Auto Parallelizing Option

1
Auto Parallelizing Option

• A Simple Introduction

2
What Is APO?

• The Auto-Parallelizing option is an optional
  compiler phase that automatically analyzes and
  attempts to parallelize loops and loop nestings
• Using data dependency analysis, DO or FOR loop
  nestings are examines and directives and pragmas
  are automatically inserted where it is determined
  safe and (hopefully) effective to do so.

3
APO Usage

• f77 apo listkeep source.f
• cc apo list source.c
• APO optionally generates intermediate files
• file_name.list diagnostic messages (if
  listkeep option)
• file_name.m transformed source file (if keep
  option)
• Before running program, specify the number of
  parallel threads
• setenv MP_SET_NUMTHREADS 4

4
Using APO an Example

• Source Code
• subroutine mat(x,y,z,m)
• real(kind8), dimension(m,m) x, y, z
• integer m
• do k 1,m
• do j 1,m
• do i 1,m
• z(i,j) z(i,j) x(i,j) y(j,k)
• end do
• end do
• end do
• return
• end

5
Using APO An Example (cont.)

• Invoke APO
• f90 apo keep O3 matmul90.f o matmul90
• Contents of matmul90.list File
• Parallelization Log of Subprogram mat_
• 5 PARALLEL (Auto) _mpdo_mat_1
• 4 Not Parallel
• Loop is contained within a parallel construct
• 6 Not Parallel
• Loop is contained within a parallel construct

6
Performance on 300 MHz Origin

• Using n1000, on the SGI Origin 2000 (at HPCC)
  with
• optimization level 3.
• PROGRAM SECONDS MFLOPS
• 1 PROCESSOR MATMUL90 3.6169 276.5
• 4 PROCESSORS MATMUL90 .9835 1016.7
• 8 PROCESSORS MATMUL90 .5287 1891.4
• 16 PROCESSORS MATMUL90 .3253 3073.8

7
Rule of thumb in APO

• Parallel loop should be outermost nesting.
• Dimension arrays with parallel dimensionrightmost
  (FORTRAN) or leftmost(C).
• Decide on best parallel dimension, considering
  number of iterations, data dependencies, etc.

8
Question

• In the source below (same as before), which loop
  should be the outermost and paralleled?
• subroutine mat(x,y,z,m)
• real(kind8), dimension(m,m) x, y, z
• integer m
• do k 1,m
• do j 1,m
• do i 1,m
• z(i,j) z(i,j) x(i,j) y(j,k)
• end do
• end do
• end do
• return
• end

9
Guidelines for Using APO

• Profile code to identify hot spots
• Run key subroutines through APO
• Examine .list and .m files
• If results look promising, test performance and
  correctness of APO transformations and quit if
  happy
• Tune using APO directives

10
Note about APO Directives

• You can manually insert APO directives to force
  parallelization the way you want . . .
• HOWEVER, once youre resorting to directives to
  try to get APO to do what you want, you might as
  well bypass APO and use MP/OMP directives which
  well learn later.

11
Reference on APO

• Man Pages (on Pleione)
• man apo
• Internet Resource
• http//techpubs.sgi.com
• Search for apo Several documents will be
  listed for C, C, Fortran90, Fortran77, etc.

12
THE END