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annotate doc/v2_planning/architecture.txt @ 1239:470beb000694

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author Yoshua Bengio <bengioy@iro.umontreal.ca>
date Thu, 23 Sep 2010 11:49:42 -0400
parents 46527ae6db53
children b9d0a326e3e7
rev   line source
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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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5
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6 SE + VM Approach
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7 =================
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8
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9 One avenue for the basic design of the library is to follow the Symbolic
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10 Expression (SE) structure + virtual machine (VM) pattern that worked for Theano.
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11
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12 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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16
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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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19
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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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29
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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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59
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60 YB: I agree that lambdas are more flexible, but from the user's point of
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61 view it is really important to know what can swap with what, so that they
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62 can easily plug-and-play. So even if informal, something in the spirit
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63 of an API must be described somewhere, and components should declare
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64 either formally or through comments what functionality 'type'
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65 they can take on.
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66
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67 Encapsulation vs. linearity
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68 ---------------------------
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69
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70 A while ago, the Apstat crew went to fight "encapsulation" to propose instead
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71 a more "linearized" approach to experiment design. I must admit I didn't
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72 really understand the deep motivations behind this, and after practicing both
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73 styles (encapsulation for PLearn / Theano, linearity @ ARL / Ubisoft), I still
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74 don't. I do find, however, some not-so-deep-but-still-significant advantages
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75 to the linear version, which hopefully can be made clear (along with a
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76 clarification of what the h*** am I talking about) in the following example:
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77
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78 * Linear version:
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79
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80 .. code-block:: python
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81
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82 my_experiment = pipeline([
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83 data,
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84 filter_samples,
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85 PCA,
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86 k_fold_split,
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87 neural_net,
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88 evaluation,
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89 ])
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90
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91 * Encapsulated version:
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92
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93 .. code-block:: python
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94
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95 my_experiment = evaluation(
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96 data=PCA(filter_samples(data)),
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97 split=k_fold_split,
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98 model=neural_net)
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99
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100 What I like in the linear version is it is much more easily human-readable
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101 (once you know what it means): you just follow the flow of the experiment by
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102 reading through a single list.
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103 On the other hand, the encapsulated version requires some deeper analysis to
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104 understand what is going on and in which order.
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105 Also, commenting out parts of the processing is simpler in the first case (it
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106 takes a single # in front of an element).
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107 However, linearity tends to break when the experiment is actually not linear,
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108 i.e. the graph of object dependencies is more complex (*).
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109
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110 I'm just bringing this up because it may be nice to be able to provide the
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111 user with the most intuitive way to design experiments. I actually don't think
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112 those approaches are mutually exclusive, and it could be possible for the
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113 underlying system to use the more flexible / powerful encapsulated
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114 representation, while having the option to write simple scripts in a form that
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115 is easier to understand and manipulate.
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116
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117 It could also be worth discussing this issue with Xavier / Christian /
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118 Nicolas.
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119
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120 (*) Note that I cheated a bit in my example above: the graph from the
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121 encapsulated version is not a simple chain, so it is not obvious how to
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122 convert it into the pipeline given in the linear version. It's still possible
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123 though, but this is probably not the place to get into the details.
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124
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125 RP comment : The way I see it, you could always have everything using the
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126 encapsulation paradigm ( which as you pointed out is a bit more powerful) and
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127 then have linear shortcuts ( functions that take a list of functions and some
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128 inputs and apply them in some order). You will not be able to have a one case
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129 cover all pipeline function, but I think it is sufficient to offer such
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130 options (linear functions) for a few widely used cases ..
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131
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132
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133 Jobman Compatibility Approach
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134 =============================
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135
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136 One basic approach for the library is to provide a set of components that are
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137 compatible with remote execution. The emphasis could be not so much on
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138 standardizing the roles and APIs of components, so much as ensuring that they
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139 can be glued together and supports parallel execution on one or more CPUs or
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140 clusters.
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141
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142 In this approach we would provide a proxy for asynchronous execution
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143 (e.g. "pylearn.call(fn, args, kwargs, backend=default_backend)"), which would
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144 come with constraints on what fn, args, and kwargs can be. Specifically, they
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145 must be picklable, and there are benefits (e.g. automatic function call caching)
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146 associated with them being hashable as well.
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147
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148
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149 Benchmark
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150 =========
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151
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152 During the general meeting on sept. 17th, we agreed to produce at least pseudo-code (if
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153 possible, actual code) for the following model:
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154 A Deep Belief Net (with greedy layerwise pre-training, and supervised
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155 fine-tuning), with preprocessing of the data, double cross-validation, and
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156 save/load of the model.
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157
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158 The different approach to be tested are:
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159 - Plugins with a global scheduler driving the experiment (Razvan's team)
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160 - Objects, with basic hooks at predefined places (Pascal L.'s team)
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161 - Existing objects and code (including dbi and Jobman), with some more
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162 pieces to tie things together (Fred B.)
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163
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164 OD comments: We were in a hurry to close the meeting and I did not have time
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165 to really explain what I meant when I suggested we should add the requirement
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166 of saving the final "best" model. What I had in mind is a typical "applied ML"
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167 experiment, i.e. the following approach that hopefully can be understood just
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168 by writing it down in the form of a processing pipeline. The double cross
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169 validation step, whose goal is to obtain an estimate of the generalization
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170 error of our final model, is:
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171 data -> k_fold_outer(preprocessing -> k_fold_inner(dbn -> evaluate) -> select_best -> retrain_on_all_data -> evaluate) -> evaluate
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172 Once this is done, the model we want to save is obtained by doing
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173 data -> preprocessing -> k_fold(dbn -> evaluate) -> select_best -> retrain_on_all_data
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174 and we save
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175 preprocessing -> best_model_selected