Perfomance slowdown with growth of operations number - numbers

Question

I have next problem. My code performance depends on number of operation! How can it be? (I use gcc v 4.3.2 under openSuse 11.1)
Here is my code:
#define N_MAX 1000000
typedef unsigned int uint;
double a[N_MAX];
double b[N_MAX];
uint n;
int main(){
for (uint i=0; i<N_MAX; i++) {
a[i]=(double)rand()/RAND_MAX;
}
for (uint n=100000; n<500000; n+=5000) {
uint time1 = time(NULL);
for (uint i=0; i<n;++i)
for (uint j=0;j<n;++j)
b[j] = a[j];
uint time2 = time(NULL);
double time = double(time2-time1);
printf("%5d ", n);
printf("%5.2f %.3f\n", time, ((double)n*n/time)/1e9);
}
return 0;
}
And here is the log of results:
n-time-Gflops (=)
200000 23.00 1.739
205000 24.00 1.751
210000 25.00 1.764
215000 26.00 1.778
220000 27.00 1.793
225000 29.00 1.746
230000 30.00 1.763
235000 32.00 1.726
240000 32.00 1.800
245000 34.00 1.765
250000 36.00 1.736
255000 37.00 1.757
260000 38.00 1.779
265000 40.00 1.756
270000 42.00 1.736
275000 44.00 1.719
280000 46.00 1.704
285000 48.00 1.692
290000 49.00 1.716
295000 51.00 1.706
300000 54.00 1.667
305000 54.00 1.723
310000 59.00 1.629
315000 61.00 1.627
320000 66.00 1.552
325000 71.00 1.488
330000 76.00 1.433
335000 79.00 1.421
340000 84.00 1.376
345000 85.00 1.400
350000 89.00 1.376
355000 96.00 1.313
360000 102.00 1.271
365000 110.00 1.211
370000 121.00 1.131
375000 143.00 0.983
380000 156.00 0.926
385000 163.00 0.909
There is also the image but I can't post it cause of new users restrictions. But here is the log plot.
What is the reason of this slowdown?
How to get rid of it? Please Help!

Answer 1

Your inner loops increase number of iterations every time - it is expected to take more time to do their job if there is more calculations to do. First time there are 100k operations to be done, second time 105k operations, and so on. It simply must take more and more time.
EDIT: To be clearer, I tried to say it looks to something that Spolsky called Shlemiel the painter's algorithm

Answer 2

Thank a lot for response!
My expectation bases on idea that operation number per time unit should remains the same. So if I increase number of operations, say twice, then computation time should increase also twice, therefore the ration of operations number and the computation time should be constant. This is something that i didn't meet. :(