CODEWITHSUNDEEP
c++arraysindexingsize-t
Giovanni Canossa
Giovanni Canossa

Reputation: 1

Code speed difference when using a random array index

Given a real number X within [0,1], after a specific binning I have to identify in what bin X falls. Given the bin size dx, I am using i = std::size_t(X/dx) , which works very well. I then look for the respective value of a given array v and set a second variable Y using double Y=v[i]. The whole code looks as follows:

double X = func();
dx=0.01;
int i = std::size_t(X/dx);
double Y = v[i];
print(Y)

This method correctly gives the expected value for the index i within the range [0, length(v)].

My main issue is not with finding the index, but using it: X is determined from an auxiliary function, and whenever I need to set Y=v[i] using the index determined above the code becomes extremely slow. Without commenting or removing any of the lines, the code becomes much faster when setting X to some random value between 0 and 1 right after its definition or by setting i to some random value between 0 and length of v after the third line.

Could anyone be able to tell why this occurs? The speed changes of a factor 1000 if not more, and since there are only additional steps in the faster method and func() is called anyway I can't understand why it should become faster.

Upvotes: 0

Views: 136

Answers (1)

huseyin tugrul buyukisik
huseyin tugrul buyukisik

Reputation: 11920

Since you have put no code in the question, there has to be a wild-guess like this:

  • You didn't sort all the X results before accessing lookup table. Processing a sorted array is faster.

  • Some of X had denormalized values which took a toll on computation time for certain CPU types including yours.

  • The dataset is too big for the L3 cache and it accessed RAM always, instead of quick cache hits that were seen in the other test.

  • Compiler was optimizing all of the expensive function calls out, but in real-world test scenario, it is not.

  • Time measurement has bugs

  • Computer is not stable in performance (like being a shared server or an antivirus intervention feeding on RAM bandwidth)

Upvotes: 1

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