Distributed Shared Memory

1
Distributed Shared Memory
2
Overview of Replicating Data

• Reasons Performance, Availability, Reliability
• Requirements
• Group management
• Consistency
• Reliable group communication
• Ordered updates
• Problems
• Reaching agreement in presence of failure
• Failure detection
• Complexity of implementation

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Distributed Shared Memory

• Middleware
• Memory that is distributed and duplicated appears
  local and as single copy
• Hides complexity

4
Designing DSM

• Data structure (byte, object, immutable)
• Synchronization constructs
• Consistency model (ordering of updates)
• Update options (update or invalidate)
• Granularity
• Thrashing

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Memory Consistency Models

• Defined for DSMs
• Consider what executions could happen
• Execution
• Sequences of read and write operations
• One sequence for each process in system.

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Uniform Consistency Models

• Only have read and write operations
• Sequential Consistency
• Pipelined-RAM
• Causal Consistency
• Coherence
• Processor Consistency

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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

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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