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v2planning - revs to requirements, added architecture
author James Bergstra <bergstrj@iro.umontreal.ca>
date Mon, 13 Sep 2010 09:42:36 -0400
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children e5306f5626d4
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1 ====================
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2 Pylearn Architecture
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3 ====================
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4
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6 Basic Design Approach
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7 =====================
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9 I propose that the basic design of the library follow the Symbolic Expression
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10 (SE) structure + virtual machine (VM) pattern that worked for Theano.
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11
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12 So the main things for the library to provide would be:
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13
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14 - a few VMs, some of which can run programs in parallel across processors,
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15 hosts, and networks [R6,R8];
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17 - MLA components as either individual Expressions (similar to Ops) or as
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18 subgraphs of SEs [R5,R7,R10,R11]
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20 - machine learning algorithms including their training and testing in the form
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21 of python functions that build SE graphs.[R1,R8].
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22
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23 This design addresses R2 (modularity) because swapping components is literally implemented by
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24 swapping subgraphs.
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25
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26 The design addresses R9 (algorithmic efficiency) because we can write
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27 Theano-style graph transformations to recognize special cases of component
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28 combinations.
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30 The design addresses R3 if we make the additional decision that the VMs (at
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31 least sometimes) cache the return value of program function calls. This cache
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32 serves as a database of experimental results, indexed by the functions that
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33 originally computed them. I think this is a very natural scheme for organizing
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34 experiment results, and ensuring experiment reproducibility [R1].
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35 At the same time, this is a clean and simple API behind which experiments can be
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36 saved using a number of database technologies.
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37
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38 APIs vs. lambda
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39 ----------------
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40
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41 Modularity in general is achieved when pieces can be substituted one for the
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42 other.
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43
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44 In an object-oriented design, modularity is achieved by agreeing on interface
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45 APIs, but in a functional design there is another possibility: the lambda.
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46
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47 In an SE these pieces are expression [applications] and the subgraphs they form.
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48 A subgraph is characterized syntactically within the program by its arguments
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49 and its return values. A lambda function allows the User to create new
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50 Expression types from arbitrary subgraphs with very few keystrokes. When a
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51 lambda is available and easy to use, there is much less pressure on the
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52 expression library to follow calling and return conventions strictly.
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53
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54 Of course, the closer are two subgraphs in terms of their inputs, outputs, and
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55 semantics, the easier it is to substitute one for the other. As library
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56 designers, we should still aim for compatibility of similar algorithms. It's
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57 just not essential to choose an API that will guarantee a match, or indeed to
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58 choose any explicit API at all.
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