[Users] OpenMP is making it slower?

Peter Diener diener at cct.lsu.edu
Wed May 18 00:14:00 CDT 2011

Hi Scott,

On Tue, 17 May 2011, Frank Loeffler wrote:

> Hi,
> On Tue, May 17, 2011 at 03:02:00PM -0700, Scott Hawley wrote:
>> Do these all be need to be declared as private?
> If the temporary variables are declared only inside the loop they are
> automatically thread-local. Oh wait, that is Fortran. Well - in that
> case you should either declare all private, or (maybe easier) put the
> include files into separate functions, declare the temporary variables
> only there and call the functions from within the loop, in which case
> they also don't have to be specified for openmp (as long as they are not
> static).
>> i certainly don't want the various processors overwriting each others'
>> work, which might be what they're doing. -- maybe they're even
>> generating NaNs which would slow things down a bit!

Alternatively you may use the DEFAULT(PRIVATE) clause, so that you only 
have to specify the shared variables. However, in that case you have to 
make sure to really declare all the shared variables as shared, since 
otherwise all processors will have to allocate storage and if they are 3d 
variables this will slow down the code and increase memory consumption. 
Also private variables have undefined values on entry to the parallel 
region so not declaring all shared variables properly can also adversely
affect the result. So be careful.

> You should see that in the results though. It might make sense to first
> make sure that the results with different numbers of threads are the
> same (depending on the problem you might actually get bit-by-bit
> identical results), and work on optimization later. I agree that your
> slow-down actually points towards some bug.
> Frank



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