Performance Optimization Getting your programs to run faster CS 691

Why optimize Better turn-around on jobs Run more programs/scenarios Release resources to other applications You want the job to finish before you retire

Ways to get more performance Run on bigger, faster hardware clock speed, more memory, … Tweak your algorithm Optimize your code

Loop Unrolling Converting passes of a loop into in-line streams of code Useful when loops do calculations on data in arrays Unrolling can take advantage of pipeline processing units in processors Compiler may preload operands into CPU registers

Loop Unrolling – disadvantages may be limited by the number of Floating point registers Pentium III: 8 Pentium 4: 8 Itanium: 128

Loop Unrolling – simple example Loop do i=1,n a(i) = b(i) +x*c(i) enddo Unrolled Loop do i=1,n,4 a(i) = b(i) +x*c(i) a(i+1) = b(i+1) +x*c(i+1) a(i+2) = b(i+2) +x*c(i+2) a(i+3) = b(i+3) +x*c(i+3) enddo

Loop Unrolling – simple example Performance – Rolled P3 550mhz – 13 mflops Itanium – 30 mflops Performance Unrolled P3 550mhz – 30 mflops Itanium – 107 mflops *from: LCI and NCSA

Loop Unrolling int a[100]; for (i=0;i<100;i++){ a[i] = a[i] * 2; } int a[100]; for (i=0;i<100;i+=5){ a[i] = a[i] * 2; a[i+1]=a[i+1]*2; a[i+2]=a[i+2]*2; a[i+3]=a[i+3]*2; a[i+4]=a[i+4]*2; }

Loop unrolling int a[10][10]; for (i=0;i<10;i++){ for (j=0;j<10;j++) { a[i][j] = a[i][j] *2;} int a[10][10]; for (i=0;i<10;i++){ a[i][0]=a[i][0]*2; a[i][1]=a[i][1]*2; a[i][2]=a[i][2]*2; a[i][3]=a[i][3]*2; a[i][4]=a[i][4]*2; a[i][5]=a[i][5]*2; a[i][6]=a[i][6]*2; a[i][7]=a[i][7]*2; a[i][8]=a[i][8]*2; a[i][9]=a[i][9]*2;}

Loop unrolling – Matrix Dot Product float a[100]; float b[100]; float z; for (i=0;i<100;i++){ z = z + a[i] * b[i]; } float a[100]; float b[100]; float z; for (i=0;i<100;i+=2){ z = z + a[i] * b[i]; z = z + a[i+1] * b[i+1]; }

Unrolling Loops You can do it automatically

Unrolling Loops – compiler options GNU Compilers -funroll-loops -funrull-all-loops (not recommended) PGI Compilers -Munroll -Munroll=c:N -Munroll=n:M

Unrolling Loops – Compiler Options Intel Compilers -unrollM (up to M times) -unroll

Taking Memory in Order Optimizing the use of cache row major order vs column major order row major --  a(1,1), a(2,1), a(3,1), a(1,2), a(2,2),… column major –  a(1,1), a(1,2), a(1,3), a(2,1), a(2,2),…

Taking Memory in Order Remember C and Fortran store arrays in the opposite manner  C – row major  Fortran – column major

Taking Memory in Order c  Fortran 

Taking Memory in Order do i=1,m do j=1,n a(i,j)=b(i,j)+c(i) end do do j=1,m do i=1,n a(i,j)=b(i,j)+c(i) end do loop time: loop runs at 4.48 Mflops loop time: 2.80 loop runs at Mflops

Floating Point Division FP Division is very expensive in terms of processor time clock cycles to compute Usually not pipelined FP Division required by IEEE “rules”

Floating point division – use reciprocal float a[100]; for (i=0;i<100;i++){ a[i]=a[i]/2; } float a[100]; Float denom; denom = 1/2; for (i=0;i<100;i++){ a[i]=a[i]*denom; }

Compiler options for IEEE Compatibility PGI Compilers  -Knoieee Intel Compilers  -mp GNU Compilers  can’t do Floating Point Division

Compilers can’t optimize if divisor is not scalar Breaks IEEE “rules” May impact portability

Function Inlining Build functions/subroutines in as inline parts of the programs code… … rather than functions/subroutines minimizes functions calls (and management of…)

Function Inlining Compile with – -Minline  compiler tries to inline what it can (meet compiler criteria) -Minline=except:func  excludes func from inlining -Minline=func  inline only func

Function Inlining …Compile with- -Minline=myfile.lib  inlines functions from inline library file -Minline=levels:n  inlines functions up to n levels of calls  usually default = 1

MPI Tuning Minimize messages Pointers/counts MPI Derived datatypes MPI_Pack/MPI_Unpack Using shared memory for message passing #PBS –l nodes=6:ppn=1 … but… #PBS –l nodes=3:ppn=2 … is better.

Compiler optimizations -O0 –no optimization -O1 –local optimization, register allocation -O2 –local/limited global optimization -O3 –aggressive global optimization -Munroll – loop unrolling -Mvect - vectorization -Minline – function inlining

gcc Compiler Optimatizations See: