Quoted from Wikipedia,
"The amount of performance gained by the use of a multi-core processor
is strongly dependent on the software algorithms and implementation. In
particular, the possible gains are limited by the fraction of the
software that can be "parallelized" to run on multiple cores
simultaneously; this effect is described by
Amdahl's law. In the best case, so-called
embarrassingly parallel
problems may realize speedup factors near the number of cores. Many
typical applications, however, do not realize such large speedup
factors and thus, the parallelization of software is a significant
on-going topic of research."
See: http://en.wikipedia.org/wiki/Multi-core_(computing)
