DSM architecture, advantages & disadvantages

Subject: DOS

CE-B

Maulik togadiya

What is shared memory?

On-Chip Memory

CPU Memory CPU1 Memory

CPU4

CPU2

CPU3

Chip package

Address and data linesConnecting the CPU to the memory

extension

A single-chip computerA hypothetical shared-memoryMultiprocessor.

Introduction

Distributed shared memory (DSM) is a form of memory architecture where the (physically separate) memories can be addressed as one (logically shared) address space. Here, the term "shared" does not mean that there is a single centralized memory but "shared" means that the address space is shared.

The shared memory model provides a virtual address space shared between all nodes.

The overcome the high cost of communication in distributed systems, DSM systems move data to the location of access.

How it works?

Data moves between main memory and secondary memory (within a node) and between main memories of different nodes.

When a process accesses data in the shared address space, the mapping manager maps shared memory address to physical memory (local or remote).

DSM architecture

High speed communication network is used for connecting nodes. A simple message passing system allows on different node to

exchange message with each other. Memory mapping manager routine in each node maps local

memory onto the shared virtual memory. For mapping operation, the shared memory space is partitioned into blocks.

The main memory of individual nodes is used to cache of shared memory space.

Memory

MappingManager

Memory

MappingManager

Memory

MappingManager

Shared Memory

NODE 1 NODE 2 NODE 3

Communication network

Systems that support DSM,data moves between secondary memory and main memory as well as between main memories of different nodes.

When process access data in shared address space, a mapping manager maps the shared memory address to the physical memory. The mapping manager is layer of software implemented either in the operating kernel or as runtime library routine.

DSM architecture

Advantages

Shields programmer from send/receive primitives. DSM is usually cheaper than using multiprocessor system. DSM provides large virtual memory space. DSM programs portable as they use common DSM programming

interface. It can handle complex and large data bases without replication or

sending the data to processes. No memory access bottleneck.

Disadvantages

Must provide for protection against simultaneous access to shared data such as lock..

It could cause a performance penalty. Little programmer control over actual messages being generated. Performance of irregular problems could be difficult. Programmers need to be understand consistency model to write

correct programs.