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Intel Threading Building Blocks
book

Intel Threading Building Blocks

by James Reinders
July 2007
Intermediate to advanced content levelIntermediate to advanced
332 pages
10h 4m
English
O'Reilly Media, Inc.
Content preview from Intel Threading Building Blocks

Lazy copying

It can be useful to copy a data structure only when another thread steals a task. Example 9-12 uses the start_reduce::execute() method to implement parallel_reduce. The code forks the loop body object you provide only when the thread runs a stolen task. The forking permits the thief to run locally afterward until it is done and joins its result to the original thread’s result. Because the forks and joins incur some overhead, they are worth doing only when stealing occurs.

Example 9-12. parallel_reduce start_reduce::execute( ) method

template<typename Range, typename Body>
  task* start_reduce<Range,Body>::execute() {
    Body* body = my_body; 
    if( is_stolen_task() ) {
      finish_reduce<Body>* p = static_cast<finish_type*>(parent() );
      body = new(p->zombie_space.begin()) Body(*body,split());
      my_body = p->right_zombie = body;
    }
    task* next_task = NULL;
    if( !my_range.is_divisible() )
      (*my_body)( my_range );
    else {
      finish_reduce<Body>& c =
        *new(allocate_continuation()) finish_type(body);
      recycle_as_child_of(c);
      c.set_ref_count(2);
      start_reduce& b =
        *new(c.allocate_child()) start_reduce(Range(my_range,split()), body);
      c.spawn(b);
      next_task = this;
    }
    return next_task;
  }
}

The method task::is_stolen_task provides a way to detect stealing. It is called on a running task, typically by the task itself. Informally speaking, it returns true if the task is stolen. Formally, it returns true if the thread that owns the task is not the thread that owns the thread’s dependent. For the usual fork-join ...

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Publisher Resources

ISBN: 9780596514808Errata Page