StarPU Handbook
Explicit Dependencies


typedef uint64_t starpu_tag_t


void starpu_task_declare_deps_array (struct starpu_task *task, unsigned ndeps, struct starpu_task *task_array[])
int starpu_task_get_task_succs (struct starpu_task *task, unsigned ndeps, struct starpu_task *task_array[])
int starpu_task_get_task_scheduled_succs (struct starpu_task *task, unsigned ndeps, struct starpu_task *task_array[])
void starpu_tag_declare_deps (starpu_tag_t id, unsigned ndeps,...)
void starpu_tag_declare_deps_array (starpu_tag_t id, unsigned ndeps, starpu_tag_t *array)
int starpu_tag_wait (starpu_tag_t id)
int starpu_tag_wait_array (unsigned ntags, starpu_tag_t *id)
void starpu_tag_restart (starpu_tag_t id)
void starpu_tag_remove (starpu_tag_t id)
void starpu_tag_notify_from_apps (starpu_tag_t id)

Detailed Description

Typedef Documentation

◆ starpu_tag_t

This type defines a task logical identifer. It is possible to associate a task with a unique tag chosen by the application, and to express dependencies between tasks by the means of those tags. To do so, fill the field starpu_task::tag_id with a tag number (can be arbitrary) and set the field starpu_task::use_tag to 1. If starpu_tag_declare_deps() is called with this tag number, the task will not be started until the tasks which holds the declared dependency tags are completed.

Function Documentation

◆ starpu_task_declare_deps_array()

void starpu_task_declare_deps_array ( struct starpu_task task,
unsigned  ndeps,
struct starpu_task task_array[] 

Declare task dependencies between a task and an array of tasks of length ndeps. This function must be called prior to the submission of the task, but it may called after the submission or the execution of the tasks in the array, provided the tasks are still valid (i.e. they were not automatically destroyed). Calling this function on a task that was already submitted or with an entry of task_array that is no longer a valid task results in an undefined behaviour. If ndeps is 0, no dependency is added. It is possible to call starpu_task_declare_deps_array() several times on the same task, in this case, the dependencies are added. It is possible to have redundancy in the task dependencies.

◆ starpu_task_get_task_succs()

int starpu_task_get_task_succs ( struct starpu_task task,
unsigned  ndeps,
struct starpu_task task_array[] 

Fills task_array with the list of tasks which are direct children of task. ndeps is the size of task_array. This function returns the number of direct children. task_array can be set to NULL if ndeps is 0, which allows to compute the number of children before allocating an array to store them. This function can only be called if task has not completed yet, otherwise the results are undefined. The result may also be outdated if some additional dependency has been added in the meanwhile.

◆ starpu_task_get_task_scheduled_succs()

int starpu_task_get_task_scheduled_succs ( struct starpu_task task,
unsigned  ndeps,
struct starpu_task task_array[] 

This behaves like starpu_task_get_task_succs(), except that it only reports tasks which will go through the scheduler, thus avoiding tasks with not codelet, or with explicit placement.

◆ starpu_tag_declare_deps()

void starpu_tag_declare_deps ( starpu_tag_t  id,
unsigned  ndeps,

Specify the dependencies of the task identified by tag id. The first argument specifies the tag which is configured, the second argument gives the number of tag(s) on which id depends. The following arguments are the tags which have to be terminated to unlock the task. This function must be called before the associated task is submitted to StarPU with starpu_task_submit().

WARNING! Use with caution. Because of the variable arity of starpu_tag_declare_deps(), note that the last arguments must be of type starpu_tag_t : constant values typically need to be explicitly casted. Otherwise, due to integer sizes and argument passing on the stack, the C compiler might consider the tag 0x200000003 instead of 0x2 and 0x3 when calling starpu_tag_declare_deps(0x1, 2, 0x2, 0x3). Using the starpu_tag_declare_deps_array() function avoids this hazard.

/* Tag 0x1 depends on tags 0x32 and 0x52 */

◆ starpu_tag_declare_deps_array()

void starpu_tag_declare_deps_array ( starpu_tag_t  id,
unsigned  ndeps,
starpu_tag_t array 

This function is similar to starpu_tag_declare_deps(), except that its does not take a variable number of arguments but an array of tags of size ndeps.

/* Tag 0x1 depends on tags 0x32 and 0x52 */
starpu_tag_t tag_array[2] = {0x32, 0x52};

◆ starpu_tag_wait()

int starpu_tag_wait ( starpu_tag_t  id)

This function blocks until the task associated to tag id has been executed. This is a blocking call which must therefore not be called within tasks or callbacks, but only from the application directly. It is possible to synchronize with the same tag multiple times, as long as the starpu_tag_remove() function is not called. Note that it is still possible to synchronize with a tag associated to a task for which the strucuture starpu_task was freed (e.g. if the field starpu_task::destroy was enabled).

◆ starpu_tag_wait_array()

int starpu_tag_wait_array ( unsigned  ntags,
starpu_tag_t id 

This function is similar to starpu_tag_wait() except that it blocks until all the ntags tags contained in the array id are terminated.

◆ starpu_tag_restart()

void starpu_tag_restart ( starpu_tag_t  id)

This function can be used to clear the already notified status of a tag which is not associated with a task. Before that, calling starpu_tag_notify_from_apps() again will not notify the successors. After that, the next call to starpu_tag_notify_from_apps() will notify the successors.

◆ starpu_tag_remove()

void starpu_tag_remove ( starpu_tag_t  id)

This function releases the resources associated to tag id. It can be called once the corresponding task has been executed and when there is no other tag that depend on this tag anymore.

◆ starpu_tag_notify_from_apps()

void starpu_tag_notify_from_apps ( starpu_tag_t  id)

This function explicitly unlocks tag id. It may be useful in the case of applications which execute part of their computation outside StarPU tasks (e.g. third-party libraries). It is also provided as a convenient tool for the programmer, for instance to entirely construct the task DAG before actually giving StarPU the opportunity to execute the tasks. When called several times on the same tag, notification will be done only on first call, thus implementing "OR" dependencies, until the tag is restarted using starpu_tag_restart().