[starpu-devel] Get / access value of a variable attached to a StarPU handle

[algiane froehly <algiane@gmail.com>]

Good morning the starPU team,

As discussed with Nathalie, I transfer you this email initially written to ask help. I finally found a solution to my problem (added at the end of the email).

TR :  

I am new to StarPU and have small difficulties.

In short, I want to taskify a code that has the following form :

```C

do {

count = f ( );

// Do other stuffs that depend on count value

} while ( test ( count )  );

```

for now, I don’t plan to target GPU architectures so I have probably made shortcuts due to the fact that I am only on CPU.

Following the examples of reduction on the gitlab repository (https://gitlab.inria.fr/starpu/starpu/-/blob/master/examples/reductions/dot_product.c) and of an EoCoE workshop (https://indico.math.cnrs.fr/event/6415/attachments/2736/3475/2021.02.24_-_exa2pro-eocoe_workshop_-_StarPU_-_S._Thibault.pdf ). I have implemented a codelet for ‘f’ and a codelet for the reduction. Now my loop has the following form: 

```C

starpu_variable_data_register(&handle_count, STARPU_MAIN_RAM, (uintptr_t)&count, sizeof(count));

starpu_data_set_reduction_methods ( handle_count, &accumulate_codelet, &izero_codelet) ;

do {

   count = 0; 

   // parallel section

   for ( i = 0; i < ntask; ++i )

      ret = starpu_task_insert( &f_cl, STARPU_REDUX, handle_count );

   }

   starpu_task_wait_for_all();

  // Serial section

  // Do other stuffs that depend on count value

}   while ( test ( count )  );

starpu_data_unregister(handle_count);

```

My issue is that StarPU fills in the count value only when I unregister it (or if I write a fake task that uses it) : I guess that I have to access the handle through one of the StarPU API but I don’t find which one. I have searched for a list of getter functions (list of API with either the « get » keyword, the  « variable » or « value » one).

EDIT :  I have found a solution (searching for the « data keyword » ;-) ). I also guess that I can remove the task_wait_for_all() call because synchronization is needed to acquire the count value :

```C

 starpu_variable_data_register(&handle_count, STARPU_MAIN_RAM, (uintptr_t)&count, sizeof(count));

 starpu_data_set_reduction_methods ( handle_count, &accumulate_codelet, &izero_codelet) ;

starpu_data_acquire(handle_count, STARPU_W);

do {

   count = 0; 

   starpu_data_release(handle_count);

   // parallel section

   for ( i = 0; i < ntask; ++i )

      ret = starpu_task_insert( &f_cl, STARPU_REDUX, handle_count );

   }

  // serial section

  starpu_data_acquire(handle_count, STARPU_RW);

  // Do other stuffs that depend on count value

}   while ( test ( count )  );

starpu_data_unregister(handle_count);

```

BR,

Algiane