upstream/ipython Files · docs/source/parallel/parallel_multiengine.txt

.. _parallel_multiengine:

==========================

IPython's Direct interface

==========================

The direct, or multiengine, interface represents one possible way of working with a set of

IPython engines. The basic idea behind the multiengine interface is that the

capabilities of each engine are directly and explicitly exposed to the user.

Thus, in the multiengine interface, each engine is given an id that is used to

identify the engine and give it work to do. This interface is very intuitive

and is designed with interactive usage in mind, and is the best place for

new users of IPython to begin.

Starting the IPython controller and engines

===========================================

To follow along with this tutorial, you will need to start the IPython

controller and four IPython engines. The simplest way of doing this is to use

the :command:`ipcluster` command::

$ ipcluster start n=4

For more detailed information about starting the controller and engines, see

our :ref:`introduction <ip1par>` to using IPython for parallel computing.

Creating a ``Client`` instance

==============================

The first step is to import the IPython :mod:`IPython.parallel`

module and then create a :class:`.Client` instance:

.. sourcecode:: ipython

In [1]: from IPython.parallel import Client

In [2]: rc = Client()

This form assumes that the default connection information (stored in

:file:`ipcontroller-client.json` found in :file:`IPYTHON_DIR/cluster_default/security`) is

accurate. If the controller was started on a remote machine, you must copy that connection

file to the client machine, or enter its contents as arguments to the Client constructor:

.. sourcecode:: ipython

# If you have copied the json connector file from the controller:

In [2]: rc = Client('/path/to/ipcontroller-client.json')

# or to connect with a specific profile you have set up:

In [3]: rc = Client(profile='mpi')

To make sure there are engines connected to the controller, users can get a list

of engine ids:

.. sourcecode:: ipython

In [3]: rc.ids

Out[3]: [0, 1, 2, 3]

Here we see that there are four engines ready to do work for us.

For direct execution, we will make use of a :class:`DirectView` object, which can be

constructed via list-access to the client:

.. sourcecode:: ipython

In [4]: dview = rc[:] # use all engines

.. seealso::

For more information, see the in-depth explanation of :ref:`Views <parallel_details>`.

Quick and easy parallelism

==========================

In many cases, you simply want to apply a Python function to a sequence of

objects, but *in parallel*. The client interface provides a simple way

of accomplishing this: using the DirectView's :meth:`~DirectView.map` method.

Parallel map

------------

Python's builtin :func:`map` functions allows a function to be applied to a

sequence element-by-element. This type of code is typically trivial to

parallelize. In fact, since IPython's interface is all about functions anyway,

you can just use the builtin :func:`map` with a :class:`RemoteFunction`, or a

DirectView's :meth:`map` method:

.. sourcecode:: ipython

In [62]: serial_result = map(lambda x:x**10, range(32))

In [63]: parallel_result = dview.map_sync(lambda x: x**10, range(32))

In [67]: serial_result==parallel_result

Out[67]: True

.. note::

The :class:`DirectView`'s version of :meth:`map` does

not do dynamic load balancing. For a load balanced version, use a

:class:`LoadBalancedView`.

.. seealso::

:meth:`map` is implemented via :class:`ParallelFunction`.

Remote function decorators

--------------------------

Remote functions are just like normal functions, but when they are called,

they execute on one or more engines, rather than locally. IPython provides

two decorators:

.. sourcecode:: ipython

In [10]: @dview.remote(block=True)

...: def getpid():

...: import os

...: return os.getpid()

...:

In [11]: getpid()

Out[11]: [12345, 12346, 12347, 12348]

The ``@parallel`` decorator creates parallel functions, that break up an element-wise

operations and distribute them, reconstructing the result.

.. sourcecode:: ipython

In [12]: import numpy as np

In [13]: A = np.random.random((64,48))

In [14]: @dview.parallel(block=True)

...: def pmul(A,B):

...: return A*B

In [15]: C_local = A*A

In [16]: C_remote = pmul(A,A)

In [17]: (C_local == C_remote).all()

Out[17]: True

.. seealso::

See the docstrings for the :func:`parallel` and :func:`remote` decorators for

options.

