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Subject: Re: [geometry] 1.57 Storing indices as values to save space.
From: Tim Finer (tim.finer_at_[hidden])
Date: 2014-11-26 14:30:37

Hi Adam,

Actually, I think what rtree is doing with the temporary vector in the
packing constructor should be considered a bug.

That problem is that the vector is created without regard to the rtree's
allocator - so even if a user gives the rtree a memory mapped file
allocator, the rtree creates a copy of the incoming data in memory,
ignoring the allocator.


On 11/26/14, 10:37 AM, Tim Finer wrote:
> Hi Adam,
> I'm trying yet another approach, heh. This time with 1.57.
> I'm going to see if I can't just store the spatial data in a memory
> mapped vector and indices into it in an in-memory rtree, as long as I
> relax my requirement for the size of the data a bit.
> What I thought I'd try is storing the smallest sized unsigned integer
> index possible (given the data) and reference a shared memory that I
> control the layout of. As a test, I set up an rtree with the value of
> std::uint32_t and passed in an IndexableGetter functor that returns an
> indexable double triplet when it receives the std::uint32_t value. I
> used the packing constructor to minimize the memory footprint. This
> all works, except, it looks like the rtree really stores a pair of an
> indexable point and a value and not just the values.
> I did some before and after memory snapshots and determined the size
> of the memory used is the same regardless if I set the value to an
> index, or if I fill the rtree with the double triplets directly. My
> expectation was that the functor would be used each time that the
> indexable is needed, because the documentation said "Important: The
> translation is done quite frequently inside the container - each time
> the rtree needs it".
> Except, that isn't really the case. What really happens is that the
> rtree caches a temporary vector of the entire size incoming data.
> pack_create.hpp, around line 155:
> std::vector<entry_type> entries;
> entries.reserve(values_count);
> Where entry_type is a std::pair<point_type, InIt>, where
> sizeof(point_type) is 24 and InIt is the incoming value iterator.
> I also tried dynamically inserting nodes too, but that used even more
> memory, although I didn't spend too much time looking at why.
> So my question is, what is the purpose of the IndexableGetter? Is
> there a way to trade off speed for space such that the rtree uses an
> IndexableGetter all the way through? Was this by design?
> Best,
> Tim

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