starpu-max/doc/doxygen/chapters/api/codelet_and_tasks.doxy

/*
 * This file is part of the StarPU Handbook.
 * Copyright (C) 2009--2011  Universit@'e de Bordeaux 1
 * Copyright (C) 2010, 2011, 2012, 2013  Centre National de la Recherche Scientifique
 * Copyright (C) 2011, 2012 Institut National de Recherche en Informatique et Automatique
 * See the file version.doxy for copying conditions.
 */

/*! \defgroup Codelet_And_Tasks Codelet And Tasks

\brief This section describes the interface to manipulate codelets and tasks.

\enum starpu_codelet_type
\ingroup Codelet_And_Tasks
\brief Describes the type of parallel task. See \ref Parallel_Tasks for details.
\var starpu_codelet_type::STARPU_SEQ
\ingroup Codelet_And_Tasks
(default) for classical sequential tasks.
\var starpu_codelet_type::STARPU_SPMD
\ingroup Codelet_And_Tasks
for a parallel task whose threads are handled by StarPU, the code has
to use starpu_combined_worker_get_size() and
starpu_combined_worker_get_rank() to distribute the work.
\var starpu_codelet_type::STARPU_FORKJOIN
\ingroup Codelet_And_Tasks
for a parallel task whose threads are started by the codelet function,
which has to use starpu_combined_worker_get_size() to determine how
many threads should be started.

\enum starpu_task_status
\ingroup Codelet_And_Tasks
\brief Task status
\var starpu_task_status::STARPU_TASK_INVALID
\ingroup Codelet_And_Tasks
The task has just been initialized.
\var starpu_task_status::STARPU_TASK_BLOCKED
\ingroup Codelet_And_Tasks
The task has just been submitted, and its dependencies has not been
checked yet.
\var starpu_task_status::STARPU_TASK_READY
\ingroup Codelet_And_Tasks
The task is ready for execution.
\var starpu_task_status::STARPU_TASK_RUNNING
\ingroup Codelet_And_Tasks
The task is running on some worker.
\var starpu_task_status::STARPU_TASK_FINISHED
\ingroup Codelet_And_Tasks
The task is finished executing.
\var starpu_task_status::STARPU_TASK_BLOCKED_ON_TAG
\ingroup Codelet_And_Tasks
The task is waiting for a tag.
\var starpu_task_status::STARPU_TASK_BLOCKED_ON_TASK
\ingroup Codelet_And_Tasks
The task is waiting for a task.
\var starpu_task_status::STARPU_TASK_BLOCKED_ON_DATA
\ingroup Codelet_And_Tasks
The task is waiting for some data.


\def STARPU_CPU
\ingroup Codelet_And_Tasks
This macro is used when setting the field starpu_codelet::where
to specify the codelet may be executed on a CPU processing unit.

\def STARPU_CUDA
\ingroup Codelet_And_Tasks
This macro is used when setting the field starpu_codelet::where
to specify the codelet may be executed on a CUDA processing unit.

\def STARPU_OPENCL
\ingroup Codelet_And_Tasks
This macro is used when setting the field starpu_codelet::where to
specify the codelet may be executed on a OpenCL processing unit.

\def STARPU_MULTIPLE_CPU_IMPLEMENTATIONS
\deprecated
\ingroup Codelet_And_Tasks
Setting the field starpu_codelet::cpu_func with this macro
indicates the codelet will have several implementations. The use of
this macro is deprecated. One should always only define the field
starpu_codelet::cpu_funcs.

\def STARPU_MULTIPLE_CUDA_IMPLEMENTATIONS
\deprecated
\ingroup Codelet_And_Tasks
Setting the field starpu_codelet::cuda_func with this macro
indicates the codelet will have several implementations. The use of
this macro is deprecated. One should always only define the field
starpu_codelet::cuda_funcs.

\def STARPU_MULTIPLE_OPENCL_IMPLEMENTATIONS
\deprecated
\ingroup Codelet_And_Tasks
Setting the field starpu_codelet::opencl_func with
this macro indicates the codelet will have several implementations.
The use of this macro is deprecated. One should always only define the
field starpu_codelet::opencl_funcs.

\def starpu_cpu_func_t
\ingroup Codelet_And_Tasks
CPU implementation of a codelet.

\def starpu_cuda_func_t
\ingroup Codelet_And_Tasks
CUDA implementation of a codelet.

