Slowdown when computing in parallel on multicore CPU
A couple of months ago I observed some unusual behaviour when I computed integer arithmetic one a single core, then on two cores, then on three cores, and so on. I own an octacore processor (AMD FX(tm)-8120, 3.1GHz). I would have assumed that as long as I have enough cores idling that adding more work to my CPU does not adversely affect performance per task. But quite the contrary, performance goes down, up to 60%. Right now I have no convincing explanation for this behaviour.
The same is true for Intel i7 (i7-2640, 2.8GHz). Performance deteriorates up to the point when all cores are used. At that moment no further slowdown can be observed.
The same is also true for floating point calculation on both processors (AMD and Intel). The problem at hand was completely artificial, and there seems to be no memory contention, as all values at hand can easily be kept within the CPU entirely. See http://eklausmeier.wordpress.com/201...ndant-on-load/.
Thank you for any comments on this.