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Subject: [Boostusers] cpp_rational memory bloat with repeated construction and destruction
From: W Randolph Franklin (boost_at_[hidden])
Date: 20140313 17:14:15
Hi,
My application, overlay, constructs and destroys millions of
cpp_rational variables. As overlay executes, its memory continually
grows. My variables don't have that many digits, but each time that
they are constructed or destroyed, they are slightly wider or narrower.
W/o studying the cpp_rational source, I surmise that it uses a heap
that gets badly fragmented. Apart from the space, each single
allocation is probably taking ever more time. That's annoying
because overlay otherwise would take linear time.
What should I do? Are there easy ways to control this problem? Are
other rational implementations better? Boost gives me a choice of
several. Are there better implementations outside boost?

Not that it's relevant, but FYI, my application is to overlay two
planar graphs, intersecting all the edges to find the output faces.
There might be 100,000 edges. Roundoff error is a serious problem
that causes topological impossibilities. The idea is to use rationals
to prevent roundoff.
Upon reading each input float, I use a continued fraction
approximation to convert it to the closest rational justified by the
number of significant digits. That entails constructing and
destroying variables of varying widths as the approximation converges.
This is probably what is thrashing the memory. However if I don't do
that, the values start by having perhaps 20 digits, which doubles each
time as line equations are computed and lines intersected.
Perhaps I could restructure my code to avoid that. However IMHO, it
is quite well structured now, and making it ugly to be fast would be a
pity.
Help?
Thanks.
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