Gnu C++ & Open MP

Tom St Denis
Wed Jan 9 02:58:00 GMT 2008

burlen wrote:
> 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

What's the affinity of the threads?  IIRC by default at least with 
pthreads they attach to the CPU that issued the thread so you have to 
set the affinity to 0xF or -1 [all].


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