Compiler and Software Distributed Shared Memory Support for Irregular Applications
We investigate the use of a software distributed shared memory (DSM) layer to support irregular computations on distributed memory machines. Software DSM supports irregular computation through demand fetching of data in response to memory access faults. With the addition of a very limited form of compiler support, namely the identification of the section of the indirection array accessed by each processor, many of these on-demand page fetches can be aggregated into a single message, and prefetched prior to the access fault. We have measured the performance of this approach for two irregular applications, moldyn and nbf, using the Tread-Marks DSM system on an 8-processor IBM SP2. We find that it has similar performance to the inspector-executor method supported by the CHAOS run-time library, while requiring much simpler compile-time support. For moldyn, it is up to 23% faster than CHAOS, depending on the input problem's characteristics; and for nbf, it is no worse than 14% slower. If we include the execution time of the inspector, the software DSM-based approach is always faster than CHAOS. The advantage of this approach increases as the frequency of changes to the indirection array increases. The disadvantage of this approach is the potential for false sharing overhead when the data set is small or has poor spatial locality.
Record created on 2005-10-20, modified on 2016-08-08