Calling Python functions

========================

The most basic type of operation that can be performed on the engines is to

execute Python code or call Python functions. Executing Python code can be

done in blocking or non-blocking mode (non-blocking is default) using the

:meth:`.View.execute` method, and calling functions can be done via the

:meth:`.View.apply` method.

apply

-----

The main method for doing remote execution (in fact, all methods that

communicate with the engines are built on top of it), is :meth:`View.apply`.

We strive to provide the cleanest interface we can, so `apply` has the following

signature:

.. sourcecode:: python

view.apply(f, *args, **kwargs)

There are various ways to call functions with IPython, and these flags are set as

attributes of the View. The ``DirectView`` has just two of these flags:

dv.block : bool

whether to wait for the result, or return an :class:`AsyncResult` object

immediately

dv.track : bool

whether to instruct pyzmq to track when

This is primarily useful for non-copying sends of numpy arrays that you plan to

edit in-place. You need to know when it becomes safe to edit the buffer

without corrupting the message.

Creating a view is simple: index-access on a client creates a :class:`.DirectView`.

.. sourcecode:: ipython

In [4]: view = rc[1:3]

Out[4]: <DirectView [1, 2]>

In [5]: view.apply<tab>

view.apply view.apply_async view.apply_sync

For convenience, you can set block temporarily for a single call with the extra sync/async methods.

Blocking execution

------------------

In blocking mode, the :class:`.DirectView` object (called ``dview`` in

these examples) submits the command to the controller, which places the

command in the engines' queues for execution. The :meth:`apply` call then

blocks until the engines are done executing the command:

.. sourcecode:: ipython

In [2]: dview = rc[:] # A DirectView of all engines

In [3]: dview.block=True

In [4]: dview['a'] = 5

In [5]: dview['b'] = 10

In [6]: dview.apply(lambda x: a+b+x, 27)

Out[6]: [42, 42, 42, 42]

You can also select blocking execution on a call-by-call basis with the :meth:`apply_sync`

method:

In [7]: dview.block=False

In [8]: dview.apply_sync(lambda x: a+b+x, 27)

Out[8]: [42, 42, 42, 42]

Python commands can be executed as strings on specific engines by using a View's ``execute``

method:

.. sourcecode:: ipython

In [6]: rc[::2].execute('c=a+b')

In [7]: rc[1::2].execute('c=a-b')

In [8]: dview['c'] # shorthand for dview.pull('c', block=True)

Out[8]: [15, -5, 15, -5]

Non-blocking execution

----------------------

In non-blocking mode, :meth:`apply` submits the command to be executed and

then returns a :class:`AsyncResult` object immediately. The

:class:`AsyncResult` object gives you a way of getting a result at a later

time through its :meth:`get` method.

.. Note::

The :class:`AsyncResult` object provides a superset of the interface in

:py:class:`multiprocessing.pool.AsyncResult`. See the

`official Python documentation <http://docs.python.org/library/multiprocessing#multiprocessing.pool.AsyncResult>`_

for more.

This allows you to quickly submit long running commands without blocking your

local Python/IPython session:

.. sourcecode:: ipython

# define our function

In [6]: def wait(t):

...: import time

...: tic = time.time()

...: time.sleep(t)

...: return time.time()-tic

# In non-blocking mode

In [7]: ar = dview.apply_async(wait, 2)

# Now block for the result

In [8]: ar.get()

Out[8]: [2.0006198883056641, 1.9997570514678955, 1.9996809959411621, 2.0003249645233154]

# Again in non-blocking mode

In [9]: ar = dview.apply_async(wait, 10)

# Poll to see if the result is ready

In [10]: ar.ready()

Out[10]: False

# ask for the result, but wait a maximum of 1 second:

In [45]: ar.get(1)

---------------------------------------------------------------------------

TimeoutError Traceback (most recent call last)

/home/you/<ipython-input-45-7cd858bbb8e0> in <module>()

----> 1 ar.get(1)

/path/to/site-packages/IPython/parallel/asyncresult.pyc in get(self, timeout)

62 raise self._exception

63 else:

---> 64 raise error.TimeoutError("Result not ready.")