\def starpu_opencl_func_t
\ingroup Codelet_And_Tasks
OpenCL implementation of a codelet.

\def starpu_mic_func_t
\ingroup Codelet_And_Tasks
MIC implementation of a codelet.

\def starpu_scc_func_t
\ingroup Codelet_And_Tasks
SCC implementation of a codelet.

\struct starpu_codelet
\brief The codelet structure describes a kernel that is possibly
implemented on various targets. For compatibility, make sure to
initialize the whole structure to zero, either by using explicit
memset, or the function starpu_codelet_init(), or by letting the
compiler implicitly do it in e.g. static storage case.
\ingroup Codelet_And_Tasks
\var starpu_codelet::where.
Optional field to indicate which types of processing units are able to
execute the codelet. The different values ::STARPU_CPU, ::STARPU_CUDA,
::STARPU_OPENCL can be combined to specify on which types of processing
units the codelet can be executed. ::STARPU_CPU|::STARPU_CUDA for instance
indicates that the codelet is implemented for both CPU cores and CUDA
devices while ::STARPU_OPENCL indicates that it is only available on
OpenCL devices. If the field is unset, its value will be automatically
set based on the availability of the XXX_funcs fields defined below.

\var starpu_codelet::can_execute
Define a function which should return 1 if the worker designated by
workerid can execute the <c>nimpl</c>th implementation of the given
task, 0 otherwise.

\var starpu_codelet::type
Optional field to specify the type of the codelet. The default is
::STARPU_SEQ, i.e. usual sequential implementation. Other values
(::STARPU_SPMD or ::STARPU_FORKJOIN declare that a parallel implementation
is also available. See \ref Parallel_Tasks for details.

\var starpu_codelet::max_parallelism
Optional field. If a parallel implementation is available, this
denotes the maximum combined worker size that StarPU will use to
execute parallel tasks for this codelet.

\var starpu_codelet::cpu_func
\deprecated
Optional field which has been made deprecated. One should use instead
the field starpu_codelet::cpu_funcs.

\var starpu_codelet::cuda_func
\deprecated
Optional field which has been made deprecated. One should use instead
the starpu_codelet::cuda_funcs field.

\var starpu_codelet::opencl_func
\deprecated
Optional field which has been made deprecated. One should use instead
the starpu_codelet::opencl_funcs field.

\var starpu_codelet::cpu_funcs
Optional array of function pointers to the CPU implementations of the
codelet. It must be terminated by a NULL value. The functions
prototype must be:
\code{.c}
void cpu_func(void *buffers[], void *cl_arg)
\endcode
The first argument being the array of data managed by the data
management library, and the second argument is a pointer to the
argument passed from the field starpu_task::cl_arg. If the field
starpu_codelet::where is set, then the field starpu_codelet::cpu_funcs
is ignored if ::STARPU_CPU does not appear in the field
starpu_codelet::where, it must be non-null otherwise.

\var starpu_codelet::cuda_funcs
Optional array of function pointers to the CUDA implementations of the
codelet. It must be terminated by a NULL value. The functions must be
host-functions written in the CUDA runtime API. Their prototype must
be:
\code{.c}
void cuda_func(void *buffers[], void *cl_arg)
\endcode
If the field starpu_codelet::where is set, then the field
starpu_codelet::cuda_funcs is ignored if ::STARPU_CUDA does not appear
in the field starpu_codelet::where, it must be non-null otherwise.

\var starpu_codelet::opencl_funcs
Optional array of function pointers to the OpenCL implementations of
the codelet. It must be terminated by a NULL value. The functions
prototype must be:
\code{.c}
void opencl_func(void *buffers[], void *cl_arg)
\endcode
If the field starpu_codelet::where field is set, then the field
starpu_codelet::opencl_funcs is ignored if ::STARPU_OPENCL does not
appear in the field starpu_codelet::where, it must be non-null
otherwise.

\var starpu_codelet::nbuffers
Specify the number of arguments taken by the codelet. These arguments
are managed by the DSM and are accessed from the <c>void *buffers[]</c>
array. The constant argument passed with the field starpu_task::cl_arg
is not counted in this number. This value should not be above
STARPU_NMAXBUFS.

