Re: [Starpu-devel] StarPU questions

[Samuel Thibault <samuel.thibault@ens-lyon.org>]

Hello,

Vergel, Julio, le Fri 29 Aug 2014 18:38:10 +0000, a écrit :
>     a) taskGain[0]
>     b) taskAddConstant which has to wait for taskGain[0] completion
>     c) taskGain[1] which has to wait for taskAddConstant completion
>     d) taskGain[2] which has to wait for taskAddConstant completion
> 
> everything works OK. However if I change the order of submitting the tasks
> 
>         starpu_task_submit( taskGain[2] );
>         starpu_task_submit( taskGain[0] );
>         starpu_task_submit( taskAddConstant );
>         starpu_task_submit( taskGain[1] );
> 
> the program hangs when i run it.

Remember that StarPU implements implicit dependencies generation from
sequential consistency, which add up to the dependencies that you create
by hand, and thus it will here add a dependency between taskGain[2] and
taskAddConstant, since taskGain[2] writes to vectorAddConstantHandle,
and taskAddConstant reads from it.  To disable implicit dependencies,
use starpu_data_set_sequential_consistency_flag, or even
starpu_data_set_default_sequential_consistency_flag.

> 2) My second question is that I noticed that if I increase the value of NX
> (line 17) to a couple of millions, the program crashes immediately.

That's not really surprising:

> int main(int argc, char **argv)
> {
>     /* We consider a vector of float that is initialized just as any of C
>        * data */
>     float vector[NX];

Theses local arrays are allocated on the stack, which is usually limited
to only a few MiB in most systems.  Increase the stack size with ulimit
-s, or make it a global variable, or allocate it dynamically from the
heap.

Samuel