65

66 def ready(self):

TimeoutError: Result not ready.

.. Note::

Note the import inside the function. This is a common model, to ensure

that the appropriate modules are imported where the task is run. You can

also manually import modules into the engine(s) namespace(s) via

:meth:`view.execute('import numpy')`.

Often, it is desirable to wait until a set of :class:`AsyncResult` objects

are done. For this, there is a the method :meth:`wait`. This method takes a

tuple of :class:`AsyncResult` objects (or `msg_ids` or indices to the client's History),

and blocks until all of the associated results are ready:

.. sourcecode:: ipython

In [72]: dview.block=False

# A trivial list of AsyncResults objects

In [73]: pr_list = [dview.apply_async(wait, 3) for i in range(10)]

# Wait until all of them are done

In [74]: dview.wait(pr_list)

# Then, their results are ready using get() or the `.r` attribute

In [75]: pr_list[0].get()

Out[75]: [2.9982571601867676, 2.9982588291168213, 2.9987530708312988, 2.9990990161895752]

The ``block`` and ``targets`` keyword arguments and attributes

--------------------------------------------------------------

Most DirectView methods (excluding :meth:`apply` and :meth:`map`) accept ``block`` and

``targets`` as keyword arguments. As we have seen above, these keyword arguments control the

blocking mode and which engines the command is applied to. The :class:`View` class also has

:attr:`block` and :attr:`targets` attributes that control the default behavior when the keyword

arguments are not provided. Thus the following logic is used for :attr:`block` and :attr:`targets`:

* If no keyword argument is provided, the instance attributes are used.

* Keyword argument, if provided override the instance attributes for

the duration of a single call.

The following examples demonstrate how to use the instance attributes:

.. sourcecode:: ipython

In [16]: dview.targets = [0,2]

In [17]: dview.block = False

In [18]: ar = dview.apply(lambda : 10)

In [19]: ar.get()

Out[19]: [10, 10]

In [16]: dview.targets = v.client.ids # all engines (4)

In [21]: dview.block = True

In [22]: dview.apply(lambda : 42)

Out[22]: [42, 42, 42, 42]

The :attr:`block` and :attr:`targets` instance attributes of the

:class:`.DirectView` also determine the behavior of the parallel magic commands.

Parallel magic commands

-----------------------

.. warning::

The magics have not been changed to work with the zeromq system. The

magics do work, but *do not* print stdin/out like they used to in IPython.kernel.

We provide a few IPython magic commands (``%px``, ``%autopx`` and ``%result``)

that make it more pleasant to execute Python commands on the engines

interactively. These are simply shortcuts to :meth:`execute` and

:meth:`get_result` of the :class:`DirectView`. The ``%px`` magic executes a single

Python command on the engines specified by the :attr:`targets` attribute of the

:class:`DirectView` instance:

.. sourcecode:: ipython

# load the parallel magic extension:

In [21]: %load_ext parallelmagic

# Create a DirectView for all targets

In [22]: dv = rc[:]

# Make this DirectView active for parallel magic commands

In [23]: dv.activate()

In [24]: dv.block=True

In [25]: import numpy

In [26]: %px import numpy

Parallel execution on engines: [0, 1, 2, 3]

In [27]: %px a = numpy.random.rand(2,2)

Parallel execution on engines: [0, 1, 2, 3]

In [28]: %px ev = numpy.linalg.eigvals(a)

Parallel execution on engines: [0, 1, 2, 3]

In [28]: dv['ev']

Out[28]: [ array([ 1.09522024, -0.09645227]),

array([ 1.21435496, -0.35546712]),

array([ 0.72180653, 0.07133042]),

array([ 1.46384341e+00, 1.04353244e-04])

]

The ``%result`` magic gets the most recent result, or takes an argument

specifying the index of the result to be requested. It is simply a shortcut to the

:meth:`get_result` method:

.. sourcecode:: ipython

In [29]: dv.apply_async(lambda : ev)

