Automatic Code Parallelization at Runtime

Director:Philippsen, M.
Period:January 1, 2011 - December 31, 2015
Coworker:Veldema, R.; Brinkers, D.
Description:

Our automatic parallelization efforts are currently focused on dynamic parallelization. While a program is running, it is analyzed to find loops where parallelization can help performance. Our current idea is to run long-running loops three times. The first two runs analyze the memory accesses of the loop and can both run in parallel. The first run stores in a shared data structure for every memory address in which loop iteration a write access happens. We do not need any synchronization for this data structure, only the guarantee that one value is written to memory, when two concurrent writes happen. In the second pass we check for every memory access, if it has a dependency to one of the stored write accesses. A write access is part of any data-dependency, so we can find all types of data dependencies. If we do not find any, the loop is actually run in parallel. If we find dependencies, the loop is executed sequentially. We can execute the analyses in parallel to a modified sequential execution of the first loop iterations.
In 2014 we enhanced the analysis so that a loop can start running while the remainder of the loop is analyzed to see if it can be run in parallel. To allow the sequential loop to execute while the tail of the loop is analyzed we needed to instrument the sequential loop slightly. The result is that the loop runs only slightly slower if the loop cannot be parallelized, but if the loop is found to be parallelizable, speedup is near to linear.
Finally, we also created a new language that uses the above library for run-time parallelization. Any loops that the programmer marked as candidates for run-time parallelization are analyzed for constructs that the library cannot yet handle. If the loop is clean, code is generated that uses the library's macros.
The ErLaDeF project is a contribution of the Chair of Computer Science 2 (Programming Systems) to the IZ ESI (Embedded Systems Initiative, http://www.esi-anwendungszentrum.de/)

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