Memory Consistency Models'

1
Memory Consistency Models.
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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

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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

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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.

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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

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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.

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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?

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Naming
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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
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Finding Entities in DS

• Name
• Unique
• Location independent
• Address
• Current location of entity
• May change
• Need service to match name with address

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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

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Locating Mobile Entities

• Location of entity not fixed
• How to find entity?

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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

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Home Based Approaches

• Home address maintains current location
• Two-tiered system

Dial number
Friends location
Home
Connection
forward connection request
Current Location