\var starpu_codelet::modes
Is an array of ::starpu_data_access_mode. It describes the required
access modes to the data neeeded by the codelet (e.g. ::STARPU_RW). The
number of entries in this array must be specified in the field
starpu_codelet::nbuffers, and should not exceed STARPU_NMAXBUFS. If
unsufficient, this value can be set with the <c>--enable-maxbuffers</c>
option when configuring StarPU.

\var starpu_codelet::dyn_modes
Is an array of ::starpu_data_access_mode. It describes the required
access modes to the data neeeded by the codelet (e.g. ::STARPU_RW).
The number of entries in this array must be specified in the field
starpu_codelet::nbuffers. This field should be used for codelets having a
number of datas greater than STARPU_NMAXBUFS (see \ref
Setting_the_Data_Handles_for_a_Task). When defining a codelet, one
should either define this field or the field starpu_codelet::modes defined above.

\var starpu_codelet::model
Optional pointer to the task duration performance model associated to
this codelet. This optional field is ignored when set to <c>NULL</c> or when
its field starpu_perfmodel::symbol is not set.

\var starpu_codelet::power_model
Optional pointer to the task power consumption performance model
associated to this codelet. This optional field is ignored when set to
<c>NULL</c> or when its field starpu_perfmodel::field is not set. In
the case of parallel codelets, this has to account for all processing
units involved in the parallel execution.

\var starpu_codelet::per_worker_stats
Optional array for statistics collected at runtime: this is filled by
StarPU and should not be accessed directly, but for example by calling
the function starpu_codelet_display_stats() (See
starpu_codelet_display_stats() for details).

\var starpu_codelet::name
Optional name of the codelet. This can be useful for debugging
purposes.

\fn void starpu_codelet_init(struct starpu_codelet *cl)
\ingroup Codelet_And_Tasks
Initialize \p cl with default values. Codelets should
preferably be initialized statically as shown in \ref
Defining_a_Codelet. However such a initialisation is not always
possible, e.g. when using C++.

\struct starpu_data_descr
\ingroup Codelet_And_Tasks
\brief This type is used to describe a data handle along with an
access mode.
\var starpu_data_descr::handle
describes a data
\var starpu_data_descr::mode
describes its access mode

\struct starpu_task
\ingroup Codelet_And_Tasks
\brief The structure describes a task that can be offloaded on the
various processing units managed by StarPU. It instantiates a codelet.
It can either be allocated dynamically with the function
starpu_task_create(), or declared statically. In the latter case, the
programmer has to zero the structure starpu_task and to fill the
different fields properly. The indicated default values correspond to
the configuration of a task allocated with starpu_task_create().
\var starpu_task::cl
Is a pointer to the corresponding structure starpu_codelet. This
describes where the kernel should be executed, and supplies the
appropriate implementations. When set to NULL, no code is executed
during the tasks, such empty tasks can be useful for synchronization
purposes.
\var starpu_task::buffers
\deprecated
This field has been made deprecated. One should use instead the
field starpu_task::handles to specify the data handles accessed
by the task. The access modes are now defined in the field
starpu_codelet::mode.
\var starpu_task::handles
Is an array of starpu_data_handle_t. It specifies the handles to the
different pieces of data accessed by the task. The number of entries
in this array must be specified in the field starpu_codelet::nbuffers,
and should not exceed STARPU_NMAXBUFS. If unsufficient, this value can
be set with the option <c>--enable-maxbuffers</c> when configuring
StarPU.
\var starpu_task::dyn_handles
Is an array of starpu_data_handle_t. It specifies the handles to the
different pieces of data accessed by the task. The number of entries
in this array must be specified in the field starpu_codelet::nbuffers.
This field should be used for tasks having a number of datas greater
than STARPU_NMAXBUFS (see \ref Setting_the_Data_Handles_for_a_Task).
When defining a task, one should either define this field or the field
starpu_task::handles defined above.

\var starpu_task::interfaces
The actual data pointers to the memory node where execution will
happen, managed by the DSM.

\var starpu_task::dyn_interfaces
The actual data pointers to the memory node where execution will
happen, managed by the DSM. Is used when the field
starpu_task::dyn_handles is defined.

\var starpu_task::cl_arg
Optional pointer which is passed to the codelet through the second
argument of the codelet implementation (e.g. starpu_codelet::cpu_func
or starpu_codelet::cuda_func). The default value is <c>NULL</c>.

