Static Determination of Synchronization Method for Slipstream Multiprocessors

Abstract

The scalability of a distributed shared memory systems is limited largely by communication overhead, most of which can be attributed to memory latency and synchronization. In systems built with dual-processor single-chip multiprocessors (CMPs) a proposed solution to this scalability limitation is the use of slipstream execution mode for certain applications. Instead of using the second on-chip processor to run a separate slice of the parallel application, slipstream mode utilizes the second processor to run a reduced version of the same task. The reduced version, known as the advanced stream, skips certain high latency events but continues to make accurate forward progress and provides future reference behavior for the unreduced version, known as the redundant stream.

Slipstream mode provides several methods of synchronization between the advanced (A-) and redundant (R-) streams to govern how far the A-stream can advance in front of the R-stream and also to provide a method of keeping the A-stream on the correct control path. A current limitation of slipstream is that the method used for A-R synchronization must be specified by the user at run time and used throughout the entire execution of the program. This is because the method that results in the best performance is application dependant and unknown beforehand.

We investigate alternate procedures to determine the A-R synchronization method by the use of static code analysis in the form of both profile- and compiler-driven techniques. A trace profile algorithm is presented that gives insight into shared memory access patterns that favor certain synchronization methods. We also discuss compiler integration of the synchronization method determination. Techniques similar to those which are used for compiler-driven prefetching and data forwarding are shown that could be used in this context as well.