Updated implementation to use 1 OpenMP thread by default and give a warning that the 'OMP_NUM_THREADS' environment varibale is unset. This allows an unaware user to run their codes without any changes, and they will see the warning and can adjust accordingly.

Author: weslowrie

src/pyclaw/classic/solver.py

@@ -612,9 +612,12 @@ def __init__(self, riemann_solver=None, claw_package=None):
         self.work = None
 
         import os
-        import multiprocessing
-        self.nthreads = int(os.environ.get('OMP_NUM_THREADS',\
-                                           multiprocessing.cpu_count()))
+        import warnings
+        if 'OMP_NUM_THREADS' in os.environ:
+            pass # Using OpenMP
+        else:
+            warnings.warn("The environment variable 'OMP_NUM_THREADS' is unset.  Set this variable to use OpenMP with more than 1 thread (i.e. export OMP_NUM_THREADS=4).")
+        self.nthreads = int(os.environ.get('OMP_NUM_THREADS',1))
 
         super(ClawSolver3D,self).__init__(riemann_solver, claw_package)
 

src/pyclaw/classic/step3.f90

@@ -40,6 +40,7 @@ subroutine step3(maxm,nthreads,num_eqn,num_waves,num_ghost,mx,my, &
   dimension method(7),mthlim(num_waves)
   dimension work(mwork)
   logical :: use_fwave
+  character :: env_value
 
   ! These block sizes must be at least 2 to avoid multiple threads
   ! trying to update the same grid cell
@@ -61,6 +62,10 @@ subroutine step3(maxm,nthreads,num_eqn,num_waves,num_ghost,mx,my, &
   ! flux3 routine.  Find starting index of each piece:
 
   nthreads=1    ! Serial
+  CALL GET_ENVIRONMENT_VARIABLE('OMP_NUM_THREADS',env_value,nlen,nstat)
+  IF(nstat >= 1)THEN
+     !$ call omp_set_num_threads(1) ! set default to 1 thread
+  END IF
   !$omp parallel
   !$omp single
   !$ nthreads = omp_get_num_threads()

src/pyclaw/classic/step3ds.f90

@@ -54,6 +54,7 @@ subroutine step3ds(maxm,nthreads,num_eqn,num_waves,num_ghost,mx,my, &
   dimension method(7),mthlim(num_waves)
   dimension work(mwork)
   logical :: use_fwave
+  character :: env_value
 
   common /comxyzt/ dtcom,dxcom,dycom,dzcom,tcom,icom,jcom,kcom
 
@@ -71,6 +72,10 @@ subroutine step3ds(maxm,nthreads,num_eqn,num_waves,num_ghost,mx,my, &
   ! flux3 routine.  Find starting index of each piece:
 
   nthreads=1 ! Serial
+  CALL GET_ENVIRONMENT_VARIABLE('OMP_NUM_THREADS',env_value,nlen,nstat)
+  IF(nstat >= 1)THEN
+     !$ call omp_set_num_threads(1) ! set default to 1 thread
+  END IF
   !$OMP PARALLEL
   !$OMP SINGLE
   !$ nthreads = omp_get_num_threads()