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

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

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. Basic principles Pareto principle Structure Parallelization
Vectorization
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14. 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 !

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

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

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

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

19. 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))

20. 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

21. 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

22. Questions ?