu_threshold: \ref options_smpi_bench
- \c smpi/async_small_thres: \ref options_model_network_asyncsend
- \c smpi/send_is_detached: \ref options_model_smpi_detached
- \c smpi/coll_selector: \ref options_model_smpi_collectives
+- \c smpi/privatize_global_variables: \ref options_smpi_global
- \c path: \ref options_generic_path
- \c verbose-exit: \ref options_generic_exit
