Multicore problem

[Anatoly Chernyshev <achernyshev@gmail.com>]

Hello, everyone,

Here is the following problem: large 3D array, which can be processed
in parallel. Naturally, I was trying to make use of 2 cores on my
processor by splitting the whole calculation into several parallel
tasks. Strangely, I could actually see that the program utilizes both
cores now (even not by 100% as I wished), but the computation time
increases more than 2 times when there are 2 tasks are running on 2
cores. During calculation any 2 or more tasks can occasionally modify
the same array member, for which a protected type is introduced.

Here are results of benchmarking for my 2 core system (Intel P4 Dual
Core, Win XP SP3):

1 task (50% total proc. load): 680 sec.
2 tasks (70-93% load): 1520 sec.
4 tasks (70-93% load): 1195 sec.

Any ideas on why it might happen and how to get around it?

Thank you.