starpu-max/doc/chapters/advanced-examples.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, 2012  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.

@menu
* Using multiple implementations of a codelet::
* Enabling implementation according to capabilities::
* Task and Worker Profiling::   
* Partitioning Data::           Partitioning Data
* Performance model example::   
* Theoretical lower bound on execution time::  
* Insert Task Utility::          
* Parallel Tasks::
* Debugging::
* The multiformat interface::
* On-GPU rendering::
* More examples::               More examples shipped with StarPU
@end menu

@node Using multiple implementations of a codelet
@section Using multiple implementations of a codelet
One may want to write multiple implementations of a codelet for a single type of
device and let StarPU choose which one to run. As an example, we will show how
to use SSE to scale a vector. The codelet can be written as follows:

@cartouche
@smallexample
#include <xmmintrin.h>

void scal_sse_func(void *buffers[], void *cl_arg)
@{
    float *vector = (float *) STARPU_VECTOR_GET_PTR(buffers[0]);
    unsigned int n = STARPU_VECTOR_GET_NX(buffers[0]);
    unsigned int n_iterations = n/4;
    if (n % 4 != 0)
        n_iterations++;

    __m128 *VECTOR = (__m128*) vector;
    __m128 factor __attribute__((aligned(16)));
    factor = _mm_set1_ps(*(float *) cl_arg);

    unsigned int i;    
    for (i = 0; i < n_iterations; i++)
        VECTOR[i] = _mm_mul_ps(factor, VECTOR[i]);
@}
@end smallexample
@end cartouche

@cartouche
@smallexample
struct starpu_codelet cl = @{
    .where = STARPU_CPU,
    .cpu_funcs = @{ scal_cpu_func, scal_sse_func, NULL @},
    .nbuffers = 1,
    .modes = @{ STARPU_RW @}
@};
@end smallexample
@end cartouche

Schedulers which are multi-implementation aware (only @code{dmda}, @code{heft}
and @code{pheft} for now) will use the performance models of all the
implementations it was given, and pick the one that seems to be the fastest.

@node Enabling implementation according to capabilities
@section Enabling implementation according to capabilities

Some implementations may not run on some devices. For instance, some CUDA
devices do not support double floating point precision, and thus the kernel
execution would just fail; or the device may not have enough shared memory for
the implementation being used. The @code{can_execute} field of the @code{struct
starpu_codelet} structure permits to express this. For instance:

@cartouche
@smallexample
static int can_execute(unsigned workerid, struct starpu_task *task, unsigned nimpl)
@{
  const struct cudaDeviceProp *props;
  if (starpu_worker_get_type(workerid) == STARPU_CPU_WORKER)
    return 1;
  /* Cuda device */
  props = starpu_cuda_get_device_properties(workerid);
  if (props->major >= 2 || props->minor >= 3)
    /* At least compute capability 1.3, supports doubles */
    return 1;
  /* Old card, does not support doubles */
  return 0;
@}

struct starpu_codelet cl = @{
    .where = STARPU_CPU|STARPU_CUDA,
    .can_execute = can_execute,
    .cpu_funcs = @{ cpu_func, NULL @},
    .cuda_funcs = @{ gpu_func, NULL @}
    .nbuffers = 1,
    .modes = @{ STARPU_RW @}
@};
@end smallexample
@end cartouche

This can be essential e.g. when running on a machine which mixes various models
of CUDA devices, to take benefit from the new models without crashing on old models.

Note: the @code{can_execute} function is called by the scheduler each time it
tries to match a task with a worker, and should thus be very fast. The
@code{starpu_cuda_get_device_properties} provides a quick access to CUDA
properties of CUDA devices to achieve such efficiency.

