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Changing references to mpirun to mpiexec in docs.
Brian Granger -
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1 1 .. _parallelmpi:
2 2
3 3 =======================
4 4 Using MPI with IPython
5 5 =======================
6 6
7 7 Often, a parallel algorithm will require moving data between the engines. One way of accomplishing this is by doing a pull and then a push using the multiengine client. However, this will be slow as all the data has to go through the controller to the client and then back through the controller, to its final destination.
8 8
9 9 A much better way of moving data between engines is to use a message passing library, such as the Message Passing Interface (MPI) [MPI]_. IPython's parallel computing architecture has been designed from the ground up to integrate with MPI. This document describes how to use MPI with IPython.
10 10
11 11 Additional installation requirements
12 12 ====================================
13 13
14 14 If you want to use MPI with IPython, you will need to install:
15 15
16 16 * A standard MPI implementation such as OpenMPI [OpenMPI]_ or MPICH.
17 17 * The mpi4py [mpi4py]_ package.
18 18
19 19 .. note::
20 20
21 21 The mpi4py package is not a strict requirement. However, you need to
22 22 have *some* way of calling MPI from Python. You also need some way of
23 23 making sure that :func:`MPI_Init` is called when the IPython engines start
24 24 up. There are a number of ways of doing this and a good number of
25 25 associated subtleties. We highly recommend just using mpi4py as it
26 26 takes care of most of these problems. If you want to do something
27 27 different, let us know and we can help you get started.
28 28
29 29 Starting the engines with MPI enabled
30 30 =====================================
31 31
32 32 To use code that calls MPI, there are typically two things that MPI requires.
33 33
34 34 1. The process that wants to call MPI must be started using
35 :command:`mpirun` or a batch system (like PBS) that has MPI support.
35 :command:`mpiexec` or a batch system (like PBS) that has MPI support.
36 36 2. Once the process starts, it must call :func:`MPI_Init`.
37 37
38 38 There are a couple of ways that you can start the IPython engines and get these things to happen.
39 39
40 Automatic starting using :command:`mpirun` and :command:`ipcluster`
40 Automatic starting using :command:`mpiexec` and :command:`ipcluster`
41 41 -------------------------------------------------------------------
42 42
43 The easiest approach is to use the `mpirun` mode of :command:`ipcluster`, which will first start a controller and then a set of engines using :command:`mpirun`::
43 The easiest approach is to use the `mpiexec` mode of :command:`ipcluster`, which will first start a controller and then a set of engines using :command:`mpiexec`::
44 44
45 $ ipcluster mpirun -n 4
45 $ ipcluster mpiexec -n 4
46 46
47 47 This approach is best as interrupting :command:`ipcluster` will automatically
48 48 stop and clean up the controller and engines.
49 49
50 Manual starting using :command:`mpirun`
50 Manual starting using :command:`mpiexec`
51 51 ---------------------------------------
52 52
53 If you want to start the IPython engines using the :command:`mpirun`, just do::
53 If you want to start the IPython engines using the :command:`mpiexec`, just do::
54 54
55 $ mpirun -n 4 ipengine --mpi=mpi4py
55 $ mpiexec -n 4 ipengine --mpi=mpi4py
56 56
57 57 This requires that you already have a controller running and that the FURL
58 58 files for the engines are in place. We also have built in support for
59 59 PyTrilinos [PyTrilinos]_, which can be used (assuming is installed) by
60 60 starting the engines with::
61 61
62 mpirun -n 4 ipengine --mpi=pytrilinos
62 mpiexec -n 4 ipengine --mpi=pytrilinos
63 63
64 64 Automatic starting using PBS and :command:`ipcluster`
65 65 -----------------------------------------------------
66 66
67 67 The :command:`ipcluster` command also has built-in integration with PBS. For more information on this approach, see our documentation on :ref:`ipcluster <parallel_process>`.
68 68
69 69 Actually using MPI
70 70 ==================
71 71
72 72 Once the engines are running with MPI enabled, you are ready to go. You can now call any code that uses MPI in the IPython engines. And, all of this can be done interactively. Here we show a simple example that uses mpi4py [mpi4py]_.
73 73
74 74 First, lets define a simply function that uses MPI to calculate the sum of a distributed array. Save the following text in a file called :file:`psum.py`:
75 75
76 76 .. sourcecode:: python
77 77
78 78 from mpi4py import MPI
79 79 import numpy as np
80 80
81 81 def psum(a):
82 82 s = np.sum(a)
83 83 return MPI.COMM_WORLD.Allreduce(s,MPI.SUM)
84 84
85 85 Now, start an IPython cluster in the same directory as :file:`psum.py`::
86 86
87 $ ipcluster mpirun -n 4
87 $ ipcluster mpiexec -n 4
88 88
89 89 Finally, connect to the cluster and use this function interactively. In this case, we create a random array on each engine and sum up all the random arrays using our :func:`psum` function:
90 90
91 91 .. sourcecode:: ipython
92 92
93 93 In [1]: from IPython.kernel import client
94 94
95 95 In [2]: mec = client.MultiEngineClient()
96 96
97 97 In [3]: mec.activate()
98 98
99 99 In [4]: px import numpy as np
100 100 Parallel execution on engines: all
101 101 Out[4]:
102 102 <Results List>
103 103 [0] In [13]: import numpy as np
104 104 [1] In [13]: import numpy as np
105 105 [2] In [13]: import numpy as np
106 106 [3] In [13]: import numpy as np
107 107
108 108 In [6]: px a = np.random.rand(100)
109 109 Parallel execution on engines: all
110 110 Out[6]:
111 111 <Results List>
112 112 [0] In [15]: a = np.random.rand(100)
113 113 [1] In [15]: a = np.random.rand(100)
114 114 [2] In [15]: a = np.random.rand(100)
115 115 [3] In [15]: a = np.random.rand(100)
116 116
117 117 In [7]: px from psum import psum
118 118 Parallel execution on engines: all
119 119 Out[7]:
120 120 <Results List>
121 121 [0] In [16]: from psum import psum
122 122 [1] In [16]: from psum import psum
123 123 [2] In [16]: from psum import psum
124 124 [3] In [16]: from psum import psum
125 125
126 126 In [8]: px s = psum(a)
127 127 Parallel execution on engines: all
128 128 Out[8]:
129 129 <Results List>
130 130 [0] In [17]: s = psum(a)
131 131 [1] In [17]: s = psum(a)
132 132 [2] In [17]: s = psum(a)
133 133 [3] In [17]: s = psum(a)
134 134
135 135 In [9]: px print s
136 136 Parallel execution on engines: all
137 137 Out[9]:
138 138 <Results List>
139 139 [0] In [18]: print s
140 140 [0] Out[18]: 187.451545803
141 141
142 142 [1] In [18]: print s
143 143 [1] Out[18]: 187.451545803
144 144
145 145 [2] In [18]: print s
146 146 [2] Out[18]: 187.451545803
147 147
148 148 [3] In [18]: print s
149 149 [3] Out[18]: 187.451545803
150 150
151 151 Any Python code that makes calls to MPI can be used in this manner, including
152 152 compiled C, C++ and Fortran libraries that have been exposed to Python.
153 153
154 154 .. [MPI] Message Passing Interface. http://www-unix.mcs.anl.gov/mpi/
155 155 .. [mpi4py] MPI for Python. mpi4py: http://mpi4py.scipy.org/
156 156 .. [OpenMPI] Open MPI. http://www.open-mpi.org/
157 157 .. [PyTrilinos] PyTrilinos. http://trilinos.sandia.gov/packages/pytrilinos/ No newline at end of file
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