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Blitz Support : |
From: Sam Kaplan (skaplan_at_[hidden])
Date: 2003-04-11 16:00:11
Hello,
I have a question concerning sum(...) vs. looping for a matrix-matrix
product. It appears that looping is faster.
In particular, please consider the following two code fragments:
1) using blitz++, sum(...):
...
Array<double,2> A(m,m);
Array<double,2> B(m,m);
Array<double,2> C(m,m);
...
firstIndex i;
secondIndex j;
thirdIndex k;
C = sum(A(i,k)*B(k,j),k);
...
2) looping:
...
Array<double,2> A(m,m);
Array<double,2> B(m,m);
Array<double,2> C(m,m);
...
int i,j,k;
for (i = 0; i < m; i++){
for (j = 0; j < m; j++){
for (k = 0; k < m; k++){
C(i,j) += A(i,k)*B(k,j);
}
}
}
...
I've attached some code for benchmarking these matrix-matrix
multiplications. One result is:
with m = 1000:
blitz with sum(...): 676.89 seconds
blitz with looping: 554.84 seconds
C style arrays: 554.57 seconds
The code was compiled with: g++ -ftemplate-depth-30 -O3
I quite like the condensed notation using sum(...). However, the speed
issue is bothersome. Have other people noticed this. If so, how do you
deal with this problem?
Thanks.
Sam