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The Liberty Research Group

The VELOCITY Compiler: Extracting Efficient Multicore Execution from Legacy Sequential Programs [abstract] (PDF)
Matthew John Bridges
Ph.D. Thesis, Department of Computer Science, Princeton University, November 2008.

Multiprocessor systems, particularly chip multiprocessors, have emerged as the predominant organization for future microprocessors. Systems with 4, 8, and 16 cores are already shipping and a future with 32 or more cores is easily conceivable. Unfortunately, multiple cores do not always directly improve application performance, particularly for a single legacy application. Consequently, parallelizing applications to execute on multiple cores is essential.

Parallel programming models and languages could be used to create multi-threaded applications. However, moving to a parallel programming model only increases the complexity and cost involved in software development. Many automatic thread extraction techniques have been explored to address these costs.

Unfortunately, the amount of parallelism that has been automatically extracted using these techniques is generally insufficient to keep many cores busy. Though there are many reasons for this, the main problem is that extensions are needed to take full advantage of these techniques. For example, many important loops are not parallelized because the compiler lacks the necessary scope to apply the optimization. Additionally, the sequential programming model forces programmers to define a single legal application outcome, rather than allowing for a range of legal outcomes, leading to conservative dependences that prevent parallelization.

This dissertation integrates the necessary parallelization techniques, extending them where necessary, to enable automatic thread extraction. In particular, this includes an expanded optimization scope, which facilitates the optimization of large loops, leading to parallelism at higher levels in the application. Additionally, this dissertation shows that many unnecessary dependences can be broken with the help of the programmer using natural, simple extensions to the sequential programming model.

Through a case study of several applications, including several C benchmarks in the SPEC CINT 2000 suite, this dissertation shows how scalable parallelism can be extracted. By changing only 38 source code lines out of 100,000, several of the applications were parallelizable by automatic thread extraction techniques, yielding a speedup of 3.64x on 32 cores.