Cache Poisoning Issue for deep nested loop
c++, performance
Solution
If the conditions are mostly true, your line of code is executed 3000x3000x3000x3x3 times. That's about 245 billion times. Depending on your hardware architecture 245 seconds might be a very reasonable timing (that's 1 iteration every 2 cycles - assuming 2GHz processor). In any case there isn't anything in the code that suggests cache poisoning.
Problem
I am writing a code for a mathematical method (Incomplete Cholesky) and I have hit a curious roadblock. Please see the following simplified code. ``` for(k=0;k<nosUnknowns;k++) { //Pieces of code for(i=k+1;i<nosUnknowns;i++) { // more code } for(j=k+1;j<nosUnknowns;j++) { for(i=j;i<nosUnknowns;i++) { //Some more code if(xOk && yOk && zOk) { if(xDF == 1 && yDF == 0 && zDF == 0) { for(row=0;row<3;row++) { for(col=0;col<3;col++) { // All 3x3 static arrays This is the line statObj->A1_[row][col] -= localFuncArr[row][col]; } } } } }//Inner loop i ends here }//Inner loop j ends here }//outer loop k ends here ``` For context, statObj is an object containing a number of 3x3 static double arrays. I am initializing statObj by a call to new function. Then I am populating the arrays inside it using some mathematical functions. One such array is A1_. The value of variable nosUnknowns is around 3000. The array localFuncArr is previously generated by matrix multiplication and is a double array. Now this is my problem: When I use the line as shown in the code, the code runs extremely sluggishly. Something like 245secs for the whole function. When I comment out the said line, the code performs extremely fast. It takes something like 6 secs. Now when I replace the said line with the following line : `localFuncArr[row][col] += 3.0`, again the code runs with the same speed as that of case(2) above. Clearly something about the call to `statObj->A1_` is making the code run slow. My question(s): Is Cache Poisoning the reason why this is happening ? If so, what could be changed in terms of array initialization/object initialization/loop unrolling or for that matter any form of code optimization that can speed this up ? Any insights to this from experienced folks is highly appreciated. EDIT: Changed the description to be more verbose and redress some of the points mentioned in the comments.