Another example is compiling CUDA code for various compute capabilities,
resulting with two CUDA functions, e.g. @code{scal_gpu_13} for compute capability
1.3, and @code{scal_gpu_20} for compute capability 2.0. Both functions can be
provided to StarPU by using @code{cuda_funcs}, and @code{can_execute} can then be
used to rule out the @code{scal_gpu_20} variant on a CUDA device which
will not be able to execute it:

@cartouche
@smallexample
static int can_execute(unsigned workerid, struct starpu_task *task, unsigned nimpl)
@{
  const struct cudaDeviceProp *props;
  if (starpu_worker_get_type(workerid) == STARPU_CPU_WORKER)
    return 1;
  /* Cuda device */
  if (nimpl == 0)
    /* Trying to execute the 1.3 capability variant, we assume it is ok in all cases.  */
    return 1;
  /* Trying to execute the 2.0 capability variant, check that the card can do it.  */
  props = starpu_cuda_get_device_properties(workerid);
  if (props->major >= 2 || props->minor >= 0)
    /* At least compute capability 2.0, can run it */
    return 1;
  /* Old card, does not support 2.0, will not be able to execute the 2.0 variant.  */
  return 0;
@}

struct starpu_codelet cl = @{
    .where = STARPU_CPU|STARPU_CUDA,
    .can_execute = can_execute,
    .cpu_funcs = @{ cpu_func, NULL @},
    .cuda_funcs = @{ scal_gpu_13, scal_gpu_20, NULL @},
    .nbuffers = 1,
    .modes = @{ STARPU_RW @}
@};
@end smallexample
@end cartouche

Note: the most generic variant should be provided first, as some schedulers are
not able to try the different variants.

@node Task and Worker Profiling
@section Task and Worker Profiling

A full example showing how to use the profiling API is available in
the StarPU sources in the directory @code{examples/profiling/}.

@cartouche
@smallexample
struct starpu_task *task = starpu_task_create();
task->cl = &cl;
task->synchronous = 1;
/* We will destroy the task structure by hand so that we can
 * query the profiling info before the task is destroyed. */
task->destroy = 0;

/* Submit and wait for completion (since synchronous was set to 1) */
starpu_task_submit(task);

/* The task is finished, get profiling information */
struct starpu_task_profiling_info *info = task->profiling_info;

/* How much time did it take before the task started ? */
double delay += starpu_timing_timespec_delay_us(&info->submit_time, &info->start_time);

/* How long was the task execution ? */
double length += starpu_timing_timespec_delay_us(&info->start_time, &info->end_time);

/* We don't need the task structure anymore */
starpu_task_destroy(task);
@end smallexample
@end cartouche

@cartouche
@smallexample
/* Display the occupancy of all workers during the test */
int worker;
for (worker = 0; worker < starpu_worker_get_count(); worker++)
@{
        struct starpu_worker_profiling_info worker_info;
        int ret = starpu_worker_get_profiling_info(worker, &worker_info);
        STARPU_ASSERT(!ret);

        double total_time = starpu_timing_timespec_to_us(&worker_info.total_time);
        double executing_time = starpu_timing_timespec_to_us(&worker_info.executing_time);
        double sleeping_time = starpu_timing_timespec_to_us(&worker_info.sleeping_time);

        float executing_ratio = 100.0*executing_time/total_time;
        float sleeping_ratio = 100.0*sleeping_time/total_time;

        char workername[128];
        starpu_worker_get_name(worker, workername, 128);
        fprintf(stderr, "Worker %s:\n", workername);
        fprintf(stderr, "\ttotal time: %.2lf ms\n", total_time*1e-3);
        fprintf(stderr, "\texec time: %.2lf ms (%.2f %%)\n", executing_time*1e-3,
                executing_ratio);
        fprintf(stderr, "\tblocked time: %.2lf ms (%.2f %%)\n", sleeping_time*1e-3,
                sleeping_ratio);
@}
@end smallexample
@end cartouche

@node Partitioning Data
@section Partitioning Data

An existing piece of data can be partitioned in sub parts to be used by different tasks, for instance:

@cartouche
@smallexample
int vector[NX];
starpu_data_handle_t handle;

/* Declare data to StarPU */
starpu_vector_data_register(&handle, 0, (uintptr_t)vector, NX, sizeof(vector[0]));

/* Partition the vector in PARTS sub-vectors */
starpu_filter f =
@{
    .filter_func = starpu_block_filter_func_vector,
    .nchildren = PARTS
@};
starpu_data_partition(handle, &f);
@end smallexample
@end cartouche

The task submission then uses @code{starpu_data_get_sub_data} to retrive the
sub-handles to be passed as tasks parameters.