In [30]: %result

Out[30]: [ [ 1.28167017 0.14197338],

[-0.14093616 1.27877273],

[-0.37023573 1.06779409],

[ 0.83664764 -0.25602658] ]

The ``%autopx`` magic switches to a mode where everything you type is executed

on the engines given by the :attr:`targets` attribute:

.. sourcecode:: ipython

In [30]: dv.block=False

In [31]: %autopx

Auto Parallel Enabled

Type %autopx to disable

In [32]: max_evals = []

<IPython.parallel.AsyncResult object at 0x17b8a70>

In [33]: for i in range(100):

....: a = numpy.random.rand(10,10)

....: a = a+a.transpose()

....: evals = numpy.linalg.eigvals(a)

....: max_evals.append(evals[0].real)

....:

<IPython.parallel.AsyncResult object at 0x17af8f0>

In [34]: %autopx

Auto Parallel Disabled

In [35]: dv.block=True

In [36]: px ans= "Average max eigenvalue is: %f"%(sum(max_evals)/len(max_evals))

Parallel execution on engines: [0, 1, 2, 3]

In [37]: dv['ans']

Out[37]: [ 'Average max eigenvalue is: 10.1387247332',

'Average max eigenvalue is: 10.2076902286',

'Average max eigenvalue is: 10.1891484655',

'Average max eigenvalue is: 10.1158837784',]

Moving Python objects around

============================

In addition to calling functions and executing code on engines, you can

transfer Python objects to and from your IPython session and the engines. In

IPython, these operations are called :meth:`push` (sending an object to the

engines) and :meth:`pull` (getting an object from the engines).

Basic push and pull

-------------------

Here are some examples of how you use :meth:`push` and :meth:`pull`:

.. sourcecode:: ipython

In [38]: dview.push(dict(a=1.03234,b=3453))

Out[38]: [None,None,None,None]

In [39]: dview.pull('a')

Out[39]: [ 1.03234, 1.03234, 1.03234, 1.03234]

In [40]: dview.pull('b', targets=0)

Out[40]: 3453

In [41]: dview.pull(('a','b'))

Out[41]: [ [1.03234, 3453], [1.03234, 3453], [1.03234, 3453], [1.03234, 3453] ]

In [43]: dview.push(dict(c='speed'))

Out[43]: [None,None,None,None]

In non-blocking mode :meth:`push` and :meth:`pull` also return

:class:`AsyncResult` objects:

.. sourcecode:: ipython

In [48]: ar = dview.pull('a', block=False)

In [49]: ar.get()

Out[49]: [1.03234, 1.03234, 1.03234, 1.03234]

Dictionary interface

--------------------

Since a Python namespace is just a :class:`dict`, :class:`DirectView` objects provide

dictionary-style access by key and methods such as :meth:`get` and

:meth:`update` for convenience. This make the remote namespaces of the engines

appear as a local dictionary. Underneath, these methods call :meth:`apply`:

.. sourcecode:: ipython

In [51]: dview['a']=['foo','bar']

In [52]: dview['a']

Out[52]: [ ['foo', 'bar'], ['foo', 'bar'], ['foo', 'bar'], ['foo', 'bar'] ]

Scatter and gather

------------------

Sometimes it is useful to partition a sequence and push the partitions to

different engines. In MPI language, this is know as scatter/gather and we

follow that terminology. However, it is important to remember that in

IPython's :class:`Client` class, :meth:`scatter` is from the

interactive IPython session to the engines and :meth:`gather` is from the

engines back to the interactive IPython session. For scatter/gather operations

between engines, MPI should be used:

.. sourcecode:: ipython

In [58]: dview.scatter('a',range(16))

Out[58]: [None,None,None,None]

In [59]: dview['a']

Out[59]: [ [0, 1, 2, 3], [4, 5, 6, 7], [8, 9, 10, 11], [12, 13, 14, 15] ]

In [60]: dview.gather('a')

Out[60]: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15]

Other things to look at

=======================

How to do parallel list comprehensions

--------------------------------------

In many cases list comprehensions are nicer than using the map function. While

we don't have fully parallel list comprehensions, it is simple to get the

basic effect using :meth:`scatter` and :meth:`gather`:

.. sourcecode:: ipython

In [66]: dview.scatter('x',range(64))

In [67]: %px y = [i**10 for i in x]

Parallel execution on engines: [0, 1, 2, 3]

Out[67]:

In [68]: y = dview.gather('y')

In [69]: print y

[0, 1, 1024, 59049, 1048576, 9765625, 60466176, 282475249, 1073741824,...]

