Automatic parallelization

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Automatic parallelization (also known as auto parallelization or Autoparallelization), refers to the use of a modern optimizing parallelizing compiler to convert sequential code into multi-threaded or vectorized (or even both) code in order to utilize multiple processors simultaneously in a shared-memory multiprocessor (SMP) machine. The goal of automatic parallelization is to relieve programers from the tedious and error-prone manual parallelization process. Though highly improved since several decades, full automatic parallelization of sequential programs by compilers remains a grand challenge due to the complex program analysis needed and the unknown factors (such as input data range) during compilation.

The programming control structures on which auto parallelization places the most focus are loops, because, in general, most of the execution time of a program takes place inside some form of loop. An auto parallelization compiler tries to split up a loop so that its iterations can be executed on separate processors concurrently.

[edit] Compiler Parallelization Analysis

The compiler usually conducts two passes of analysis before actual parallelization in order to determine the following:

• Is it safe to parallelize the loop? Answering this question needs accurate dependence analysis and alias analysis
• Is it worthwhile to parallelize it? Reliable estimation of workload of programs and capacity of parallel systems is required.

The first pass of the compiler performs a data dependence analysis of the loop to determine whether each iteration of the loop can be executed independently of the others. Data dependence can sometimes be dealt with, but it may incur additional overhead in the form of message passing, synchronization of shared memory, or some other method of processor communication.

The second pass attempts to justify the parallelization effort by comparing the theoretical execution time of the code after parallelization to the code's sequential execution time. Somewhat counterintuitively, code does not always benefit from parallel execution. The extra overhead that can be associated with using multiple processors can eat into the potential speedup of parallelized code.

[edit] A brief example of Auto-Parallelization

Code 1 below can be auto-parallelized by a compiler because each iteration is independent of the others, and the final result of array z will be correct regardless of the execution order of the other iterations.

!code 1
      do i=1,n
         z(i) = x(i) + y(i)
      enddo

On the other hand, code 2 below cannot be auto-parallelized, because the value of z(i) depends on the result of the previous iteration z(i-1).

!code 2
      do i=1,n
         z(i) = z(i -1)*2
      enddo

This does not mean that the code cannot be parallelized. However, current parallelizing compilers are not capable of bringing out these parallelisms automatically, and it is very questionable as to whether this code would benefit from parallelization in the first place.

[edit] Workaround

Due to the inherent difficulties in full automatic parallelization, several easier approaches exist to get a parallel program in higher quality. They are:

• let programmers to add "hints" into programs to guide compiler parallelization, such as HPF for distributed memory systems and OpenMP for shared memory systems.
• build an interactive system between programmers and parallelizing tools/compilers. Notable examples are SUIF explorer, Polaris compiler, and CAPTOOLS(now Parawise).
• hardware-supported speculative threading