Document how to permit simgrid execution on desktop without CUDA/OpenCL

doc/chapters/perf-optimization.texi

@@ -495,7 +495,11 @@ visualize the task graph more easily.
 @section Simulated performance
 
 StarPU can use Simgrid in order to simulate execution on an arbitrary
-platform. The idea is to first compile StarPU normally, and run the application,
+platform.
+
+@subsection Calibration
+
+The idea is to first compile StarPU normally, and run the application,
 so as to automatically benchmark the bus and the codelets.
 
 @smallexample
@@ -513,6 +517,8 @@ Note that we force to use the dmda scheduler to generate performance
 models for the application. The application may need to be run several
 times before the model is calibrated.
 
+@subsection Simulation
+
 Then, recompile StarPU, passing @code{--enable-simgrid} to @code{./configure}, and re-run the
 application, specifying the requested number of devices:
 
@@ -556,3 +562,35 @@ Note: of course, if the application uses @code{gettimeofday} to make its
 performance measurements, the real time will be used, which will be bogus. To
 get the simulated time, it has to use @code{starpu_timing_now} which returns the
 virtual timestamp in ms.
+
+@subsection Simulation on another machine
+
+The simgrid support even permits to perform simulations on another machine, your
+desktop, typically. To achieve this, one still needs to perform the Calibration
+step on the actual machine to be simulated, then copy them to your desktop
+machine (the @code{$STARPU_HOME/.starpu} directory). One can then performa the
+Simulation step on the desktop machine, by setting the @code{STARPU_HOSTNAME}
+environment variable to the name of the actual machine, to make StarPU use the
+performance models of the simulated machine even on the desktop machine.
+
+If the desktop machine does not have CUDA or OpenCL, StarPU is still able to
+use simgrid to simulate execution with CUDA/OpenCL devices, but the application
+source code will probably disable the CUDA and OpenCL codelets in that
+case. Since during simgrid execution, the functions of the codelet are actually
+not called, one can use dummy functions such as the following to still permit
+CUDA or OpenCL execution:
+
+@smallexample
+static struct starpu_codelet cl11 =
+{
+	.cpu_funcs = {chol_cpu_codelet_update_u11, NULL},
+#ifdef STARPU_USE_CUDA
+	.cuda_funcs = {chol_cublas_codelet_update_u11, NULL},
+#elif defined(STARPU_SIMGRID)
+	.cuda_funcs = {(void*)1, NULL},
+#endif
+	.nbuffers = 1,
+	.modes = {STARPU_RW},
+	.model = &chol_model_11
+};
+@end smallexample