Re: What limits the largest size of matrix?

[Peter Cloetens]

David Bateman wrote:
> Michael Creel wrote:
> > I'm using two different machines, both of which run  Ubuntu AMD64. Using
> > the command a = rand(b,b), I find that I can set b=17000 on a machine
> > that has 2GB of RAM. It goes into swap, but it works. That machine has
> > Octave 2.9.12 installed, and the CPU is AMD Turion dual core. On another
> > machine that has 8GB RAM and plenty of swap space, I get the message
> > "memory exhausted or requested size too large for range of Octave's
> > index type". That machine has 2.9.14 installed, and the CPUs are two
> > Xeons, a couple of years old. I believe that the compile options I used
> > for Octave were --enable-shared --disable-static on both machines. I
> > don't understand why the machine with more RAM is reluctant to create
> > the matrix.
> > Thanks, MIchael
>
> Octave uses "int" for indexing, unless --enable-64  is used. You can
> address 2GB of memory with the int type as only 31 bits are available.
> Now a basic operation like a(i,j) in Array.h becomes data[i + nc*j] and
> so its the product of the dimensions that count. Now the real type uses
> 8 bytes per value. and log2(17000.^2 * 8) = 31.106, and so therefore yes
> it does exceed the addressable size of a matrix and 2.9.14 detects it
> correctly. Probably the issue is in 2.9.12 rather than 2.9.14
>
> D.
>
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I tried this on a version compiled with --enable-64.
In this case I could successfully increase to
b=37660;a=rand(b,b);
Above the following error message appears:

error: memory exhausted or requested size too large for range of 
Octave's index type -- trying to return to prompt

This corresponds to 10.7 GB (the machine has physically 8 GB),
or log2(37660.^2 * 8) = 33.4

I'm not sure where this limitation comes from. Anyway not very relevant, 
probably nobody wants to work on matrices that size.

The very nice thing about the 64-bits compilation is that we can now 
handle correctly files that are larger than 2 GB. fseek and so on works :-)

Peter