@cartouche
@smallexample
/* Submit a task on each sub-vector */
for (i=0; i<starpu_data_get_nb_children(handle); i++) @{
    /* Get subdata number i (there is only 1 dimension) */
    starpu_data_handle_t sub_handle = starpu_data_get_sub_data(handle, 1, i);
    struct starpu_task *task = starpu_task_create();

    task->handles[0] = sub_handle;
    task->cl = &cl;
    task->synchronous = 1;
    task->cl_arg = &factor;
    task->cl_arg_size = sizeof(factor);

    starpu_task_submit(task);
@}
@end smallexample
@end cartouche

Partitioning can be applied several times, see
@code{examples/basic_examples/mult.c} and @code{examples/filters/}.

Wherever the whole piece of data is already available, the partitioning will
be done in-place, i.e. without allocating new buffers but just using pointers
inside the existing copy. This is particularly important to be aware of when
using OpenCL, where the kernel parameters are not pointers, but handles. The
kernel thus needs to be also passed the offset within the OpenCL buffer:

@cartouche
@smallexample
void opencl_func(void *buffers[], void *cl_arg)
@{
    cl_mem vector = (cl_mem) STARPU_VECTOR_GET_DEV_HANDLE(buffers[0]);
    unsigned offset = STARPU_BLOCK_GET_OFFSET(buffers[0]);

    ...
    clSetKernelArg(kernel, 0, sizeof(vector), &vector);
    clSetKernelArg(kernel, 1, sizeof(offset), &offset);
    ...
@}
@end smallexample
@end cartouche

And the kernel has to shift from the pointer passed by the OpenCL driver:

@cartouche
@smallexample
__kernel void opencl_kernel(__global int *vector, unsigned offset)
@{
    block = (__global void *)block + offset;
    ...
@}
@end smallexample
@end cartouche

@node Performance model example
@section Performance model example

To achieve good scheduling, StarPU scheduling policies need to be able to
estimate in advance the duration of a task. This is done by giving to codelets
a performance model, by defining a @code{starpu_perfmodel} structure and
providing its address in the @code{model} field of the @code{struct starpu_codelet}
structure. The @code{symbol} and @code{type} fields of @code{starpu_perfmodel}
are mandatory, to give a name to the model, and the type of the model, since
there are several kinds of performance models.

@itemize
@item
Measured at runtime (@code{STARPU_HISTORY_BASED} model type). This assumes that for a
given set of data input/output sizes, the performance will always be about the
same. This is very true for regular kernels on GPUs for instance (<0.1% error),
and just a bit less true on CPUs (~=1% error). This also assumes that there are
few different sets of data input/output sizes. StarPU will then keep record of
the average time of previous executions on the various processing units, and use
it as an estimation. History is done per task size, by using a hash of the input
and ouput sizes as an index.
It will also save it in @code{~/.starpu/sampling/codelets}
for further executions, and can be observed by using the
@code{starpu_perfmodel_display} command, or drawn by using
the @code{starpu_perfmodel_plot}.  The models are indexed by machine name. To
share the models between machines (e.g. for a homogeneous cluster), use
@code{export STARPU_HOSTNAME=some_global_name}. Measurements are only done when using a task scheduler which makes use of it, such as @code{heft} or @code{dmda}.

The following is a small code example.

If e.g. the code is recompiled with other compilation options, or several
variants of the code are used, the symbol string should be changed to reflect
that, in order to recalibrate a new model from zero. The symbol string can even
be constructed dynamically at execution time, as long as this is done before
submitting any task using it.

@cartouche
@smallexample
static struct starpu_perfmodel mult_perf_model = @{
    .type = STARPU_HISTORY_BASED,
    .symbol = "mult_perf_model"
@};

struct starpu_codelet cl = @{
    .where = STARPU_CPU,
    .cpu_funcs = @{ cpu_mult, NULL @},
    .nbuffers = 3,
    .modes = @{ STARPU_R, STARPU_R, STARPU_W @},
    /* for the scheduling policy to be able to use performance models */
    .model = &mult_perf_model
@};
@end smallexample
@end cartouche

@item
Measured at runtime and refined by regression (@code{STARPU_*REGRESSION_BASED}
model type). This still assumes performance regularity, but can work
with various data input sizes, by applying regression over observed
execution times. STARPU_REGRESSION_BASED uses an a*n^b regression
form, STARPU_NL_REGRESSION_BASED uses an a*n^b+c (more precise than
STARPU_REGRESSION_BASED, but costs a lot more to compute). For instance,
@code{tests/perfmodels/regression_based.c} uses a regression-based performance
model for the @code{memset} operation. Of course, the application has to issue
tasks with varying size so that the regression can be computed. StarPU will not
trust the regression unless there is at least 10% difference between the minimum
and maximum observed input size. For non-linear regression, since computing it
is quite expensive, it is only done at termination of the application. This
means that the first execution uses history-based performance model to perform
scheduling.

