Distributed Shared Memory in Distributed Computing

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Distributed SharedMemory


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as c para gms or nDC

Shared Memory

Message passing.


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Introduction to DSM

Shared memory is used in tightlycoupled systems.

loosely coupled systems are usingmsg passing or RPC.

But how to use shared memory on

loosely coupled??


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DSM History

Memory mapped files started in theMULTICS operating system in the1960s

One of the first DSM implementationswas Apollo. One of the first system touse Apollo was Integrated shared

Virtual memory at Yale (IVY) DSM developed in parallel with

shared-memory multiprocessors


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Distributed shared memory

Topics covered :

Architecture

design issues structure of shared memory space

replacement strategy

thrashing


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How DSM implemented???

A Software layer on top of messagepassing system to provide sharedmemory abstraction.

Gives an illusion of physically sharedmemory.

The software layer can beimplemented on os kernel or runtime library with kernel support.

DSM is the shared memory paradigma lied to loosel cou led distributed


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DSM.

That is DSM is basically anabstraction that integrates the localmemory of different machines in a

network environment into a singlelogical entity shared by cooperatingprocess executing on multiple site.

Shared memory exists only virtually. Sharing with the help of

replication/migration

So DSM also called as Distributed


8/232008, Pham Quoc Cuon Phan DinhDistributed Shared Memory 8Distributed Shared Memory

DSM vs Message passing

DSM Message passing

Variables are shared directly Variables have to be marshalledyourself

Processes can cause error to oneanother by altering data

Processes are protected from oneanother by having private address

spaces

Processes may execute with non-overlapping lifetimes

Processes must execute at thesame time

Invisibility of communicationscost

Cost of communication is obvious


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Distributed Shared Memory

Distributed Shared Memory (DSM) allowsprograms running on separate computersto share data without the programmerhaving to deal with sending messages

Instead underlying technology will sendthe messages to keep the DSM consistent(or relatively consistent) betweencomputers

DSM allows programs that used to operateon the same computer to be easilyadapted to operate on separate computers


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General Architecture of DSMsystems

A software memory mappingmanager routine in each node mapsthe local memory onto the shared

virtual memory . To facilitate the mapping operation,

the shared memory space is

partitioned into blocks Memory access latency- network

latency can be reduced by data

caching.


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The distributed shared memoryabstractionDistributed Shared Memory

(exists only virtually)

CPU 1

CPU n

Memory

Memory-MappingManager

CPU 1

CPU n

Memory


CPU 1

CPU n

Memory


Communication Network


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Design and Implementationissue of DSM.

1. Granularity: refers to block size. Unit of sharing or unit of data transfer

Possible units are few words , a page ora few pages.

Determines the measure of parallelismand amount of traffic

2. Structure of Shared Memoryspace:

layout of shared data in memory


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Design and Implementationissue of DSM cont..

3. Memory Coherence andaccess Synchronization:

Coherence means consistency : relatedto replicated data.

Concurrent access to the shared datamay be there

Synchronization primitives likesemephores, event count, and locketc..may be used


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4. Data location and access: Data block locating mechanism.

5. Replacement Strategy: When local memory of a node is full and

Cache miss occurs

6. Thrashing: Data block migrating between nodes on

demand.

Blocks transfer at high rate so that no


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7. Heterogeneity:

To function properly with machines

with different architectures.


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Structure of shared memory

Structure and granularity are closelyrelated

Commonly used approaches for

structuring are : No structuring :

unstructured shared memory, can choosesuitable page size and a fixes grain size

Structuring by data type :

Structured as collection of objects orvariables ,so variable grain size

Structuring as a database


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Replacement strategy

In DSM blocks are dynamicallymigrated / replicated

Issues addressed

Which block should be replaced to makespace for newly required block?

Where should the replaced block beplaced?


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Which block to replace?

Usage based versus non usagebased(LRU, FIFO, Rand)

Fixed space versus variable space

Based priority mechanism unUsed : block that is not currently

being used

Nil : block that has been invalidated

Read only : block for which the nodehas only read access right

Read owned : block for which the node


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Which block to replace

Replacement Priority

Used and nil : highest replacementpriority

Read only : next replacement priority,copy of the block will be there availableat the owner , can be brought if

required. Read owned and writable blocks for

which replicas exit on other nodes : nextreplacement priority


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Where to replace??

Blocks selected for replacementhaving useful information need notbe discarded eg : writable, read

owned blocks etc. Approaches for storing replaced

blocks are:

Using secondary mry

Using the mry space of other nodes

By keeping tack of free mry space at other

nodes in the system


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Thrashing

Serious performance problem

Because of poor locality of reference

Large amount of time wastedtransferring shared data blocks fromone node to another, than doinguseful work

Thrashing occurs when networkresources are exhausted, and moretime is spent invalidating data andsendin u dates than is used doin


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Thrashing occurs..

When interleaved data access madeby processes on two or more nodescauses a data block to move back

and forth from one node to anotheron quick succession

When blocks with read only

permission are repeatedlyinvalidated soon after they arereplicated.


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Methods to solve trashing

Providing application controlled locks

Nailing a block to a node for aminimum amount of time

Tailoring the coherence algorithm tothe shared data usage patterns.

Distributed Shared Memory in Distributed Computing

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