parallel_phase Interface for Task Arena

parallel_phase Interface for Task Arena#

Note

To enable this feature, set TBB_PREVIEW_PARALLEL_PHASE macro to 1.

Description#

This feature extends the tbb::task_arena specification with the following API:

  • Adds the leave_policy enumeration class to task_arena.

  • Adds leave_policy as the last parameter in task_arena constructors and task_arena::initialize methods. This allows you to inform the scheduler about the preferred policy for worker threads when they are about to leave task_arena due to a lack of available work.

  • Adds new start_parallel_phase and end_parallel_phase interfaces to the task_arena class and the this_task_arena namespace. These interfaces work as hints to the scheduler to mark the start and end of parallel work submission into the arena, enabling different worker thread retention policies.

  • Adds the Resource Acquisition is Initialization (RAII) class scoped_parallel_phase to task_arena.

More details about motivation, semantics and conditions for becoming fully supported functionality can be found in the corresponding Request For Comments document for parallel_phase.

API#

Synopsis#

namespace oneapi {
    namespace tbb {

        class task_arena {
        public:

            enum class leave_policy : /* unspecified type */ {
                automatic = /* unspecifed */,
                fast = /* unspecifed */,
            };

            task_arena(int max_concurrency = automatic, unsigned reserved_for_masters = 1,
                       priority a_priority = priority::normal,
                       leave_policy a_leave_policy = leave_policy::automatic);

            task_arena(const constraints& constraints_, unsigned reserved_for_masters = 1,
                       priority a_priority = priority::normal,
                       leave_policy a_leave_policy = leave_policy::automatic);

            void initialize(int max_concurrency, unsigned reserved_for_masters = 1,
                            priority a_priority = priority::normal,
                            leave_policy a_leave_policy = leave_policy::automatic);

            void initialize(constraints a_constraints, unsigned reserved_for_masters = 1,
                            priority a_priority = priority::normal,
                            leave_policy a_leave_policy = leave_policy::automatic);

            void start_parallel_phase();
            void end_parallel_phase(bool with_fast_leave = false);

            class scoped_parallel_phase {
            public:
                scoped_parallel_phase(task_arena& ta, bool with_fast_leave = false);
            };
        }; // class task_arena

        namespace this_task_arena {
            void start_parallel_phase();
            void end_parallel_phase(bool with_fast_leave = false);
        } // namespace this_task_arena

    } // namespace tbb
} // namespace oneapi

Member Types#

enum leave_policy::automatic#

When passed to a constructor or the initialize method, the initialized task_arena has the default (possibly system specific) policy for how quickly worker threads leave the arena when there is no more work available in the arena and when the arena is not in a parallel phase.

Note

Worker threads in task_arena might be retained based on internal heuristics.

enum leave_policy::fast#

When passed to a constructor or the initialize method, the initialized task_arena has policy that allows worker threads to more quickly leave the arena when there is no more work available in the arena and when the arena is not in a parallel phase.

class scoped_parallel_phase#

The RAII class to map a parallel phase to a code scope.

scoped_parallel_phase::scoped_parallel_phase(task_arena &ta, bool with_fast_leave = false)#

Constructs a scoped_parallel_phase object that starts a parallel phase in the specified task_arena. If with_fast_leave is true, the worker threads leave policy is temporarily set to fast.

Note

For task_arena initialized with leave_policy::fast, with_fast_leave setting has no effect.

Note

When worker threads enter the arena with no active parallel phases, the leave policy is reset to the value set during the initialization of the arena.

Member Functions#

task_arena(const task_arena&)#

Copies settings from task_arena instance including the leave_policy.

void start_parallel_phase()#

Indicates a point from where the scheduler can use a hint to keep threads in the arena for longer.

Note

This function can also be a warm-up hint for the scheduler. It allows the scheduler to wake up worker threads in advance.

void end_parallel_phase(bool with_fast_leave = false)#

Indicates the point when the scheduler may drop a hint and no longer retain threads in the arena. If with_fast_leave is true, worker threads leave policy is temporarily set to fast.

Note

For task_arena initialized with leave_policy::fast, with_fast_leave setting has no effect.

Note

When worker threads enter the arena with no active parallel phases, the leave policy is reset to the value set during the initialization of the arena.

Functions#

void this_task_arena::start_parallel_phase()#

Indicates the start of the parallel phase in the current task_arena.

void this_task_arena::end_parallel_phase(bool with_fast_leave = false)#

Indicates the end of the parallel phase in the current task_arena. If with_fast_leave is true, worker threads leave policy is temporarily set to fast.

Example#

In this example, task_arena is created with leave_policy::fast. It means that worker threads are not expected to remain in task_arena once parallel work is completed.

However, the workflow includes a sequence of parallel work (initializing and sorting data) interceded by serial work (prefix sum). To hint the start and end of parallel work, scoped_parallel_phase is used. This provides a hint to the scheduler that worker threads might need to remain in task_arena since there is more parallel work to come.

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