Gnu C++ & Open MP

[burlen <bloring@xxxxxxx>]

Hi,
I notice some strange result when I use Gnu OpenMP, I have an existing 
C++ serial code which has some time consuming for loops where some 
calculations are made on a large arrays of doubles,(12 arrays of length 
1E6 for example) and also some of these arrays are copied to create a 
vector field(also an array of doubles). This particular for loop is 
where the code takes 90% of its time and typically can take 1 or 2 
minutes here.  What I notice is that when I run open MP its actually 
slower than the serial code! Not by much but still slower. Here I use 
the command 'time' to compare. Also I notice by watching top, that in 
both cases only 1 core is taxed, while in the OpenMP build memory usage 
doubles(actually a second core goes up from 0% to about 10%). I have a 
quad core system(dual cpu dual core, and 4G of ram), so this result is 
totally unexpected. In my code I explicitly set the number of threads to 
4. I was expecting to see all 4 cores running at 80-90%. I have verified 
that 4 threads are being launched by runing the OpenMP build in GDB. 
What is going on here? how can I figure what the problem is?
Thanks Burlen