645 research outputs found
Linear Convergence on Positively Homogeneous Functions of a Comparison Based Step-Size Adaptive Randomized Search: the (1+1) ES with Generalized One-fifth Success Rule
In the context of unconstraint numerical optimization, this paper
investigates the global linear convergence of a simple probabilistic
derivative-free optimization algorithm (DFO). The algorithm samples a candidate
solution from a standard multivariate normal distribution scaled by a step-size
and centered in the current solution. This solution is accepted if it has a
better objective function value than the current one. Crucial to the algorithm
is the adaptation of the step-size that is done in order to maintain a certain
probability of success. The algorithm, already proposed in the 60's, is a
generalization of the well-known Rechenberg's Evolution Strategy (ES)
with one-fifth success rule which was also proposed by Devroye under the name
compound random search or by Schumer and Steiglitz under the name step-size
adaptive random search. In addition to be derivative-free, the algorithm is
function-value-free: it exploits the objective function only through
comparisons. It belongs to the class of comparison-based step-size adaptive
randomized search (CB-SARS). For the convergence analysis, we follow the
methodology developed in a companion paper for investigating linear convergence
of CB-SARS: by exploiting invariance properties of the algorithm, we turn the
study of global linear convergence on scaling-invariant functions into the
study of the stability of an underlying normalized Markov chain (MC). We hence
prove global linear convergence by studying the stability (irreducibility,
recurrence, positivity, geometric ergodicity) of the normalized MC associated
to the -ES. More precisely, we prove that starting from any initial
solution and any step-size, linear convergence with probability one and in
expectation occurs. Our proof holds on unimodal functions that are the
composite of strictly increasing functions by positively homogeneous functions
with degree (assumed also to be continuously differentiable). This
function class includes composite of norm functions but also non-quasi convex
functions. Because of the composition by a strictly increasing function, it
includes non continuous functions. We find that a sufficient condition for
global linear convergence is the step-size increase on linear functions, a
condition typically satisfied for standard parameter choices. While introduced
more than 40 years ago, we provide here the first proof of global linear
convergence for the -ES with generalized one-fifth success rule and the
first proof of linear convergence for a CB-SARS on such a class of functions
that includes non-quasi convex and non-continuous functions. Our proof also
holds on functions where linear convergence of some CB-SARS was previously
proven, namely convex-quadratic functions (including the well-know sphere
function)
Linear Convergence of Comparison-based Step-size Adaptive Randomized Search via Stability of Markov Chains
In this paper, we consider comparison-based adaptive stochastic algorithms
for solving numerical optimisation problems. We consider a specific subclass of
algorithms that we call comparison-based step-size adaptive randomized search
(CB-SARS), where the state variables at a given iteration are a vector of the
search space and a positive parameter, the step-size, typically controlling the
overall standard deviation of the underlying search distribution.We investigate
the linear convergence of CB-SARS on\emph{scaling-invariant} objective
functions. Scaling-invariantfunctions preserve the ordering of points with
respect to their functionvalue when the points are scaled with the same
positive parameter (thescaling is done w.r.t. a fixed reference point). This
class offunctions includes norms composed with strictly increasing functions
aswell as many non quasi-convex and non-continuousfunctions. On
scaling-invariant functions, we show the existence of ahomogeneous Markov
chain, as a consequence of natural invarianceproperties of CB-SARS (essentially
scale-invariance and invariance tostrictly increasing transformation of the
objective function). We thenderive sufficient conditions for \emph{global
linear convergence} ofCB-SARS, expressed in terms of different stability
conditions of thenormalised homogeneous Markov chain (irreducibility,
positivity, Harrisrecurrence, geometric ergodicity) and thus define a general
methodologyfor proving global linear convergence of CB-SARS algorithms
onscaling-invariant functions. As a by-product we provide aconnexion between
comparison-based adaptive stochasticalgorithms and Markov chain Monte Carlo
algorithms.Comment: SIAM Journal on Optimization, Society for Industrial and Applied
Mathematics, 201
Markov Chain Analysis of Evolution Strategies on a Linear Constraint Optimization Problem
This paper analyses a -Evolution Strategy, a randomised
comparison-based adaptive search algorithm, on a simple constraint optimisation
problem. The algorithm uses resampling to handle the constraint and optimizes a
linear function with a linear constraint. Two cases are investigated: first the
case where the step-size is constant, and second the case where the step-size
is adapted using path length control. We exhibit for each case a Markov chain
whose stability analysis would allow us to deduce the divergence of the
algorithm depending on its internal parameters. We show divergence at a
constant rate when the step-size is constant. We sketch that with step-size
adaptation geometric divergence takes place. Our results complement previous
studies where stability was assumed.Comment: Amir Hussain; Zhigang Zeng; Nian Zhang. IEEE Congress on Evolutionary
Computation, Jul 2014, Beijing, Chin
Markov Chain Analysis of Cumulative Step-size Adaptation on a Linear Constrained Problem
