Axpy
====

This benchmark runs several AXPY operations. AXPY is a Level 1 operation in the
Basic Linear Algebra Subprograms (BLAS) package, and is a common operation in
computations with vector processors. AXPY is a combination of scalar
multiplication and vector addition.

.. highlight:: c
We want to parallelize the code using the ``task`` directive in order to
express parallelism. We will probably need to synchronize task execution across
AXPY operations (using ``taskwait``) The sequential version is already using
the blocking technique, and blocks are usually good candidates to become
tasks. Ideally we should focus our effort in the following two functions::

   void axpy_block(double *x, double *y, double alpha, long N) {
   	for (long i=0; i < N; ++i) {
   		y[i] += alpha * x[i];
   	}
   }
   
   void axpy(double *x, double *y, double alpha, long N, long TS) {
   	for (long i=0; i < N; i+=TS) {
   		axpy_block(x+i, y+i, alpha, std::min(TS, N-i));
   	}
   }

Goals of this exercise
----------------------
* Try to understand what is doing each part of the code. Look for candidates to become a task and taskify them.
* When creating tasks, pay special attention to data-sharing attributes.
* Include synchroniztion directives when required (e.g., you should no start the verification until all the computation has been executed).
* Check several runtime options when executing the program (versions, schedulers, etc.).
* Check scalability. Execute the program using different number of cpus and compute the speed-up.
* Change program's arguments that may impact on task granularity (block size, tile size, etc.).
* Change program's arguments that may impact on the number of tasks (matrix sizes and/or block/tile sizes).
* Get different paraver traces using different runtime options or program arguments and compare them.


Execution instructions
----------------------

You should run the program as::

  ./saxpy SIZE BLOCK_SIZE INTERATIONS

where:

* ``SIZE`` is the number of elements of the vectors used on the SAXPY operation.
* The SAXPY operation will be applied to the vector in blocks that contains ``BLOCK_SIZE`` elements.
* ``ITERATIONS`` is the number of times the SAXPY operation is executed.

References
----------
* http://en.wikipedia.org/wiki/AXPY