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Memory Consistency Models'

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All processes order all writes in program order. Causal Consistency: ... DNS Domain Name System. Internet registry service. NFS Network File System ... – PowerPoint PPT presentation

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Title: Memory Consistency Models'


1
Memory Consistency Models.
2
Uniform Consistency Models
  • Only have read and write operations
  • Sequential Consistency
  • Pipelined-RAM
  • Causal Consistency
  • Coherence
  • Processor Consistency

3
Uniform Model Definitions
  • Sequential Consistency
  • All processes order all writes in the same order
  • Pipelined RAM
  • All processes order all writes in program order
  • Causal Consistency
  • All processes order writes such that it satisfies
    Lamports happens-before relation
  • Coherence
  • All processes order writes to the same location
    in the same order

4
Release Consistency
  • Hybrid Model
  • Operations available
  • Read read local copy
  • Write write to local copy
  • Acquire update local copy
  • Release send updates of local copy
  • Acquire and Release ops are sequentially
    consistent

5
Acquire-release Order
  • (O,ltaro) acquire-release order
  • o1 ltaro o2 if o1 ltpo o2 and
  • o1 and o2 are on the same data item, or
  • o1 is an acquire, or
  • o2 is a release, or
  • There exists o? st o1 ltaro o? and o? ltaro o2

6
Formal Definition of RC
  • A computation is Release Consistent if
  • Each process orders its own ops, all writes,
    release and acquire operations
  • Such that the order satisfies the acquire-release
    order
  • The acquire and release ops are ordered such that
  • All processes agree on the ordering (SC)
  • Each acquire is immediately followed by its
    matching release.

7
Hybrid Models
  • Distinguish between different types of memory
    accesses
  • Release Consistency
  • Provides acquire and release operations
  • Entry Consistency
  • Each shared variable has a lock
  • Scope Consistency
  • Variables are associated with locks automatically
  • Weak Consistency
  • Orders synchronization operations

8
Questions (1/3)
  • What does Distributed Shared Memory middleware
    provide?
  • Give 6 issues to consider when designing a DSM
    implementation.
  • Give an advantage of providing sequential
    consistency as the memory consistency model.

9
Questions (2/3)
  • Give a disadvantage of providing sequential
    consistency as the memory consistency model.
  • Give three other uniform memory consistency
    models that attempt to overcome this
    disadvantage.
  • Why are hybrid consistency models often
    preferable over uniform memory models?

10
Questions (3/3)
  • What is a disadvantage of hybrid consistency
    models?

11
Naming
12
Example Internet DNS
  • http//www.company.com8080/dir/index.html

Protocol
Name of internet host
Port number
Browser contacts DNS server for address
DNS server returns address (eg 204.148.170.161)
Browser contacts server located at
204.148.170.161, listening at port 8080
13
Finding Entities in DS
  • Name
  • Unique
  • Location independent
  • Address
  • Current location of entity
  • May change
  • Need service to match name with address

14
Implementations
  • DNS Domain Name System
  • Internet registry service
  • NFS Network File System
  • Suns file server configuration
  • Directories organized as trees
  • Linking files
  • Mounting drives
  • GNS Global Name Service

15
Locating Mobile Entities
  • Location of entity not fixed
  • How to find entity?

16
Simple Solutions
  • Broadcasting and Multicasting
  • Inefficient for large or widely dispersed
    networks
  • Forwarding Pointers
  • Chain may become very long
  • Each link must be maintained
  • Broken links

17
Home Based Approaches
  • Home address maintains current location
  • Two-tiered system

Dial number
Friends location
Home
Connection
forward connection request
Current Location
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