This paper analyzes a (1, )-Evolution Strategy, a randomized
comparison-based adaptive search algorithm, optimizing a linear function with a
linear constraint. The algorithm uses resampling to handle the constraint. Two
cases are investigated: first the case where the step-size is constant, and
second the case where the step-size is adapted using cumulative step-size
adaptation. We exhibit for each case a Markov chain describing the behaviour of
the algorithm. Stability of the chain implies, by applying a law of large
numbers, either convergence or divergence of the algorithm. Divergence is the
desired behaviour. In the constant step-size case, we show stability of the
Markov chain and prove the divergence of the algorithm. In the cumulative
step-size adaptation case, we prove stability of the Markov chain in the
simplified case where the cumulation parameter equals 1, and discuss steps to
obtain similar results for the full (default) algorithm where the cumulation
parameter is smaller than 1. The stability of the Markov chain allows us to
deduce geometric divergence or convergence , depending on the dimension,
constraint angle, population size and damping parameter, at a rate that we
estimate. Our results complement previous studies where stability was assumed.Comment: Evolutionary Computation, Massachusetts Institute of Technology Press
(MIT Press): STM Titles, 201
Information-Geometric Optimization Algorithms: A Unifying Picture via Invariance Principles
We present a canonical way to turn any smooth parametric family of
probability distributions on an arbitrary search space into a
continuous-time black-box optimization method on , the
\emph{information-geometric optimization} (IGO) method. Invariance as a design
principle minimizes the number of arbitrary choices. The resulting \emph{IGO
flow} conducts the natural gradient ascent of an adaptive, time-dependent,
quantile-based transformation of the objective function. It makes no
assumptions on the objective function to be optimized.
The IGO method produces explicit IGO algorithms through time discretization.
It naturally recovers versions of known algorithms and offers a systematic way
to derive new ones. The cross-entropy method is recovered in a particular case,
and can be extended into a smoothed, parametrization-independent maximum
likelihood update (IGO-ML). For Gaussian distributions on , IGO
is related to natural evolution strategies (NES) and recovers a version of the
CMA-ES algorithm. For Bernoulli distributions on , we recover the
PBIL algorithm. From restricted Boltzmann machines, we obtain a novel algorithm
for optimization on . All these algorithms are unified under a
single information-geometric optimization framework.
Thanks to its intrinsic formulation, the IGO method achieves invariance under
reparametrization of the search space , under a change of parameters of the
probability distributions, and under increasing transformations of the
objective function.
Theory strongly suggests that IGO algorithms have minimal loss in diversity
during optimization, provided the initial diversity is high. First experiments
using restricted Boltzmann machines confirm this insight. Thus IGO seems to
provide, from information theory, an elegant way to spontaneously explore
several valleys of a fitness landscape in a single run.Comment: Final published versio
Identification of the Isotherm Function in Chromatography Using CMA-ES
This paper deals with the identification of the flux for a system of
conservation laws in the specific example of analytic chromatography. The
fundamental equations of chromatographic process are highly non linear. The
state-of-the-art Evolution Strategy, CMA-ES (the Covariance Matrix Adaptation
Evolution Strategy), is used to identify the parameters of the so-called
isotherm function. The approach was validated on different configurations of
simulated data using either one, two or three components mixtures. CMA-ES is
then applied to real data cases and its results are compared to those of a
gradient-based strategy
COCO: The Experimental Procedure
We present a budget-free experimental setup and procedure for benchmarking
numericaloptimization algorithms in a black-box scenario. This procedure can be
applied with the COCO benchmarking platform. We describe initialization of and
input to the algorithm and touch upon therelevance of termination and restarts.Comment: ArXiv e-prints, arXiv:1603.0877
On the adaptation of the noise level for stochastic optimization
International audienceThis paper deals with the optimization of noisy fitness functions, where the noise level can be reduced by increasing the computational effort. We theoretically investigate the question of the control of the noise level. We analyse two different schemes for an adaptive control and prove sufficient conditions ensuring the existence of an homogeneous Markov chain, which is the first step to prove linear convergence when dealing with non-noisy fitness functions. We experimentally validate the relevance of the homogeneity criterion. Large-scale experiments conclude to the efficiency in a difficult framework
COCO: Performance Assessment
We present an any-time performance assessment for benchmarking numerical
optimization algorithms in a black-box scenario, applied within the COCO
benchmarking platform. The performance assessment is based on runtimes measured
in number of objective function evaluations to reach one or several quality
indicator target values. We argue that runtime is the only available measure
with a generic, meaningful, and quantitative interpretation. We discuss the
choice of the target values, runlength-based targets, and the aggregation of
results by using simulated restarts, averages, and empirical distribution
functions
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