Hi,
I assume that someone here could maybe help me, and I'm hoping it's
not too much off topic.
I have 2 arrays of 2d point coordinates and would like to calculate
all pairwise distances as fast as possible.
Going from Python/Numpy to a (Swigged) C extension already gave me a
55x speedup.
(.9ms vs. 50ms for arrays of length 329 and 340).
I'm using gcc on Linux.
Now I'm wondering if I could go even faster !?
My hope that the compiler might automagically do some SSE2
optimization got disappointed.
Since I have a 4 core CPU I thought OpenMP might be an option;
I never used that, and after some playing around I managed to get
(only) 50% slowdown(!) :-(
My code in short is this:
(My SWIG typemaps use obj_to_array_no_conversion() from numpy.i)
-------<Ccode> ----------
void dists2d(
double *a_ps, int nx1, int na,
double *b_ps, int nx2, int nb,
double *dist, int nx3, int ny3) throw (char*)
{
if(nx1 != 2) throw (char*) "a must be of shape (n,2)";
if(nx2 != 2) throw (char*) "b must be of shape (n,2)";
if(nx3 != nb || ny3 != na) throw (char*) "c must be of shape (na,nb)";
double *dist_;
int i, j;
#pragma omp parallel private(dist_, j, i)
{
#pragma omp for nowait
for(i=0;i<na;i++)
{
//num_threads=omp_get_num_threads(); --> 4
dist_ = dist+i*nb; // dists_ is only
introduced for OpenMP
for(j=0;j<nb;j++)
{
*dist_++ = sqrt( sq(a_ps[i*nx1] - b_ps[j*nx2]) +
sq(a_ps[i*nx1+1] -
b_ps[j*nx2+1]) );
}
}
}
}
-------</Ccode> ----------
There is probably a simple mistake in this code - as I said I never
used OpenMP before.
It should be not too difficult to use OpenMP correctly here
or - maybe better -
is there a simple SSE(2,3,4) version that might be even better than OpenMP... !?
I supposed, that I did not get the #pragma omp lines right - any idea ?
Or is it in general not possible to speed this kind of code up using OpenMP !?
Thanks,
Sebastian Haase
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