@item
Provided as an estimation from the application itself (@code{STARPU_COMMON} model type and @code{cost_function} field),
see for instance
@code{examples/common/blas_model.h} and @code{examples/common/blas_model.c}.

@item
Provided explicitly by the application (@code{STARPU_PER_ARCH} model type): the
@code{.per_arch[arch][nimpl].cost_function} fields have to be filled with pointers to
functions which return the expected duration of the task in micro-seconds, one
per architecture.

@end itemize

For the @code{STARPU_HISTORY_BASED} and @code{STARPU_*REGRESSION_BASE},
the total size of task data (both input and output) is used as an index by
default. The @code{size_base} field of @code{struct starpu_perfmodel} however
permits the application to override that, when for instance some of the data
do not matter for task cost (e.g. mere reference table), or when using sparse
structures (in which case it is the number of non-zeros which matter), or when
there is some hidden parameter such as the number of iterations, etc.

How to use schedulers which can benefit from such performance model is explained
in @ref{Task scheduling policy}.

The same can be done for task power consumption estimation, by setting the
@code{power_model} field the same way as the @code{model} field. Note: for
now, the application has to give to the power consumption performance model
a name which is different from the execution time performance model.

The application can request time estimations from the StarPU performance
models by filling a task structure as usual without actually submitting
it. The data handles can be created by calling @code{starpu_data_register}
functions with a @code{NULL} pointer (and need to be unregistered as usual)
and the desired data sizes. The @code{starpu_task_expected_length} and
@code{starpu_task_expected_power} functions can then be called to get an
estimation of the task duration on a given arch. @code{starpu_task_destroy}
needs to be called to destroy the dummy task afterwards. See
@code{tests/perfmodels/regression_based.c} for an example.

@node Theoretical lower bound on execution time
@section Theoretical lower bound on execution time

For kernels with history-based performance models, StarPU can very easily provide a theoretical lower
bound for the execution time of a whole set of tasks. See for
instance @code{examples/lu/lu_example.c}: before submitting tasks,
call @code{starpu_bound_start}, and after complete execution, call
@code{starpu_bound_stop}. @code{starpu_bound_print_lp} or
@code{starpu_bound_print_mps} can then be used to output a Linear Programming
problem corresponding to the schedule of your tasks. Run it through
@code{lp_solve} or any other linear programming solver, and that will give you a
lower bound for the total execution time of your tasks. If StarPU was compiled
with the glpk library installed, @code{starpu_bound_compute} can be used to
solve it immediately and get the optimized minimum, in ms. Its @code{integer}
parameter allows to decide whether integer resolution should be computed
and returned too.

The @code{deps} parameter tells StarPU whether to take tasks and implicit data
dependencies into account. It must be understood that the linear programming
problem size is quadratic with the number of tasks and thus the time to solve it
will be very long, it could be minutes for just a few dozen tasks. You should
probably use @code{lp_solve -timeout 1 test.pl -wmps test.mps} to convert the
problem to MPS format and then use a better solver, @code{glpsol} might be
better than @code{lp_solve} for instance (the @code{--pcost} option may be
useful), but sometimes doesn't manage to converge. @code{cbc} might look
slower, but it is parallel. Be sure to try at least all the @code{-B} options
of @code{lp_solve}. For instance, we often just use
@code{lp_solve -cc -B1 -Bb -Bg -Bp -Bf -Br -BG -Bd -Bs -BB -Bo -Bc -Bi} , and
the @code{-gr} option can also be quite useful.

Setting @code{deps} to 0 will only take into account the actual computations
on processing units. It however still properly takes into account the varying
performances of kernels and processing units, which is quite more accurate than
just comparing StarPU performances with the fastest of the kernels being used.

The @code{prio} parameter tells StarPU whether to simulate taking into account
the priorities as the StarPU scheduler would, i.e. schedule prioritized
tasks before less prioritized tasks, to check to which extend this results
to a less optimal solution. This increases even more computation time.

