Optimization: The Art of Computing Intel Challenge experience and other tricks … Mathieu Gravey

Golden principle of Optimizing - t e r m Algorithm Implementation Hardware P e r f o m a n c

Example: Prime Number Algorithm For i=2 to N bool isPrime=true; For j=2 to N If (mod(i,j)==0 and i != j) isPrime=false; break; end if end for if (isPrime) add i to the listOfPrimeNumber End for

Example: Prime Number Algorithm For i=2 to N bool isPrime=true; For j=2 to i If (mod(i,j)==0) isPrime=false; break; end if end for if (isPrime) add i to the listOfPrimeNumber End for

Example: Prime Number Algorithm For i=2 to N bool isPrime=true; For j=2 to √i If (mod(i,j)==0) isPrime=false; break; end if end for if (isPrime) add i to the listOfPrimeNumber End for

Example: Prime Number Algorithm // the job For i=2 to N bool isPrime=true; For j=2 to √i If (mod(i,j)==0) isPrime=false; break; end if end for if (isPrime) add i to the listOfPrimeNumber End for

Example: Prime Number Algorithm // the job For i=2 to N bool isPrime=true; For j=2 to √i If (mod(i,j)==0) isPrime=false; break; end if end for if (isPrime) add i to the listOfPrimeNumber End for

Example: Prime Number Algorithm // the job For i=2 to N bool isPrime=true; vectorize the job For j=2 to √i isPrime = isPrime && (mod(i,j)!=0); end for if (isPrime) add i to the listOfPrimeNumber End for

Example: Prime Number Algorithm // the job For i=3 to N step 2 bool isPrime=true; vectorize the job For j in √i step 2 isPrime = isPrime && (mod(i,j)!=0); end for if (isPrime) add i to the listOfPrimeNumber End for

Example: Prime Number Algorithm // the job For i=2 to N step 2 bool isPrime=true; vectorize the job For j=2 to √i step 2 isPrime = isPrime && (mod(i,j)!=0); end for if (isPrime) add i to the listOfPrimeNumber End for

Example: Prime Number Algorithm // the job For i==2 to N bool isPrime=true; vectorize the job For j in listOfPrimeNumber and j<√i isPrime = isPrime && (mod(i,j)!=0); end for if (isPrime) add i to the listOfPrimeNumber in order End for

Example: Prime Number Algorithm // the job For i==1 or i==5 in base 6, to N bool isPrime=true; vectorize the job For j in listOfPrimeNumber and j<√i isPrime = isPrime && (mod(i,j)!=0); end for if (isPrime) add i to the listOfPrimeNumber in order End for

Basic principles Pareto principle Structure Parallelization Vectorization inotes4you.files.wordpress.com

Basic principles Start by the main issues Global view  critical issue Monkey development Start simple  go to complex Iterative process Optimizing, start by slowing down Global picture ! http://bestofpicture.com/

Rules Guidelines Be lazy Don’t reinvent the wheel Don’t be idle Design pattern Global variables are your enemies Don’t Overgeneralize

Rules Guidelines Trust the compiler Simple for you = simple for compiler | computer Share your knowledge Compiler

Rules Guidelines Think different, try, change and try again … Don’t aim for the Best, but something Good and Better

Concrete trick : Memory Array vs. List Prefetch | random access

Concrete trick : First step Optimization Compiler optimization icpc myCodeFile –O3 -xhost –o myCompiledProgram ⚠ -g const No-writes inline restrict/__restrict__ No read updates Loop-unroll __builtin_expect((x),(y))

Concrete trick : OpenMP Vectorization => SIMD #pragma omp simd Multi-operation with one instruction ⚠ non-aligned data Multi-Thread L3 cache-communication Shared memory How to use : #pragma omp parallel for default(none) shared(x,y) fisratPrivate(array) reduction(max:MaxValue) schedule(static) for(int i=0; i< 10000; i++){ something … } #pragma omp critical #pragma omp barrier

Multi-Chip | Multi-Sockets NUMA (Non-uniform memory access) slower than local memory Position in memory => first touch Parallelize the initialisation with : schedule(static) read only data => copy in each local memory Thread Affinity

Questions ?