.. _Task_Scheduler_Init:

Migrating from tbb::task_scheduler_init

tbb::task_scheduler_init was a multipurpose functionality in the previous versions of Threading Building Blocks (TBB). This section considers different use cases and how they can be covered with oneTBB.

Managing the number of threads

Querying the default number of threads ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

:onetbb-spec:oneapi::tbb::info::default_concurrency() <info_namespace> returns the maximum concurrency that will be created by default in implicit or explicit task_arena.
:onetbb-spec:oneapi::tbb::this_task_arena::max_concurrency() <task_scheduler/task_arena/this_task_arena_ns> returns the maximum number of threads available for the parallel algorithms within the current context (or default if an implicit arena is not initialized)
:onetbb-spec:oneapi::tbb::global_control::active_value(tbb::global_control::max_allowed_parallelism) <task_scheduler/scheduling_controls/global_control_cls> returns the current limit of the thread pool (or default if oneTBB scheduler is not initialized)

Setting the maximum concurrency ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

:onetbb-spec:task_arena(/* max_concurrency */) <task_scheduler/task_arena/this_task_arena_ns> limits the maximum concurrency of the parallel algorithm running inside task_arena
:onetbb-spec:tbb::global_control(tbb::global_control::max_allowed_parallelism, /* max_concurrency */) <task_scheduler/scheduling_controls/global_control_cls> limits the total number of oneTBB worker threads

Examples ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The default parallelism:

.. code:: cpp

#include <oneapi/tbb/info.h>
#include <oneapi/tbb/parallel_for.h>
#include <oneapi/tbb/task_arena.h>
#include <cassert>

int main() {
    // Get the default number of threads
    int num_threads = oneapi::tbb::info::default_concurrency();

    // Run the default parallelism
    oneapi::tbb::parallel_for( /* ... */ [] {
        // Assert the maximum number of threads
        assert(num_threads == oneapi::tbb::this_task_arena::max_concurrency());
    });

    // Create the default task_arena
    oneapi::tbb::task_arena arena;
    arena.execute([]{
        oneapi::tbb::parallel_for( /* ... */ [] {
            // Assert the maximum number of threads
            assert(num_threads == oneapi::tbb::this_task_arena::max_concurrency());
        });
    });

    return 0;
}

The limited parallelism:

.. code:: cpp

#include <oneapi/tbb/info.h>
#include <oneapi/tbb/parallel_for.h>
#include <oneapi/tbb/task_arena.h>
#include <oneapi/tbb/global_control.h>
#include <cassert>

int main() {
    // Create the custom task_arena with four threads
    oneapi::tbb::task_arena arena(4);
    arena.execute([]{
        oneapi::tbb::parallel_for( /* ... */ [] {
            // This arena is limited with for threads
            assert(oneapi::tbb::this_task_arena::max_concurrency() == 4);
        });
    });

    // Limit the number of threads to two for all oneTBB parallel interfaces
    oneapi::tbb::global_control global_limit(oneapi::tbb::global_control::max_allowed_parallelism, 2);

    // the default parallelism
    oneapi::tbb::parallel_for( /* ... */ [] {
        // No more than two threads is expected; however, tbb::this_task_arena::max_concurrency() can return a bigger value
        int thread_limit = oneapi::tbb::global_control::active_value(oneapi::tbb::global_control::max_allowed_parallelism);
        assert(thread_limit == 2);
    });

    arena.execute([]{
        oneapi::tbb::parallel_for( /* ... */ [] {
            // No more than two threads is expected; however, tbb::this_task_arena::max_concurrency() is four
            int thread_limit = oneapi::tbb::global_control::active_value(oneapi::tbb::global_control::max_allowed_parallelism);
            assert(thread_limit == 2);
            assert(tbb::this_task_arena::max_concurrency() == 4);
        });
    });

    return 0;
}

Setting thread stack size

Use :onetbb-spec:oneapi::tbb::global_control(oneapi::tbb::global_control::thread_stack_size, /* stack_size */) <task_scheduler/scheduling_controls/global_control_cls> to set the stack size for oneTBB worker threads:

.. code:: cpp

#include <oneapi/tbb/parallel_for.h>
#include <oneapi/tbb/global_control.h>

int main() {
    // Set 16 MB of the stack size for oneTBB worker threads.
    // Note that the stack size of the main thread should be configured in accordace with the
    // system documentation, e.g. at application startup moment
    oneapi::tbb::global_control global_limit(tbb::global_control::thread_stack_size, 16 * 1024 * 1024);

    oneapi::tbb::parallel_for( /* ... */ [] {
        // Create a big array in the stack
        char big_array[10*1024*1024];
    });

    return 0;
}

Terminating oneTBB scheduler

:onetbb-spec:task_scheduler_handle <task_scheduler/scheduling_controls/task_scheduler_handle_cls> allows waiting for oneTBB worker threads completion:

.. code:: cpp

#include <oneapi/tbb/global_control.h>
#include <oneapi/tbb/parallel_for.h>

int main() {
    oneapi::tbb::task_scheduler_handle handle{tbb::attach{}};
    // Do some parallel work here
    oneapi::tbb::parallel_for(/* ... */);
    oneapi::tbb::finalize(handle);
    return 0;
}