starpu-max/doc/chapters/using.texi

@c -*-texinfo-*-

@c This file is part of the StarPU Handbook.
@c Copyright (C) 2009--2011  Universit@'e de Bordeaux 1
@c Copyright (C) 2010, 2011  Centre National de la Recherche Scientifique
@c Copyright (C) 2011 Institut National de Recherche en Informatique et Automatique
@c See the file starpu.texi for copying conditions.

@node Using StarPU
@chapter Using StarPU

@menu
* Setting flags for compiling and linking applications::  
* Running a basic StarPU application::  
* Kernel threads started by StarPU::
* Enabling OpenCL::
@end menu

@node Setting flags for compiling and linking applications
@section Setting flags for compiling and linking applications

Compiling and linking an application against StarPU may require to use
specific flags or libraries (for instance @code{CUDA} or @code{libspe2}).
To this end, it is possible to use the @code{pkg-config} tool.

If StarPU was not installed at some standard location, the path of StarPU's
library must be specified in the @code{PKG_CONFIG_PATH} environment variable so
that @code{pkg-config} can find it. For example if StarPU was installed in
@code{$prefix_dir}:

@example
% PKG_CONFIG_PATH=$PKG_CONFIG_PATH:$prefix_dir/lib/pkgconfig
@end example

The flags required to compile or link against StarPU are then
accessible with the following commands:

@example
% pkg-config --cflags libstarpu  # options for the compiler
% pkg-config --libs libstarpu    # options for the linker
@end example

@node Running a basic StarPU application
@section Running a basic StarPU application

Basic examples using StarPU are built in the directory
@code{examples/basic_examples/} (and installed in
@code{$prefix_dir/lib/starpu/examples/}). You can for example run the example
@code{vector_scal}.

@example
% ./examples/basic_examples/vector_scal
BEFORE : First element was 1.000000
AFTER First element is 3.140000
%
@end example

When StarPU is used for the first time, the directory
@code{$HOME/.starpu/} is created, performance models will be stored in
that directory.

Please note that buses are benchmarked when StarPU is launched for the
first time. This may take a few minutes, or less if @code{hwloc} is
installed. This step is done only once per user and per machine.

@node Kernel threads started by StarPU
@section Kernel threads started by StarPU

StarPU automatically binds one thread per CPU core. It does not use
SMT/hyperthreading because kernels are usually already optimized for using a
full core, and using hyperthreading would make kernel calibration rather random.

Since driving GPUs is a CPU-consuming task, StarPU dedicates one core per GPU

While StarPU tasks are executing, the application is not supposed to do
computations in the threads it starts itself, tasks should be used instead.

TODO: add a StarPU function to bind an application thread (e.g. the main thread)
to a dedicated core (and thus disable the corresponding StarPU CPU worker).

@node Enabling OpenCL
@section Enabling OpenCL

When both CUDA and OpenCL drivers are enabled, StarPU will launch an
OpenCL worker for NVIDIA GPUs only if CUDA is not already running on them.
This design choice was necessary as OpenCL and CUDA can not run at the
same time on the same NVIDIA GPU, as there is currently no interoperability
between them.

To enable OpenCL, you need either to disable CUDA when configuring StarPU:

@example
% ./configure --disable-cuda
@end example

or when running applications:

@example
% STARPU_NCUDA=0 ./application
@end example

OpenCL will automatically be started on any device not yet used by
CUDA. So on a machine running 4 GPUS, it is therefore possible to
enable CUDA on 2 devices, and OpenCL on the 2 other devices by doing
so:

@example
% STARPU_NCUDA=2 ./application
@end example