\var starpu_task::cl_arg_size
Optional field. For some specific drivers, the pointer
starpu_task::cl_arg cannot not be directly given to the driver
function. A buffer of size starpu_task::cl_arg_size needs to be
allocated on the driver. This buffer is then filled with the
starpu_task::cl_arg_size bytes starting at address
starpu_task::cl_arg. In this case, the argument given to the codelet
is therefore not the starpu_task::cl_arg pointer, but the address of
the buffer in local store (LS) instead. This field is ignored for CPU,
CUDA and OpenCL codelets, where the starpu_task::cl_arg pointer is
given as such.

\var starpu_task::callback_func
Optional field, the default value is <c>NULL</c>. This is a function
pointer of prototype <c>void (*f)(void *)</c> which specifies a
possible callback. If this pointer is non-null, the callback function
is executed on the host after the execution of the task. Tasks which
depend on it might already be executing. The callback is passed the
value contained in the starpu_task::callback_arg field. No callback is
executed if the field is set to NULL.

\var starpu_task::callback_arg (optional) (default: NULL)
Optional field, the default value is <c>NULL</c>. This is the pointer
passed to the callback function. This field is ignored if the
callback_func is set to <c>NULL</c>.

\var starpu_task::use_tag
Optional field, the default value is 0. If set, this flag indicates
that the task should be associated with the tag contained in the
starpu_task::tag_id field. Tag allow the application to synchronize
with the task and to express task dependencies easily.

\var starpu_task::tag_id
This optional field contains the tag associated to the task if the
field starpu_task::use_tag is set, it is ignored otherwise.

\var starpu_task::sequential_consistency
If this flag is set (which is the default), sequential consistency is
enforced for the data parameters of this task for which sequential
consistency is enabled. Clearing this flag permits to disable
sequential consistency for this task, even if data have it enabled.

\var starpu_task::synchronous
If this flag is set, the function starpu_task_submit() is blocking and
returns only when the task has been executed (or if no worker is able
to process the task). Otherwise, starpu_task_submit() returns
immediately.

\var starpu_task::priority
Optional field, the default value is STARPU_DEFAULT_PRIO. This field
indicates a level of priority for the task. This is an integer value
that must be set between the return values of the function
starpu_sched_get_min_priority() for the least important tasks, and
that of the function starpu_sched_get_max_priority() for the most
important tasks (included). The STARPU_MIN_PRIO and STARPU_MAX_PRIO
macros are provided for convenience and respectively returns the value
of starpu_sched_get_min_priority() and
starpu_sched_get_max_priority(). Default priority is
STARPU_DEFAULT_PRIO, which is always defined as 0 in order to allow
static task initialization. Scheduling strategies that take priorities
into account can use this parameter to take better scheduling
decisions, but the scheduling policy may also ignore it.

\var starpu_task::execute_on_a_specific_worker
Default value is 0. If this flag is set, StarPU will bypass the
scheduler and directly affect this task to the worker specified by the
field starpu_task::workerid.

\var starpu_task::workerid
Optional field. If the field starpu_task::execute_on_a_specific_worker
is set, this field indicates the identifier of the worker that should
process this task (as returned by starpu_worker_get_id()). This field
is ignored if the field starpu_task::execute_on_a_specific_worker is
set to 0.

\var starpu_task::bundle
Optional field. The bundle that includes this task. If no bundle is
used, this should be NULL.

\var starpu_task::detach
Optional field, default value is 1. If this flag is set, it is not
possible to synchronize with the task by the means of starpu_task_wait()
later on. Internal data structures are only guaranteed to be freed
once starpu_task_wait() is called if the flag is not set.

\var starpu_task::destroy
Optional value. Default value is 0 for starpu_task_init(), and 1 for
starpu_task_create(). If this flag is set, the task structure will
automatically be freed, either after the execution of the callback if
the task is detached, or during starpu_task_wait() otherwise. If this
flag is not set, dynamically allocated data structures will not be
freed until starpu_task_destroy() is called explicitly. Setting this
flag for a statically allocated task structure will result in
undefined behaviour. The flag is set to 1 when the task is created by
calling starpu_task_create(). Note that starpu_task_wait_for_all()
will not free any task.

\var starpu_task::regenerate
Optional field. If this flag is set, the task will be re-submitted to
StarPU once it has been executed. This flag must not be set if the
destroy flag is set.

\var starpu_task::status
Optional field. Current state of the task.

