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From "Derek Scherger (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (MATH-1134) unsafe initialization in BicubicSplineInterpolatingFunction
Date Wed, 02 Jul 2014 03:21:25 GMT

    [ https://issues.apache.org/jira/browse/MATH-1134?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14049576#comment-14049576
] 

Derek Scherger commented on MATH-1134:
--------------------------------------

I'm all for callers having the option to not initialize for partial derivatives on construction,
as long as there is some explicit and obvious way to ensure that everything has been initialized
before unleashing a bunch of threads on the interpolator.

At the moment, it's not obvious that there is any internal state that requires initialization
before using the class, the only way to find out is to experience the problem of incomplete
initialization, and then to look at the source to see what is going on and discover that there
is lazy initialization being done in both the partialDerivative[XY...] and value methods.
The solution I've landed on for now is to make an initial partialDerivativeX(x,y) call as
the objects are constructed, but some sort of explicit init() or alternative constructor with
eager/lazy init options would be better.

I'm not particularly fond of the idea of declaring methods synchronized unless that's really
what is required to make them safe. The overhead there may be small, but that's relative and
when running on many threads doing lots of computation it adds up, and sometimes becomes a
significant problem, so if it can be avoided all the better. Similarly, recalculating the
partials on every call is not very good if they can be computed once and re-used for every
subsequent call, which seems like it could be a major performance win.



> unsafe initialization in BicubicSplineInterpolatingFunction
> -----------------------------------------------------------
>
>                 Key: MATH-1134
>                 URL: https://issues.apache.org/jira/browse/MATH-1134
>             Project: Commons Math
>          Issue Type: Bug
>    Affects Versions: 3.3
>            Reporter: Derek Scherger
>            Priority: Minor
>
> The lazy initialization of the internal array of partialDerivatives in BicubicSplineInterpolatingFunction
is not thread safe. If multiple threads call any of the partialDerivative functions concurrently
one thread may start the initialization and others will see the array is non-null and assume
it is fully initialized. If the internal array of partial derivatives was initialized in the
constructor this would not be a problem.
> i.e. the following check in partialDerivative(which, x, y)
>         if (partialDerivatives == null) {
>             computePartialDerivatives();
>         }
> will start the initialization. However in computePartialDerivatives()
>         partialDerivatives = new BivariateFunction[5][lastI][lastJ];
> makes it appear to other threads as the the initialization has completed when it may
not have.



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