Note that for simplicity, all this however doesn't take into account data
transfers, which are assumed to be completely overlapped.

@node Insert Task Utility
@section Insert Task Utility

StarPU provides the wrapper function @code{starpu_insert_task} to ease
the creation and submission of tasks.

@deftypefun int starpu_insert_task (struct starpu_codelet *@var{cl}, ...)
Create and submit a task corresponding to @var{cl} with the following
arguments.  The argument list must be zero-terminated.

The arguments following the codelets can be of the following types:

@itemize
@item
@code{STARPU_R}, @code{STARPU_W}, @code{STARPU_RW}, @code{STARPU_SCRATCH}, @code{STARPU_REDUX} an access mode followed by a data handle;
@item
the specific values @code{STARPU_VALUE}, @code{STARPU_CALLBACK},
@code{STARPU_CALLBACK_ARG}, @code{STARPU_CALLBACK_WITH_ARG},
@code{STARPU_PRIORITY}, followed by the appropriated objects as
defined below.
@end itemize

Parameters to be passed to the codelet implementation are defined
through the type @code{STARPU_VALUE}. The function
@code{starpu_codelet_unpack_args} must be called within the codelet
implementation to retrieve them.
@end deftypefun

@defmac STARPU_VALUE
this macro is used when calling @code{starpu_insert_task}, and must be
followed by a pointer to a constant value and the size of the constant
@end defmac

@defmac STARPU_CALLBACK
this macro is used when calling @code{starpu_insert_task}, and must be
followed by a pointer to a callback function
@end defmac

@defmac STARPU_CALLBACK_ARG
this macro is used when calling @code{starpu_insert_task}, and must be
followed by a pointer to be given as an argument to the callback
function
@end defmac

@defmac  STARPU_CALLBACK_WITH_ARG
this macro is used when calling @code{starpu_insert_task}, and must be
followed by two pointers: one to a callback function, and the other to
be given as an argument to the callback function; this is equivalent
to using both @code{STARPU_CALLBACK} and
@code{STARPU_CALLBACK_WITH_ARG}
@end defmac

@defmac STARPU_PRIORITY
this macro is used when calling @code{starpu_insert_task}, and must be
followed by a integer defining a priority level
@end defmac

@deftypefun void starpu_codelet_pack_args ({char **}@var{arg_buffer}, {size_t *}@var{arg_buffer_size}, ...)
Pack arguments of type @code{STARPU_VALUE} into a buffer which can be
given to a codelet and later unpacked with the function
@code{starpu_codelet_unpack_args} defined below.
@end deftypefun

@deftypefun void starpu_codelet_unpack_args ({void *}@var{cl_arg}, ...)
Retrieve the arguments of type @code{STARPU_VALUE} associated to a
task automatically created using the function
@code{starpu_insert_task} defined above.
@end deftypefun

Here the implementation of the codelet:

@smallexample
void func_cpu(void *descr[], void *_args)
@{
        int *x0 = (int *)STARPU_VARIABLE_GET_PTR(descr[0]);
        float *x1 = (float *)STARPU_VARIABLE_GET_PTR(descr[1]);
        int ifactor;
        float ffactor;

        starpu_codelet_unpack_args(_args, &ifactor, &ffactor);
        *x0 = *x0 * ifactor;
        *x1 = *x1 * ffactor;
@}

struct starpu_codelet mycodelet = @{
        .where = STARPU_CPU,
        .cpu_funcs = @{ func_cpu, NULL @},
        .nbuffers = 2,
        .modes = @{ STARPU_RW, STARPU_RW @}
@};
@end smallexample

And the call to the @code{starpu_insert_task} wrapper:

@smallexample
starpu_insert_task(&mycodelet,
                   STARPU_VALUE, &ifactor, sizeof(ifactor),
                   STARPU_VALUE, &ffactor, sizeof(ffactor),
                   STARPU_RW, data_handles[0], STARPU_RW, data_handles[1],
                   0);
@end smallexample

The call to @code{starpu_insert_task} is equivalent to the following
code:

