Help with Fibonacci Recursive

[Gregory_G_]

Hi anyone can help me? why it's not running?

#include <stdio.h>
#include <tbb/tbb.h>

using namespace tbb;

class FibTask: public task {
    public:
    const long n;
    long* const sum;
    FibTask(long n_,long* sum_ ) : n(n_), sum(sum_)
    {}
     task* execute() {
        
            long x, y;
            FibTask& a = *new( allocate_child() )
            FibTask(n-1,&x);
            FibTask& b = *new( allocate_child() )
            FibTask(n-2,&y);
            set_ref_count(3);
            spawn( b );
            spawn_and_wait_for_all( a );
            *sum = x+y;
        
        return NULL;
    }
};

long ParallelFib(long n ) {
    long sum;
    FibTask& a = *new(task::allocate_root())
    FibTask(n,&sum);
    task::spawn_root_and_wait(a);
    
    return sum;
}

// Fibonacci paralelo.
int    main() {

    long int n = 30;
    task_scheduler_init init(task_scheduler_init::automatic);    
    init.initialize();
    printf("%ld",ParallelFib(n));
    init.terminate();
    
    return 0;
}

[Gregory_G_]

ooops,

#include <stdio.h>
#include <tbb/tbb.h>

using namespace tbb;

class FibTask: public task {
    public:
    const long n;
    long* const sum;
    FibTask(long n_,long* sum_ ) : n(n_), sum(sum_)
    {}
     task* execute() {
        
            long x, y;
            FibTask& a = *new( allocate_child() )
            FibTask(n-1,&x);
            FibTask& b = *new( allocate_child() )
            FibTask(n-2,&y);
            set_ref_count(3);
            spawn( b );
            spawn_and_wait_for_all( a );
            *sum = x+y;
        
        return NULL;
    }
};

long ParallelFib(long n ) {
    long sum;
    FibTask& a = *new(task::allocate_root())
    FibTask(n,&sum);
    task::spawn_root_and_wait(a);
    
    return sum;
}

// Fibonacci paralelo.
int    main() {

    long int n = 30;
    task_scheduler_init init(task_scheduler_init::automatic);    
    init.initialize();
    ParallelFib(n);
    init.terminate();
    
    return 0;
}

[Gregory_G_]

Segmentation falt

[RafSchietekat]

You don't terminate the calculation. The User Guide does so by delegating the calculation to SerialFib below Cutoff, and SerialFib returns n below 2: 0, 1, … You can eliminate SerialFib, but not the special treatment below n=2.

[Gregory_G_]

Thank's Raf,

Now it works!

the final version is

#include <stdio.h>
#include <tbb/tbb.h>

using namespace tbb;

long SerialFib(long n ) {
    if( n<2 )
        return n;
    else
    return SerialFib(n-1)+SerialFib(n-2);
}

class FibTask: public task {
    public:
    const long n;
    long* const sum;
    FibTask(long n_,long* sum_ ) : n(n_), sum(sum_)
    {}
     task* execute() {
        if ( n < 2) {
            *sum = SerialFib(n);
        } else {
            long x, y;
            FibTask& a = *new( allocate_child() ) FibTask(n-1,&x);
            FibTask& b = *new( allocate_child() ) FibTask(n-2,&y);
            set_ref_count(3);
            spawn( b );
            spawn_and_wait_for_all( a );
            *sum = x+y;
        }        
        return NULL;
    }
};

long ParallelFib(long n ) {
    long sum;
    FibTask& a = *new(task::allocate_root()) FibTask(n,&sum);
    task::spawn_root_and_wait(a);
    
    return sum;
}

// Fibonacci paralelo.
int    main() {
    
    task_scheduler_init init(task_scheduler_init::automatic);    
    init.initialize();
    printf("%ld",ParallelFib(40));
    init.terminate();

    return 0;
}

It's right isn't it?

[RafSchietekat]

Except that you shouldn't switch to SerialFib only for n smaller than 2 (now you might as well return n instead and get rid of SerialFib). The role of SerialFib in the User Guide example is to give at least tasks at the lower levels enough work that average parallel overhead, even with tasks already orders of magnitude cheaper than threads, remains limited. Here it dominates because no task has more than a few instructions of useful work. See "User Guide>Parallelizing Simple Loops>parallel_for>Controlling Chunking" for a nice explanation of the principle.