[Users] Generalising to OpenMP
c.stevens at ru.ac.za
Wed Mar 28 03:24:04 CDT 2018
I am new to OpenMP programming, and have quickly come across an (I
think) interesting task that I want to parallelise with OpenMP that does
not seem straightforwardly generalisable. I would like to hear your
feedback on the potential solutions and whether there is a better way to
Task: Create a scratch array, say of CCTK_REAL, during an initialisation
phase that is shared with other code using a header file. This array is
defined only once. This scratch array is then used as temporary storage
space in a function that does some computation within a for loop. i.e.
function blah ()
computation involving scratch array
This works fine when OpenMP is turned off. However when you add to the
for loop above #pragma omp parallel for, then one sees that this one
scratch array will be being used by multiple threads at once, thus
Solution 1: Each thread has its own scratch space. I am not sure if it
is within the OpenMP paradigm to create scratch spaces for each thread
within the initialisation file, and share them through a header. This
doesn't seem to work and I am thinking this shouldn't be possible?
Solution 2: Create one serial scratch space during initialisation that
has size dependent on omp_get_num_threads(). One would then have to
change the computation involving the scratch array to be dependent on
omp_get_thread_num() so as to use its allocated part of the scratch space.
Solution 3: One could do something like
#pragma omp parallel
create scratch space
#pragma omp parallel for
computation involving thread specific scratch space
In summary: Solution 1 doesn't seem to work and probably doesn't fit
into the OpenMP paradigm. Solution 2 would work but doesn't seem very
nice. Solution 3 is nicer but inefficient as I don't want to be creating
scratch spaces all the time.
Is there perhaps another way that fits better with the OpenMP paradigm?
Dr Chris Stevens
Claude Leon Postdoctoral Fellow
Department of Mathematics
Ph: +27 46 603 8932
Web: www.chrisdoesmaths.com <http://www.chrisdoesmaths.com>
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