On Fri, Nov 1, 2013 at 7:31 AM, John Peterson <jwpeter...@gmail.com> wrote:
>
>
> > On Nov 1, 2013, at 5:56 AM, Roy Stogner <royst...@ices.utexas.edu>
> wrote:
> >
> >
> >> On Thu, 31 Oct 2013, John Peterson wrote:
> >>
> >> We build a std::map<> from Elem* to a unique sorted contiguous ID,
> as Metis only considers the active elements and needs
> >> some contiguous numbering. I expect that gets quite big, and maybe
> should be refactored to use a sorted
> >> std::vector<std::pair<Elem*, dof_id_type> > instead?
> >> You are quite correct about this. I ran our memory logger to compare
> the size of memory of a map<int,int> and a vector of
> >> pair<int,int>, both with 8M elements. The results were pretty striking:
> >>
> https://drive.google.com/file/d/0B9BK7pg8se_iaWFzbHlEemxmX0U/edit?usp=sharing
> >> The map's peak memory usage is almost exactly 6X that of the vector's...
> >
> > Wow, nasty. I was expecting 3X; I forgot about the sizeof(int*) ==
> > 2*sizeof(int) issue.
> >
> > It's not really relevant here, but out of curiosity, could you give
> > unordered_map a try?
>
> Sure, I had a plot with several different containers, but I forgot about
> the unordered ones. I'll make one later today.
unordered_map ends up somewhere between vector<pair> and map in terms of
memory usage.
https://drive.google.com/file/d/0B9BK7pg8se_iTFBBSTZzSm8xSTA/edit?usp=sharing
This is an interesting article on different implementations of the
unordered containers:
http://bannalia.blogspot.com/2013/10/implementation-of-c-unordered.html
Looks like the three main ones are doubly-linked list between all values,
or storing the hashes, to satisfy the complexity requirements for iterating
over the data...
--
John
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