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.. _faq:
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========================================
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Frequently asked questions
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========================================
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General questions
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=================
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Questions about parallel computing with IPython
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================================================
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Will IPython speed my Python code up?
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Yes and no. When converting a serial code to run in parallel, there often many
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difficulty questions that need to be answered, such as:
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* How should data be decomposed onto the set of processors?
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* What are the data movement patterns?
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* Can the algorithm be structured to minimize data movement?
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* Is dynamic load balancing important?
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We can't answer such questions for you. This is the hard (but fun) work of parallel
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computing. But, once you understand these things IPython will make it easier for you to
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implement a good solution quickly. Most importantly, you will be able to use the
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resulting parallel code interactively.
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With that said, if your problem is trivial to parallelize, IPython has a number of
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different interfaces that will enable you to parallelize things is almost no time at
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all. A good place to start is the ``map`` method of our `multiengine interface`_.
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.. _multiengine interface: ./parallel_multiengine
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What is the best way to use MPI from Python?
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--------------------------------------------
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What about all the other parallel computing packages in Python?
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---------------------------------------------------------------
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Some of the unique characteristic of IPython are:
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* IPython is the only architecture that abstracts out the notion of a
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parallel computation in such a way that new models of parallel computing
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can be explored quickly and easily. If you don't like the models we
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provide, you can simply create your own using the capabilities we provide.
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* IPython is asynchronous from the ground up (we use `Twisted`_).
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* IPython's architecture is designed to avoid subtle problems
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that emerge because of Python's global interpreter lock (GIL).
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* While IPython'1 architecture is designed to support a wide range
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of novel parallel computing models, it is fully interoperable with
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traditional MPI applications.
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* IPython has been used and tested extensively on modern supercomputers.
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* IPython's networking layers are completely modular. Thus, is
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straightforward to replace our existing network protocols with
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high performance alternatives (ones based upon Myranet/Infiniband).
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* IPython is designed from the ground up to support collaborative
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parallel computing. This enables multiple users to actively develop
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and run the *same* parallel computation.
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* Interactivity is a central goal for us. While IPython does not have
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to be used interactivly, is can be.
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.. _Twisted: http://www.twistedmatrix.com
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Why The IPython controller a bottleneck in my parallel calculation?
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-------------------------------------------------------------------
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A golden rule in parallel computing is that you should only move data around if you
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absolutely need to. The main reason that the controller becomes a bottleneck is that
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too much data is being pushed and pulled to and from the engines. If your algorithm
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is structured in this way, you really should think about alternative ways of
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handling the data movement. Here are some ideas:
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1. Have the engines write data to files on the locals disks of the engines.
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2. Have the engines write data to files on a file system that is shared by
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the engines.
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3. Have the engines write data to a database that is shared by the engines.
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4. Simply keep data in the persistent memory of the engines and move the
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computation to the data (rather than the data to the computation).
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5. See if you can pass data directly between engines using MPI.
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Isn't Python slow to be used for high-performance parallel computing?
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---------------------------------------------------------------------
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