Memory footprint of multi-dimensional arrays

[Fawzi Mohamed]

On 2008-04-18 18:04:17 +0200, Ben Mitchell <Arisian at gmail.com> said:

> So, I was watching my D program eat up ~1GB of ram doing something my 
> back of the envelope calculation said should only be taking a few 
> hundred MB max, and I started playing around.  Turns out that if I do:
> 
> ubyte[][] b = new ubyte[][](3, 10485760);
> 
> It takes about 30MB of main memory, as would be expected.  If, on the 
> other hand, I reverse the order of the indexing and do:
> 
> ubyte[][] b = new ubyte[][](10485760, 3);
> 
> It takes about 250MB of main memory.  This seems totally absurd to me; 
> I can only assume this is because it's an array of (potentially 
> arbitrary size) dynamic arrays, and there's ~220MB of bookkeeping 
> information being stored.   Is there any way around this?  Can I 
> somehow tell the compiler it's a symmetric array?  Can I make a dynamic 
> array of static arrays of size 3?  I can reverse the indexing if I have 
> to, but it would make the rest of my code very counter-intuitive, and 
> it just doesn't seem like it should make an order of magnitude 
> difference in memory usage.
> 
>    -  Ben

you probably want to look at the multiarray library:
http://www.dsource.org/projects/multiarray/

Fawzi