Ebook Mixed Speculative Multithreaded Execution Models
The current trend toward chip multiprocessor architectures has placed great pressure on programmers and compilers to generate thread-parallel programs. Improved execution performance can no longer be obtained via traditional single-thread instruction level parallelism (ILP), but, instead, via multithreaded execution. One notable technique that facilitates the extraction of parallel threads from sequential applications is thread-level speculation (TLS). This technique allows programmers/compilers to generate threads without checking for inter-thread data and control dependences, which are then transparently enforced by the hardware. Most prior work on TLS has concentrated on thread selection and mechanisms to efficiently support the main TLS operations, such as squashes, data versioning, and commits.
This thesis seeks to enhance TLS functionality by combining it with other speculative multithreaded execution models. The main idea is that TLS already requires extensive hardware support, which when slightly augmented can accommodate other speculative multithreaded techniques. Recognizing that for different applications, or even program phases, the application bottlenecks may be different, it is reasonable to assume that the more versatile a system is, the more efficiently it will be able to execute the given program.
As mentioned above, generating thread-parallel programs is hard and TLS has been suggested as an execution model that can speculatively exploit thread-level parallelism (TLP) even when thread independence cannot be guaranteed by the programmer/compiler. Alternatively, the helper threads (HT) execution model has been proposed where subordinate threads are executed in parallel with a main thread in order to improve the execution efficiency (i.e., ILP) of the latter. Yet another execution model, runahead execution (RA), has also been proposed where subordinate versions of the main thread are dynamically created especially to copewith long-latency operations, again with the aim of improving the execution efficiency of the mainthread (ILP).
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