Remote imports

--------------

Sometimes you will want to import packages both in your interactive session

and on your remote engines. This can be done with the :class:`ContextManager`

created by a DirectView's :meth:`sync_imports` method:

.. sourcecode:: ipython

In [69]: with dview.sync_imports():

...: import numpy

importing numpy on engine(s)

Any imports made inside the block will also be performed on the view's engines.

sync_imports also takes a `local` boolean flag that defaults to True, which specifies

whether the local imports should also be performed. However, support for `local=False`

has not been implemented, so only packages that can be imported locally will work

this way.

You can also specify imports via the ``@require`` decorator. This is a decorator

designed for use in Dependencies, but can be used to handle remote imports as well.

Modules or module names passed to ``@require`` will be imported before the decorated

function is called. If they cannot be imported, the decorated function will never

execution, and will fail with an UnmetDependencyError.

.. sourcecode:: ipython

In [69]: from IPython.parallel import require

In [70]: @requre('re'):

...: def findall(pat, x):

...: # re is guaranteed to be available

...: return re.findall(pat, x)

# you can also pass modules themselves, that you already have locally:

In [71]: @requre(time):

...: def wait(t):

...: time.sleep(t)

...: return t

Parallel exceptions

-------------------

In the multiengine interface, parallel commands can raise Python exceptions,

just like serial commands. But, it is a little subtle, because a single

parallel command can actually raise multiple exceptions (one for each engine

the command was run on). To express this idea, we have a

:exc:`CompositeError` exception class that will be raised in most cases. The

:exc:`CompositeError` class is a special type of exception that wraps one or

more other types of exceptions. Here is how it works:

.. sourcecode:: ipython

In [76]: dview.block=True

In [77]: dview.execute('1/0')

---------------------------------------------------------------------------

CompositeError Traceback (most recent call last)

/home/you/<ipython-input-10-15c2c22dec39> in <module>()

----> 1 dview.execute('1/0', block=True)

/path/to/site-packages/IPython/parallel/view.py in execute(self, code, block)

460 default: self.block

461 """

--> 462 return self.apply_with_flags(util._execute, args=(code,), block=block)

463

464 def run(self, filename, block=None):

/home/you/<string> in apply_with_flags(self, f, args, kwargs, block, track)

/path/to/site-packages/IPython/parallel/view.py in sync_results(f, self, *args, **kwargs)

46 def sync_results(f, self, *args, **kwargs):

47 """sync relevant results from self.client to our results attribute."""

---> 48 ret = f(self, *args, **kwargs)

49 delta = self.outstanding.difference(self.client.outstanding)

50 completed = self.outstanding.intersection(delta)

/home/you/<string> in apply_with_flags(self, f, args, kwargs, block, track)

/path/to/site-packages/IPython/parallel/view.py in save_ids(f, self, *args, **kwargs)

35 n_previous = len(self.client.history)

36 try:

---> 37 ret = f(self, *args, **kwargs)

38 finally:

39 nmsgs = len(self.client.history) - n_previous

/path/to/site-packages/IPython/parallel/view.py in apply_with_flags(self, f, args, kwargs, block, track)

398 if block:

399 try:

--> 400 return ar.get()

401 except KeyboardInterrupt:

402 pass

/path/to/site-packages/IPython/parallel/asyncresult.pyc in get(self, timeout)

87 return self._result

88 else:

---> 89 raise self._exception

90 else:

91 raise error.TimeoutError("Result not ready.")

CompositeError: one or more exceptions from call to method: _execute

[0:apply]: ZeroDivisionError: integer division or modulo by zero

[1:apply]: ZeroDivisionError: integer division or modulo by zero

[2:apply]: ZeroDivisionError: integer division or modulo by zero

[3:apply]: ZeroDivisionError: integer division or modulo by zero

Notice how the error message printed when :exc:`CompositeError` is raised has

information about the individual exceptions that were raised on each engine.

