[Python] Add multiprocessing example

interfaces/cython/cantera/examples/transport/multiprocessing_viscosity.py

@@ -0,0 +1,99 @@
+"""
+This example demonstrates how Cantera can be used with the 'multiprocessing'
+module. 
+
+Because Cantera Python objects are built on top of C++ objects which cannot be
+passed between Python processes, it is necessary to set up the computation so
+that each process has its own copy of the relevant Cantera objects. One way to
+do this is by storing the objects in (module) global variables, which are
+initialized once per worker process.
+"""
+
+import multiprocessing
+import numpy as np
+import cantera as ct
+import itertools
+from time import time
+
+# Global storage for Cantera Solution objects
+gases = {}
+
+def init_process(mech):
+    """
+    This function is called once for each process in the Pool. We use it to
+    initialize any Cantera objects we need to use.
+    """
+    gases[mech] = ct.Solution(mech)
+    gases[mech].transport_model = 'Multi'
+
+def get_thermal_conductivity(args):
+    # Pool.imap only permits a single argument, so we pack all of the needed
+    # arguments into the tuple 'args'
+    mech, T, P, X = args
+    gas = gases[mech]
+    gas.TPX = T, P, X
+    return gas.thermal_conductivity
+
+def get_viscosity(args):
+    # Pool.imap only permits a single argument, so we pack all of the needed
+    # arguments into the tuple 'args'
+    mech, T, P, X = args
+    gas = gases[mech]
+    gas.TPX = T, P, X
+    return gas.enthalpy_mass
+
+def parallel(mech, predicate, nProcs, nTemps):
+    """
+    Call the function ``predicate`` on ``nProcs`` processors for ``nTemps``
+    different temperatures.
+    """
+    P = ct.one_atm
+    X = 'CH4:1.0, O2:1.0, N2:3.76'
+    pool = multiprocessing.Pool(processes=nProcs, 
+                                initializer=init_process, 
+                                initargs=(mech,))
+
+    y = pool.map(predicate, 
+                 zip(itertools.repeat(mech),
+                     np.linspace(300, 900, nTemps),
+                     itertools.repeat(P),
+                     itertools.repeat(X)))
+    return y
+
+def serial(mech, predicate, nTemps):
+    P = ct.one_atm
+    X = 'CH4:1.0, O2:1.0, N2:3.76'
+    init_process(mech)
+    y = map(predicate, 
+            zip(itertools.repeat(mech),
+                np.linspace(300, 900, nTemps),
+                itertools.repeat(P),
+                itertools.repeat(X)))
+    return y
+    
+if __name__ == '__main__':
+    # For functions where the work done in each subprocess is substantial,
+    # significant speedup can be obtained using the multiprocessing module.
+    print('Thermal conductivity')
+    t1 = time()
+    parallel('gri30.xml', get_thermal_conductivity, 4, 1000)
+    t2 = time()
+    print('Parallel: {0:.3f} seconds'.format(t2-t1))
+
+    t1 = time()
+    serial('gri30.xml', get_thermal_conductivity, 1000)
+    t2 = time()
+    print('Serial: {0:.3f} seconds'.format(t2-t1))
+
+    # On the other hand, if the work done per call to the predicate function is
+    # small, there may be no advantage to using multiprocessing.
+    print('\nViscosity')
+    t1 = time()
+    parallel('gri30.xml', get_viscosity, 4, 1000)
+    t2 = time()
+    print('Parallel: {0:.3f} seconds'.format(t2-t1))
+
+    t1 = time()
+    serial('gri30.xml', get_viscosity, 1000)
+    t2 = time()
+    print('Serial: {0:.3f} seconds'.format(t2-t1))