Re: [Boost-users] cpp_rational memory bloat with repeated construction and destruction

Subject: Re: [Boost-users] cpp_rational memory bloat with repeated construction and destruction
From: John Maddock (boost.regex_at_[hidden])
Date: 2014-03-14 04:31:17

> 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?

Good question, cpp_rational is just a pair of cpp_int's, and cpp_int's 
either:

* Allocate no memory if the digit count is small enough then no memory 
allocation is performed.  Exactly where the cutoff occurs depends on the 
platform, but should be at least 1024 bits.  You can also control this 
cutoff via the template parameters.
* Otherwise allocation use std::allocator, you can obviously provide 
your own allocator if you think you can do better ;-)

So if memory fragmentation is the issue, then it sounds like you need to 
tweak the template parameters to cpp_int and then compose your own 
rational type from that:

typedef cpp_int_backend<MAX_BITS_BEFORE_ALLOC> my_int_backend;
typedef rational_adaptor<my_int_backend > my_rational_backend;
typedef number<my_rational_backend> my_rational;

You could also give gmp_rational a try, but I would expect even more 
memory allocation and fragmentation I'm afraid :-(

HTH, John.