\var starpu_task::profiling_info
Optional field. Profiling information for the task.

\var starpu_task::predicted
Output field. Predicted duration of the task. This field is only set
if the scheduling strategy used performance models.

\var starpu_task::predicted_transfer
Optional field. Predicted data transfer duration for the task in
microseconds. This field is only valid if the scheduling strategy uses
performance models.

\var starpu_task::prev
\private
A pointer to the previous task. This should only be used by StarPU.

\var starpu_task::next
\private
A pointer to the next task. This should only be used by StarPU.

\var starpu_task::mf_skip
\private
This is only used for tasks that use multiformat handle. This should
only be used by StarPU.

\var starpu_task::flops
This can be set to the number of floating points operations that the
task will have to achieve. This is useful for easily getting GFlops
curves from starpu_perfmodel_plot(), and for the hypervisor load
balancing.

\var starpu_task::starpu_private
\private
This is private to StarPU, do not modify. If the task is allocated by
hand (without starpu_task_create()), this field should be set to NULL.

\var starpu_task::magic
\private
This field is set when initializing a task. The function
starpu_task_submit() will fail if the field does not have the right
value. This will hence avoid submitting tasks which have not been
properly initialised.

\var starpu_task::sched_ctx
Scheduling context.

\var starpu_task::hypervisor_tag
Helps the hypervisor monitor the execution of this task.

\var starpu_task::scheduled
Whether the scheduler has pushed the task on some queue

\fn void starpu_task_init(struct starpu_task *task)
\ingroup Codelet_And_Tasks
Initialize task with default values. This function is
implicitly called by starpu_task_create(). By default, tasks initialized
with starpu_task_init() must be deinitialized explicitly with
starpu_task_clean(). Tasks can also be initialized statically, using
STARPU_TASK_INITIALIZER.

\def STARPU_TASK_INITIALIZER
\ingroup Codelet_And_Tasks
It is possible to initialize statically allocated tasks with
this value. This is equivalent to initializing a structure starpu_task
with the function starpu_task_init() function.

\def STARPU_TASK_GET_HANDLE(struct starpu_task *task, int i)
\ingroup Codelet_And_Tasks
Return the \p i th data handle of the given task. If the task
is defined with a static or dynamic number of handles, will either
return the \p i th element of the field starpu_task::handles or the \p
i th element of the field starpu_task::dyn_handles (see \ref
Setting_the_Data_Handles_for_a_Task)

\def STARPU_TASK_SET_HANDLE(struct starpu_task *task, starpu_data_handle_t handle, int i)
\ingroup Codelet_And_Tasks
Set the \p i th data handle of the given task with the given
dat handle. If the task is defined with a static or dynamic number of
handles, will either set the \p i th element of the field
starpu_task::handles or the \p i th element of the field
starpu_task::dyn_handles (see \ref
Setting_the_Data_Handles_for_a_Task)

\def STARPU_CODELET_GET_MODE(struct starpu_codelet *codelet, int i)
\ingroup Codelet_And_Tasks
Return the access mode of the \p i th data handle of the given
codelet. If the codelet is defined with a static or dynamic number of
handles, will either return the \p i th element of the field
starpu_codelet::modes or the \p i th element of the field
starpu_codelet::dyn_modes (see \ref
Setting_the_Data_Handles_for_a_Task)

\def STARPU_CODELET_SET_MODE(struct starpu_codelet *codelet, enum starpu_data_access_mode mode, int i)
\ingroup Codelet_And_Tasks
Set the access mode of the \p i th data handle of the given
codelet. If the codelet is defined with a static or dynamic number of
handles, will either set the \p i th element of the field
starpu_codelet::modes or the \p i th element of the field
starpu_codelet::dyn_modes (see \ref
Setting_the_Data_Handles_for_a_Task)

\fn struct starpu_task * starpu_task_create(void)
\ingroup Codelet_And_Tasks
Allocate a task structure and initialize it with default
values. Tasks allocated dynamically with starpu_task_create() are
automatically freed when the task is terminated. This means that the
task pointer can not be used any more once the task is submitted,
since it can be executed at any time (unless dependencies make it
wait) and thus freed at any time. If the field starpu_task::destroy is
explicitly unset, the resources used by the task have to be freed by
calling starpu_task_destroy().

\fn struct starpu_task * starpu_task_dup(struct starpu_task *task)
\ingroup Codelet_And_Tasks
Allocate a task structure which is the exact duplicate of the
given task.

