On 29 April 2013 12:40, Ed Meyer <eem2...@gmail.com> wrote:
>
>
> On Mon, Apr 29, 2013 at 6:10 AM, Jordi Gutiérrez Hermoso
> <jord...@octave.org> wrote:
>>
>> On 29 April 2013 06:25, Miroslaw Kwasniak <miroslaw.kwasn...@pwr.wroc.pl>
>> wrote:
>> > it's something wrong whith sparse matrices A(n,n) when n is a multiple
>> > of 65536=2^16.
>> >
>> > Demonstration code ======================================
>> >
>> > for i=1:3;
>> > for n=i*2^16+(-1:1);
>> > A=spdiags(ones(n,1),0,n,n);
>> > t=trace(A);
>> > printf("n=%8d trace=%8d %s\n",n,t,["ERR";"ok"]((t==n)+1,:));
>> > endfor;
>> > endfor
>> >
>> > Results ======================================
>> >
>> > n= 65535 trace= 65535 ok
>> > n= 65536 trace= 0 ERR
>> > n= 65537 trace= 65537 ok
>> > n= 131071 trace= 131071 ok
>> > n= 131072 trace= 0 ERR
>> > n= 131073 trace= 131073 ok
>> > n= 196607 trace= 196607 ok
>> > n= 196608 trace= 0 ERR
>> > n= 196609 trace= 196609 ok
>>
>> Confirmed. The problem is that the numel function is limited to
>> returning octave_idx_type, which ordinarily of size 2^32, and
>> certainly is so for Debian. This makes sense, since you can only index
>> that many elements in a matrix. You're hitting the indexing limit. To
>> get 64-bit indexing, you would need to recompile all of Octave's
>> Fortran dependencies with -fdefault-integer-8.
>>
>> I'm not sure exactly what the bug is here. For instance, you can't
>> index your matrix A either, and this is checked for correctly:
>>
>> A(end)
>>
>> Perhaps the best thing to do would be to forbid creation of sparse
>> matrices where numel(A) > std::numeric_limits<int>::max().
>>
>> Your matrix is simply too large to be indexed, and this breaks
>> assumptions elsewhere in our code.
>>
>> - Jordi G. H.
>
>
> I'm confused - this is a diagonal sparse matrix so you should be able to
> trace() (or any other op)
> up to n = 2^32, not n^2 = 2^32. The limit on sparse matrices should be
> number of non-zeros < 2^32
All matrices need to be linearly indexable, and of course, this is how
they are actually stored in memory, as a single long array indexed by
a single index. Thus, the total number of indexable elements of a
matrix can't be larger than
std::numeric_limits<octave_idx_type>::max().
There could be some tricks we could do to relax this requirement for
sparse matrices, but it would require some pretty deep surgery of the
current code.
- Jordi G. H.
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