The reason why parallel programming is hard is, like John Hennessy said: "Many of the early ideas were motivated by observations of what was easy to implement

The reason why parallel programming is hard is, like John Hennessy said: "Many of the early ideas were motivated by observations of what was easy to implement in the hardware rather than what was easy to use" (see http://www.acmqueue.com/modules.php?name=Content&pa=showpage&pid=445&page=5 http://www.acmqueue.com/modules.php?name=Content&pa=show...) i.e parallel programming was always an afterthought. To architects, building a parallel system was viewed primarily as a hardware hurdle to overcome.