@smallexample
struct starpu_task *task = starpu_task_create();
task->cl = &mycodelet;
task->handles[0] = data_handles[0];
task->handles[1] = data_handles[1];
char *arg_buffer;
size_t arg_buffer_size;
starpu_codelet_pack_args(&arg_buffer, &arg_buffer_size,
                    STARPU_VALUE, &ifactor, sizeof(ifactor),
                    STARPU_VALUE, &ffactor, sizeof(ffactor),
                    0);
task->cl_arg = arg_buffer;
task->cl_arg_size = arg_buffer_size;
int ret = starpu_task_submit(task);
@end smallexample

If some part of the task insertion depends on the value of some computation,
the @code{STARPU_DATA_ACQUIRE_CB} macro can be very convenient. For
instance, assuming that the index variable @code{i} was registered as handle
@code{i_handle}:

@smallexample
/* Compute which portion we will work on, e.g. pivot */
starpu_insert_task(&which_index, STARPU_W, i_handle, 0);

/* And submit the corresponding task */
STARPU_DATA_ACQUIRE_CB(i_handle, STARPU_R, starpu_insert_task(&work, STARPU_RW, A_handle[i], 0));
@end smallexample

The @code{STARPU_DATA_ACQUIRE_CB} macro submits an asynchronous request for
acquiring data @code{i} for the main application, and will execute the code
given as third parameter when it is acquired. In other words, as soon as the
value of @code{i} computed by the @code{which_index} codelet can be read, the
portion of code passed as third parameter of @code{STARPU_DATA_ACQUIRE_CB} will
be executed, and is allowed to read from @code{i} to use it e.g. as an
index. Note that this macro is only avaible when compiling StarPU with
the compiler @code{gcc}.

@node Parallel Tasks
@section Parallel Tasks

StarPU can leverage existing parallel computation libraries by the means of
parallel tasks. A parallel task is a task which gets worked on by a set of CPUs
(called a parallel or combined worker) at the same time, by using an existing
parallel CPU implementation of the computation to be achieved. This can also be
useful to improve the load balance between slow CPUs and fast GPUs: since CPUs
work collectively on a single task, the completion time of tasks on CPUs become
comparable to the completion time on GPUs, thus relieving from granularity
discrepancy concerns.

Two modes of execution exist to accomodate with existing usages.

@subsection Fork-mode parallel tasks

In the Fork mode, StarPU will call the codelet function on one
of the CPUs of the combined worker. The codelet function can use
@code{starpu_combined_worker_get_size()} to get the number of threads it is
allowed to start to achieve the computation. The CPU binding mask is already
enforced, so that threads created by the function will inherit the mask, and
thus execute where StarPU expected. For instance, using OpenMP (full source is
available in @code{examples/openmp/vector_scal.c}):

@example
void scal_cpu_func(void *buffers[], void *_args)
@{
    unsigned i;
    float *factor = _args;
    struct starpu_vector_interface *vector = buffers[0];
    unsigned n = STARPU_VECTOR_GET_NX(vector);
    float *val = (float *)STARPU_VECTOR_GET_PTR(vector);

#pragma omp parallel for num_threads(starpu_combined_worker_get_size())
    for (i = 0; i < n; i++)
        val[i] *= *factor;
@}

static struct starpu_codelet cl =
@{
    .modes = @{ STARPU_RW @},
    .where = STARPU_CPU,
    .type = STARPU_FORKJOIN,
    .max_parallelism = INT_MAX,
    .cpu_funcs = @{scal_cpu_func, NULL@},
    .nbuffers = 1,
@};
@end example

Other examples include for instance calling a BLAS parallel CPU implementation
(see @code{examples/mult/xgemm.c}).

@subsection SPMD-mode parallel tasks

In the SPMD mode, StarPU will call the codelet function on
each CPU of the combined worker. The codelet function can use
@code{starpu_combined_worker_get_size()} to get the total number of CPUs
involved in the combined worker, and thus the number of calls that are made in
parallel to the function, and @code{starpu_combined_worker_get_rank()} to get
the rank of the current CPU within the combined worker. For instance:

@example
static void func(void *buffers[], void *args)
@{
    unsigned i;
    float *factor = _args;
    struct starpu_vector_interface *vector = buffers[0];
    unsigned n = STARPU_VECTOR_GET_NX(vector);
    float *val = (float *)STARPU_VECTOR_GET_PTR(vector);

