OpenMP compiler for a Software Distributed Shared Memory System SCASH

In this paper, we present an implementation of OpenMP compiler for a page-based software distributed shared memory system, SCASH on a cluster of PCs. For programming distributed memory multiprocessors such as clusters of PC/WS and MPP, message passing is usually used. A message passing system requires programmers to explicitly code the communication and makes writing parallel programs cumbersome. OpenMP is attracting wide-spread interests because of its easy-to-use parallel programming model. While OpenMP is designed as a programming model for shared memory hardware, one way to support OpenMP in a distributed memory environment is to use a software distributed shared memory system (SDSM) as an underlying runtime system for OpenMP. Our target SDSM is a page-based software distributed shared memory system, SCASH[2], which is running on a cluster of PCs connected by a high speed network Myrinet. In the most SDSMs, only part of the address space is shared. In SCASH, the address space allocated by a shared memory allocation primitive can be shared among the processors. Variables declared in global scope are private in the processor. We call this memory model, \shmem memory model". Parallel programs using the Unix \shmem" system calls use this memory model. In this model, all shared variables must be allocated at run-time at the beginning of execution. We have implemented the OpenMP compiler for \shmem memory model" using Omni OpenMP compiler system [4]. The compiler detects references to a shared data object, and rewrite it to the reference into objects re-allocated in shared memory area. H. Lu, et.al. [3] present an OpenMP implementation for TreadMarks[1] software DSM. Their compiler only supports a subset of OpenMP. Instead, they propose some modi cations of OpenMP standard to make OpenMP programs run easier and more e cient. This approach may loose portability of OpenMP programs. Our compiler supports a full set of OpenMP so that OpenMP compliant programs runs on SDSMs without any modi cations. In the next section, we present overview of the Omni OpenMP compiler system and SCASH as background. Section 3 describes how to translate the OpenMP programs for \shmem memory model" and some preliminary results. Section 4 presents our concluding remarks.