Gnu C++ & Open MP

[burlen <burlen at apollo dot sr dot unh dot edu>]

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