    /* Compute slice to compute */
    unsigned m = starpu_combined_worker_get_size();
    unsigned j = starpu_combined_worker_get_rank();
    unsigned slice = (n+m-1)/m;

    for (i = j * slice; i < (j+1) * slice && i < n; i++)
        val[i] *= *factor;
@}

static struct starpu_codelet cl =
@{
    .modes = @{ STARPU_RW @},
    .where = STARP_CPU,
    .type = STARPU_SPMD,
    .max_parallelism = INT_MAX,
    .cpu_funcs = @{ func, NULL @},
    .nbuffers = 1,
@}
@end example

Of course, this trivial example will not really benefit from parallel task
execution, and was only meant to be simple to understand.  The benefit comes
when the computation to be done is so that threads have to e.g. exchange
intermediate results, or write to the data in a complex but safe way in the same
buffer.

@subsection Parallel tasks performance

To benefit from parallel tasks, a parallel-task-aware StarPU scheduler has to
be used. When exposed to codelets with a Fork or SPMD flag, the @code{pheft}
(parallel-heft) and @code{pgreedy} (parallel greedy) schedulers will indeed also
try to execute tasks with several CPUs. It will automatically try the various
available combined worker sizes and thus be able to avoid choosing a large
combined worker if the codelet does not actually scale so much.

@subsection Combined worker sizes

By default, StarPU creates combined workers according to the architecture
structure as detected by hwloc. It means that for each object of the hwloc
topology (NUMA node, socket, cache, ...) a combined worker will be created. If
some nodes of the hierarchy have a big arity (e.g. many cores in a socket
without a hierarchy of shared caches), StarPU will create combined workers of
intermediate sizes.

@subsection Concurrent parallel tasks

Unfortunately, many environments and librairies do not support concurrent
calls.

For instance, most OpenMP implementations (including the main ones) do not
support concurrent @code{pragma omp parallel} statements without nesting them in
another @code{pragma omp parallel} statement, but StarPU does not yet support
creating its CPU workers by using such pragma.

Other parallel libraries are also not safe when being invoked concurrently
from different threads, due to the use of global variables in their sequential
sections for instance.

The solution is then to use only a single combined worker, scoping all
the CPUs.  This can be done by setting @code{single_combined_worker}
to 1 in the @code{starpu_conf} structure, or setting the
@code{STARPU_SINGLE_COMBINED_WORKER} environment variable to 1. StarPU will then
use parallel tasks only over all the CPUs at the same time.

@node Debugging
@section Debugging

StarPU provides several tools to help debugging aplications. Execution traces
can be generated and displayed graphically, see @ref{Generating traces}. Some
gdb helpers are also provided to show the whole StarPU state:

@smallexample
(gdb) source tools/gdbinit
(gdb) help starpu
@end smallexample

@node The multiformat interface
@section The multiformat interface
It may be interesting to represent the same piece of data using two different
data structures: one that would only be used on CPUs, and one that would only
be used on GPUs. This can be done by using the multiformat interface. StarPU
will be able to convert data from one data structure to the other when needed.
Note that the heft scheduler is the only one optimized for this interface. The
user must provide StarPU with conversion codelets:

@cartouche
@smallexample
#define NX 1024
struct point array_of_structs[NX];
starpu_data_handle_t handle;

/*
 * The conversion of a piece of data is itself a task, though it is created,
 * submitted and destroyed by StarPU internals and not by the user. Therefore,
 * we have to define two codelets.
 * Note that for now the conversion from the CPU format to the GPU format has to
 * be executed on the GPU, and the conversion from the GPU to the CPU has to be
 * executed on the CPU.
 */
#ifdef STARPU_USE_OPENCL
void cpu_to_opencl_opencl_func(void *buffers[], void *args);
struct starpu_codelet cpu_to_opencl_cl = @{
    .where = STARPU_OPENCL,
    .opencl_funcs = @{ cpu_to_opencl_opencl_func, NULL @},
    .nbuffers = 1,
    .modes = @{ STARPU_RW @}
@};

void opencl_to_cpu_func(void *buffers[], void *args);
struct starpu_codelet opencl_to_cpu_cl = @{
    .where = STARPU_CPU,
    .cpu_funcs = @{ opencl_to_cpu_func, NULL @},
    .nbuffers = 1,
    .modes = @{ STARPU_RW @}
@};
#endif

struct starpu_multiformat_data_interface_ops format_ops = @{
#ifdef STARPU_USE_OPENCL
    .opencl_elemsize = 2 * sizeof(float),
    .cpu_to_opencl_cl = &cpu_to_opencl_cl,
    .opencl_to_cpu_cl = &opencl_to_cpu_cl,
#endif
    .cpu_elemsize = 2 * sizeof(float),
    ...
@};
starpu_multiformat_data_register(handle, 0, &array_of_structs, NX, &format_ops);
@end smallexample
@end cartouche

