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From: Mattias Flodin (flodin_at_[hidden])
Date: 2004-08-06 00:31:21

In December last year, Jim Apple asked if there was interest in a
memoization library he was working on. [1] It seems there were several
people interested in this, but I have seen no more posts about it. Was
this project discontinued? Are there other boosters working on a
library like this?

Dave Abrahams pointed out (in [2]) that a library such as this may
need the ability to invalidate parts of the cache, but was met with
confusion and I don't think the motivation for the need was widely
understood. I have had increasing need for a memoization library such
as this in our current project, so I thought I'd try to summarize it
as a kind of use case.

The project is a medical diagnosis application using in the order of a
hundred image processing step for diagnosis of digitized images. Some
of these steps can take a very significant amount of time, and many of
them may need to use intermediate results that were already computed
in other steps. I am currently obtaining the result image recursively
through a very rudimentary non-generic memoization cache class, where
recursive calls are explicitly routed through the cache object (by a
string->funptr mapping). Thanks to the caching I am now probably
cutting away a good 90% of processing time.

Although all processing-step functions now (of necessity) have the
same signature, many of them use various globally set parameters
related to the algorithm, such as threshold settings for
neural-network outputs. Some parameters are not used until late in the
process, whereas other parameters affect early steps.

To optimize the parameter values we must perform a lot of searches
through the parameter space using various parameter optimization
techniques. Currently, with every parameter change we must re-run the
entire process, even if the parameter actually only affects one of the
very last steps.

With a memoization library that knows about parameter dependencies, we
would be able to try many different values of this parameter and only
the dependent parts of the cache would be invalidated, which could
save hours if not days of processing time.


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