# Speed of elementary operations on different grids

This code is compiled using either the default tree implementation or the ‘multigrid’ regular Cartesian grid implementation.

We use a square, regular, Cartesian grid and vary the resolution from 162 to 20482 (to check for the influence of memory caching).

#include "utils.h"

scalar a[], b[];

int main (int argc, char * argv[])
{
int start, end;
if (argc == 1) {
start = 4; end = 11;
}
else
start = end = atoi(argv);
for (int l = start; l <= end; l++) {
init_grid (1 << l);
int nloops, i;
clock_t start, end;

We fill a with a simple function.

    foreach()
a[] = cos(2.*pi*x)*sin(2.*pi*y);

We set a number of loops proportional to the number of grid points so that times are comparable on all grids.

    nloops = i = (1 << 25) >> 2*l;

Here we compute \displaystyle b = \nabla^2 a using a 5-points Laplacian operator.

    start = clock();
while (i--)
foreach()
b[] = (a[0,1] + a[1,0] + a[0,-1] + a[-1,0] - 4.*a[])/sq(Delta);
end = clock();
printf ("lap %d %g\n", l,
1e9*(end - start)/(double)CLOCKS_PER_SEC/(nloops*(1 << 2*l)));

Something simpler: the sum of a over the entire mesh.

    nloops = i = (1 << 25) >> 2*l;
double sum = 0.;
start = clock();
while (i--)
foreach()
sum += a[];
end = clock();
printf ("sum %d %g %g\n", l,
1e9*(end - start)/(double)CLOCKS_PER_SEC/(nloops*(1 << 2*l)), sum);

And finally the restriction operator.

    nloops = i = (1 << 25) >> 2*l;
start = clock();
while (i--)
restriction ({b});
end = clock();
printf ("res %d %g %g\n", l,
1e9*(end - start)/(double)CLOCKS_PER_SEC/(nloops*(1 << 2*l)), sum);
}
}

## Results

This graph shows the speed of the tree implementation for each operation relative to the speed on the Cartesian mesh. As expected, the overhead is relatively larger for the simpler operations (e.g. sum of all elements). This graph is quite sensitive to the exact machine architecture (cache hierarchy etc…).

set xlabel 'Level'
set ylabel 'Cartesian speed / Quadtree speed'
set key center right
plot '< paste out cout | grep lap' u 2:($3/$6) w lp t '5-points Laplacian', \
'< paste out cout | grep sum' u 2:($3/$7) w lp t 'Sum', \
'< paste out cout | grep res' u 2:($3/$7) w lp t 'Restriction' Relative speed of simple operations on a tree mesh (script)

The absolute speed for the Laplacian operator on both grid implementations is shown below. Note that Cartesian meshes are fast! (i.e. hundreds of million of grid points per second).

set ylabel 'nanoseconds per grid point'
set yrange [0:]
plot '< grep lap out' u 2:3 w lp t 'Quadtree', \
'< grep lap cout' u 2:3 w lp t 'Cartesian' Absolute speed of the 5-points Laplacian on Cartesian and tree meshes (script)