Distributed shared memory

RGPV: Distributed System: Unit 2
Definition of DSM-
A distributed shared memory is a mechanism allowing end-users' processes to access shared data without using inter-process communications. In other words, the goal of a DSM system is to make inter-process communications transparent to end-users.
Message passing vs. DSM
Message passing
Distributed shared memory
Variables have to be marshaled
Variables are shared directly
Cost of communication is obvious
Cost of communication is invisible
Processes are protected by having private address space
Processes could cause error by altering data
Processes should execute at the same time
Executing the processes may happen with non-overlapping lifetimes
Abstract view of DSM
Advantages of DSM-
        System scalable
           Hides the message passing
           Can handle complex and large data bases without replication or sending the data to processes
           DSM is usually cheaper than using multiprocessor system
           No memory access bottleneck, as no single bus
           DSM provides large virtual memory space
           DSM programs portable as they use common DSM programming interface
           Shields programmer from sending or receive primitives
           DSM can (possibly) improve performance by speeding up data access.
Disadvantages of DSM-
        Could cause a performance penalty
        Should provide for protection against simultaneous access to shared data such as lock
        Performance of irregular problems could be difficult.

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