\fn void starpu_task_clean(struct starpu_task *task)
\ingroup Codelet_And_Tasks
Release all the structures automatically allocated to execute
task, but not the task structure itself and values set by the user
remain unchanged. It is thus useful for statically allocated tasks for
instance. It is also useful when users want to execute the same
operation several times with as least overhead as possible. It is
called automatically by starpu_task_destroy(). It has to be called
only after explicitly waiting for the task or after starpu_shutdown()
(waiting for the callback is not enough, since StarPU still
manipulates the task after calling the callback).

\fn void starpu_task_destroy(struct starpu_task *task)
\ingroup Codelet_And_Tasks
Free the resource allocated during starpu_task_create() and
associated with task. This function is already called automatically
after the execution of a task when the field starpu_task::destroy is
set, which is the default for tasks created by starpu_task_create().
Calling this function on a statically allocated task results in an
undefined behaviour.

\fn int starpu_task_wait(struct starpu_task *task)
\ingroup Codelet_And_Tasks
This function blocks until \p task has been executed. It is not
possible to synchronize with a task more than once. It is not possible
to wait for synchronous or detached tasks. Upon successful completion,
this function returns 0. Otherwise, <c>-EINVAL</c> indicates that the
specified task was either synchronous or detached.

\fn int starpu_task_submit(struct starpu_task *task)
\ingroup Codelet_And_Tasks
This function submits task to StarPU. Calling this function
does not mean that the task will be executed immediately as there can
be data or task (tag) dependencies that are not fulfilled yet: StarPU
will take care of scheduling this task with respect to such
dependencies. This function returns immediately if the field
starpu_task::synchronous is set to 0, and block until the
termination of the task otherwise. It is also possible to synchronize
the application with asynchronous tasks by the means of tags, using
the function starpu_tag_wait() function for instance. In case of
success, this function returns 0, a return value of <c>-ENODEV</c>
means that there is no worker able to process this task (e.g. there is
no GPU available and this task is only implemented for CUDA devices).
starpu_task_submit() can be called from anywhere, including codelet
functions and callbacks, provided that the field
starpu_task::synchronous is set to 0.

\fn int starpu_task_wait_for_all(void)
\ingroup Codelet_And_Tasks
This function blocks until all the tasks that were submitted
(to the current context or the global one if there aren't any) are
terminated. It does not destroy these tasks.

\fn int starpu_task_wait_for_all_in_ctx(unsigned sched_ctx_id)
\ingroup Codelet_And_Tasks
This function waits until all the tasks that were already
submitted to the context \p sched_ctx_id have been executed

\fn int starpu_task_nready(void)
\ingroup Codelet_And_Tasks
TODO

\fn int starpu_task_nsubmitted(void)
\ingroup Codelet_And_Tasks
Return the number of submitted tasks which have not completed yet.

\fn int starpu_task_nready(void)
\ingroup Codelet_And_Tasks
Return the number of submitted tasks which are ready for
execution are already executing. It thus does not include tasks
waiting for dependencies.

\fn struct starpu_task * starpu_task_get_current(void)
\ingroup Codelet_And_Tasks
This function returns the task currently executed by the
worker, or <c>NULL</c> if it is called either from a thread that is not a
task or simply because there is no task being executed at the moment.

\fn void starpu_codelet_display_stats(struct starpu_codelet *cl)
\ingroup Codelet_And_Tasks
Output on stderr some statistics on the codelet \p cl.

\fn int starpu_task_wait_for_no_ready(void)
\ingroup Codelet_And_Tasks
This function waits until there is no more ready task.

\fn void starpu_task_set_implementation(struct starpu_task *task, unsigned impl)
\ingroup Codelet_And_Tasks
This function should be called by schedulers to specify the
codelet implementation to be executed when executing the task.

\fn unsigned starpu_task_get_implementation(struct starpu_task *task)
\ingroup Codelet_And_Tasks
This function return the codelet implementation to be executed
when executing the task.

\fn void starpu_create_sync_task(starpu_tag_t sync_tag, unsigned ndeps, starpu_tag_t *deps,	void (*callback)(void *), void *callback_arg)
\ingroup Codelet_And_Tasks
This creates (and submits) an empty task that unlocks a tag once all
its dependencies are fulfilled.


*/