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api_optimization: Couple questions
author Olivier Delalleau <delallea@iro>
date Fri, 10 Sep 2010 10:28:34 -0400
parents 9fe0f0755b03
children 2cf3ad953bf9
rev   line source
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1 =========================
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2 Optimization for Learning
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3 =========================
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4
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5 Members: Bergstra, Lamblin, Delalleau, Glorot, Breuleux, Bordes
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6 Leader: Bergstra
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7
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9
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10 Initial Writeup by James
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11 =========================================
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12
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14
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15 Previous work - scikits, openopt, scipy provide function optimization
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16 algorithms. These are not currently GPU-enabled but may be in the future.
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17
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18
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19 IS PREVIOUS WORK SUFFICIENT?
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20 --------------------------------
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21
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22 In many cases it is (I used it for sparse coding, and it was ok).
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23
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24 These packages provide batch optimization, whereas we typically need online
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25 optimization.
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26
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27 It can be faster (to run) and more convenient (to implement) to have
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28 optimization algorithms as Theano update expressions.
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29
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30
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31 What optimization algorithms do we want/need?
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32 ---------------------------------------------
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33
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34 - sgd
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35 - sgd + momentum
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36 - sgd with annealing schedule
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37 - TONGA
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38 - James Marten's Hessian-free
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39 - Conjugate gradients, batch and (large) mini-batch [that is also what Marten's thing does]
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40
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41 Do we need anything to make batch algos work better with Pylearn things?
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42 - conjugate methods? yes
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43 - L-BFGS? maybe, when needed
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44
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48
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49 Proposal for API
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50 ================
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51
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52 Stick to the same style of API that we've used for SGD so far. I think it has
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53 worked well. It takes theano expressions as inputs and returns theano
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54 expressions as results. The caller is responsible for building those
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55 expressions into a callable function that does the minimization (and other
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56 things too maybe).
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57
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58
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59 def stochastic_gradientbased_optimization_updates(parameters, cost=None, grads=None, **kwargs):
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60 """
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61 :param parameters: list or tuple of Theano variables (typically shared vars)
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62 that we want to optimize iteratively algorithm.
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63
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64 :param cost: scalar-valued Theano variable that computes noisy estimate of
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65 cost (what are the conditions on the noise?). The cost is ignored if
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66 grads are given.
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67
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68 :param grads: list or tuple of Theano variables representing the gradients on
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69 the corresponding parameters. These default to tensor.grad(cost,
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70 parameters).
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71
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72 :param kwargs: algorithm-dependent arguments
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73
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74 :returns: a list of pairs (v, new_v) that indicate the value (new_v) each
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75 variable (v) should take in order to carry out the optimization procedure.
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76
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77 The first section of the return value list corresponds to the terms in
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78 `parameters`, and the optimization algorithm can return additional update
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79 expression afterward. This list of pairs can be passed directly to the
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80 dict() constructor to create a dictionary such that dct[v] == new_v.
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81 """
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82
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83
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84 Why not a class interface with an __init__ that takes the kwargs, and an
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85 updates() that returns the updates? It would be wrong for auxiliary shared
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86 variables to be involved in two updates, so the interface should not encourage
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87 separate methods for those two steps.
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90