Kernels can be written almost as for any other interface. Note that
STARPU_MULTIFORMAT_GET_CPU_PTR shall only be used for CPU kernels. CUDA kernels
must use STARPU_MULTIFORMAT_GET_CUDA_PTR, and OpenCL kernels must use
STARPU_MULTIFORMAT_GET_OPENCL_PTR. STARPU_MULTIFORMAT_GET_NX may be used in any
kind of kernel.
@cartouche
@smallexample
static void
multiformat_scal_cpu_func(void *buffers[], void *args)
@{
    struct point *aos;
    unsigned int n;

    aos = STARPU_MULTIFORMAT_GET_CPU_PTR(buffers[0]);
    n = STARPU_MULTIFORMAT_GET_NX(buffers[0]);
    ...
@}

extern "C" void multiformat_scal_cuda_func(void *buffers[], void *_args)
@{
    unsigned int n;
    struct struct_of_arrays *soa;

    soa = (struct struct_of_arrays *) STARPU_MULTIFORMAT_GET_CUDA_PTR(buffers[0]);
    n = STARPU_MULTIFORMAT_GET_NX(buffers[0]);

    ...
@}
@end smallexample
@end cartouche

A full example may be found in @code{examples/basic_examples/multiformat.c}.

@node On-GPU rendering
@section On-GPU rendering

Graphical-oriented applications need to draw the result of their computations,
typically on the very GPU where these happened. Technologies such as OpenGL/CUDA
interoperability permit to let CUDA directly work on the OpenGL buffers, making
them thus immediately ready for drawing, by mapping OpenGL buffer, textures or
renderbuffer objects into CUDA. To achieve this with StarPU, it simply needs to
be given the CUDA pointer at registration, for instance:

@cartouche
@smallexample
for (workerid = 0; workerid < starpu_worker_get_count(); workerid++)
        if (starpu_worker_get_type(workerid) == STARPU_CUDA_WORKER)
                break;

cudaSetDevice(starpu_worker_get_devid(workerid));
cudaGraphicsResourceGetMappedPointer((void**)&output, &num_bytes, resource);
starpu_vector_data_register(&handle, starpu_worker_get_memory_node(workerid), output, num_bytes / sizeof(float4), sizeof(float4));

starpu_insert_task(&cl, STARPU_RW, handle, 0);

starpu_data_unregister(handle);

cudaSetDevice(starpu_worker_get_devid(workerid));
cudaGraphicsUnmapResources(1, &resource, 0);

/* Now display it */
@end smallexample
@end cartouche

@node More examples
@section More examples


More examples are available in the StarPU sources in the @code{examples/}
directory. Simple examples include:

@table @asis
@item @code{incrementer/}:
    Trivial incrementation test.
@item @code{basic_examples/}:
        Simple documented Hello world (as shown in @ref{Hello World}), vector/scalar product (as shown
        in @ref{Vector Scaling on an Hybrid CPU/GPU Machine}), matrix
        product examples (as shown in @ref{Performance model example}), an example using the blocked matrix data
        interface, an example using the variable data interface, and an example
        using different formats on CPUs and GPUs.
@item @code{matvecmult/}:
    OpenCL example from NVidia, adapted to StarPU.
@item @code{axpy/}:
    AXPY CUBLAS operation adapted to StarPU.
@item @code{fortran/}:
    Example of Fortran bindings.
@end table

More advanced examples include:

@table @asis
@item @code{filters/}:
    Examples using filters, as shown in @ref{Partitioning Data}.
@item @code{lu/}:
    LU matrix factorization, see for instance @code{xlu_implicit.c}
@item @code{cholesky/}:
    Cholesky matrix factorization, see for instance @code{cholesky_implicit.c}.
@end table