Parallel computing those that use concurrency, are more difficult to write than sequential ones, because concurrency introduces several new classes of potential software bugs, of which race conditions are the most common. Communication and synchronization between the different subtasks are typically some of the greatest obstacles to getting good parallel program performance. A theoretical upper bound on the speed-up of a single program as a result of parallelization is given by Amdahl's law. Traditionally, computer software has been written for serial computation. To solve a problem, an algorithm is constructed and implemented as a serial stream of instructions. These instructions are executed
Parallel universes in fiction or unintentionally – as in "Angel Heart". Less often, the alternative narratives are given equal weight in the story, making them truly alternative universes, such as in the German film "Run Lola Run", the short-lived British West End musical "Our House" and the British film "Sliding Doors". More recent films that have explicitly explored parallel universes are: the 2000 film "The Family Man", the 2001 cult movie "Donnie Darko", which deals with what it terms a "tangent universe" that erupts from our own universe; "Super Mario Bros." (1993) has the eponymous heroes cross over into a parallel universe ruled by