Re: Newbie: uBLAS slow?

From: Thomas Willhalm (yg-boost-users_at_[hidden])
Date: 2002-09-12 16:57:36

Kresimir Fresl wrote:
> 
> Thomas Willhalm wrote:
> 
>> I'm new to ublas, so it's easily possible that I overlooked something
>> obvious. The first thing I tried to calculate was the following line:
>> 
>> numerics::matrix<double> M(dimension,n);
>> numerics::matrix<double> Covariance
>>    = numerics::prod(M,numerics::trans(M))/n;
>> 
>> where dimension is 50 and n is 8000.
>> 
>> Unfortunately. it turned out that the following "stupid" code actually
>> performs better:
>> 
>> numerics::matrix<double> Covariance(dimension,dimension);
>> for (int i=0; i<dimension; ++i)
>>    for (int j=0; j<dimension; ++j) {
>>       Covariance(i,j)=0;
>>       for (int k=0; k<n; ++k)
>>          Covariance(i,j) += M(i,k)*M(j,k);
>>       Covariance(i,j) /= n;
>>    }
>> 
>> What am I doing wrong?
>> 
>> I'm using gcc 2.95.3 under Linux 2.4.18 on a mobile Pentium III, if it
>> matters.
> 
> First of all, it seems that you have fairly old version of ublas.
> New version is in namespace `boost::numeric::ublas'.
> But I don't think that this influences the performance ;o)

Seems like the reason is that I have an old version of gcc:
2.95.3 doesn't check the namespaces...
 
> You must define NDEBUG. Otherwise expression templates
> are not enabled.

That's it, thank you very much! Without -DNDEBUG I had:

ublas 5.28
loop 1.33

Now, it's:

ublas 0.58
loop 0.94

(Upgrading ublas didn't change much, but the version I had was only some
weeks old.)

Thanks again
Thomas