If you want, you can even raise one of these original exceptions:

.. sourcecode:: ipython

In [80]: try:

....: dview.execute('1/0')

....: except client.CompositeError, e:

....: e.raise_exception()

....:

---------------------------------------------------------------------------

ZeroDivisionError Traceback (most recent call last)

/ipython1-client-r3021/docs/examples/<ipython console> in <module>()

/ipython1-client-r3021/ipython1/kernel/error.pyc in raise_exception(self, excid)

156 raise IndexError("an exception with index %i does not exist"%excid)

157 else:

--> 158 raise et, ev, etb

159

160 def collect_exceptions(rlist, method):

ZeroDivisionError: integer division or modulo by zero

If you are working in IPython, you can simple type ``%debug`` after one of

these :exc:`CompositeError` exceptions is raised, and inspect the exception

instance:

.. sourcecode:: ipython

In [81]: dview.execute('1/0')

---------------------------------------------------------------------------

CompositeError Traceback (most recent call last)

/home/you/<ipython-input-10-15c2c22dec39> in <module>()

----> 1 dview.execute('1/0', block=True)

/path/to/site-packages/IPython/parallel/view.py in execute(self, code, block)

460 default: self.block

461 """

--> 462 return self.apply_with_flags(util._execute, args=(code,), block=block)

463

464 def run(self, filename, block=None):

/home/you/<string> in apply_with_flags(self, f, args, kwargs, block, track)

/path/to/site-packages/IPython/parallel/view.py in sync_results(f, self, *args, **kwargs)

46 def sync_results(f, self, *args, **kwargs):

47 """sync relevant results from self.client to our results attribute."""

---> 48 ret = f(self, *args, **kwargs)

49 delta = self.outstanding.difference(self.client.outstanding)

50 completed = self.outstanding.intersection(delta)

/home/you/<string> in apply_with_flags(self, f, args, kwargs, block, track)

/path/to/site-packages/IPython/parallel/view.py in save_ids(f, self, *args, **kwargs)

35 n_previous = len(self.client.history)

36 try:

---> 37 ret = f(self, *args, **kwargs)

38 finally:

39 nmsgs = len(self.client.history) - n_previous

/path/to/site-packages/IPython/parallel/view.py in apply_with_flags(self, f, args, kwargs, block, track)

398 if block:

399 try:

--> 400 return ar.get()

401 except KeyboardInterrupt:

402 pass

/path/to/site-packages/IPython/parallel/asyncresult.pyc in get(self, timeout)

87 return self._result

88 else:

---> 89 raise self._exception

90 else:

91 raise error.TimeoutError("Result not ready.")

CompositeError: one or more exceptions from call to method: _execute

[0:apply]: ZeroDivisionError: integer division or modulo by zero

[1:apply]: ZeroDivisionError: integer division or modulo by zero

[2:apply]: ZeroDivisionError: integer division or modulo by zero

[3:apply]: ZeroDivisionError: integer division or modulo by zero

In [82]: %debug

> /path/to/site-packages/IPython/parallel/asyncresult.py(80)get()

79 else:

---> 80 raise self._exception

81 else:

# With the debugger running, e is the exceptions instance. We can tab complete

# on it and see the extra methods that are available.

ipdb> e.

e.__class__ e.__getitem__ e.__new__ e.__setstate__ e.args

e.__delattr__ e.__getslice__ e.__reduce__ e.__str__ e.elist

e.__dict__ e.__hash__ e.__reduce_ex__ e.__weakref__ e.message

e.__doc__ e.__init__ e.__repr__ e._get_engine_str e.print_tracebacks

e.__getattribute__ e.__module__ e.__setattr__ e._get_traceback e.raise_exception

ipdb> e.print_tracebacks()

[0:apply]:

Traceback (most recent call last):

File "/path/to/site-packages/IPython/parallel/streamkernel.py", line 332, in apply_request

exec code in working, working