Presently all programmers want to perform their tasks much faster than before. So, Parallel Processing comes into the picture to satisfy the increasing demands. Till a long time, parallel programs were only written either for multiprocessing environment or multi-computing environment. However, both of these parallel processing systems have some relative advantages and disadvantages. Distributed Shared Memory (DSM) system is a new and attractive area of research which combines the advantages of both shared-memory parallel processors (multiprocessors) and distributed systems (multi-computers). However, in DSM environment there are some critical issues like memory consistency that should be handled carefully. In this paper, an overview of DSM is given after a brief description of Distributed Computing Systems. Later various implementation issues and consistency models related to DSM are shown. Then an example of a simple program is given that can be implemented in DSM environment using Open SHMEM.

Parallel Programming; Multiprocessing; Multicomputing; Distributed Shared Memory (DSM); Consistency Models

[1] Pradip K.Sinha, �Distributed Operating Systems: Concepts and Designs� published by IEEE Computer Society Press,2004.
[2] Andrew S. Tanenbaum, �Distributed Operating Systems� published by PEARSON Education, Fifth Impression,2008.
[3] Ryan Saptarshi Ray, Utpal Kumar Ray, Ashish Anand,Parama Bhaumik, �Distributed Shared Memory � A Survey and Implementation Using Openshmem� published in Int. Journal of Engineering Research and Applications, ISSN: 2248-9622, Vol. 6, Issue 2, (Part - 1) February 2016, pp.49-52.
[4] David Mosberger, �Memory Consistency Models�, Paper submitted to Department of Computer Science, The University of Arizona, Tucson, AZ 85721.
[5] Radhika Gogia, Preeti Chhabra, Rupa Kumari, �CONSISTENCY MODELS IN DISTRIBUTED SHARED MEMORY SYSTEMS�, Research Article published in IJCSMC, Vol. 3, Issue. 9, September 2014, pg.196 � 201.
[6] Kai Li, �Shared Virtual Memory on Loosely Coupled Microprocessors�, PhD Thesis submitted to Yale University, September 1986.
[7] M. J. Flynn, �Computer Architecture: Pipelined and Parallel Processor Design� published by Jones and Barlett, Boston, 1995.
[8] Jelica Protic, Milo Tomasevic, Veljko Milutinovic, �A Survey of Distributed Shared Memory Systems� published in Proceedings of the 28th Annual Hawaii International Conference on System Sciences,1995.
[9] PGAS Forum,http://www.pgas.org/
[10] B. Chapman, T. Curtis, S. Pophale, S. Poole, J. Kuehn, C. Koelbel, L. Smith �Introducing OpenSHMEM, SHMEM for the PGASCommunity�, published in Proceedings of Partitioned Global AddressSpace Conference, 2010.
[11] Message Passing Interface (MPI) standard, http://www-unix.mcs.anl.gov/mpi/
[12] http://openshmem.org/site/Documentation/Manpages/Browse
[13] Faizul Navi Khan, Kapil Govil, �Reliability Based Task Allocation Scheme to Enhance the Performance of Distributed Environment�, Research Article published in IJCSE Vol. 2, Issue. 8, September 2014, pg.99.