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How is Sequential consistency model implemented if Replicated Migrating Blocks are used in Distributed Shared Memory

Mumbai University > Information Technology > Sem6 > Distributed System

Marks: 10M

Year: Dec 2015

1 Answer
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Implementing Sequential Consistency Model:

  • Most commonly used consistency model in DSM.
  • Protocols depends on whether the DSM system allows replication/migration of shared –memory blocks.
  • Different Strategies are,

    • Non-Replicated, Non-Migrating Blocks(NRNMBs)
    • Non-Replicated ,Migrating blocks (NRMBs)
    • Replicated, Migrating blocks(RMBs)
    • Replicated ,Non-Migrating blocks(RNMBs)
  • Non-Replicated, Non-Migrating Blocks(NRNMBs)

    • Simplest strategy.
    • Each block of the shared memory has a single copy whose location is always fixed.
    • All access request to a block from any node are sent to the owner node of the block, which has only copy of the block.
    • On receiving, a request from a client node, the MMU and OS of the owner node return a responsed to the client.
    • Serializing data access creates a bottleneck.
    • Parallelism is not possible.
    • There is single copy of each block in the system.
    • The location of the block never changes.

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There is a single copy of each block and the location of a block changing dynamically.

  • Broadcasting: Each node maintains an owned blocks table that contains an entry for each block for which the node is the current owner.

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  • Replicated Migrating Blocks:

    • To increase parallelism, all DSM systems replicated blocks.
    • With replicated blocks, read operations can be carried out in parallel with multiples nodes, the average cost of read operation is reduced.
    • However , replication tends to increase the cost of write operations because for a write to a block all its replicas must be invalidated or update to maintain consistency.
    • Replication complicates the memory coherence protocol.

Basically there are two protocols for enhancing sequential consistency

  • Write Invalidate:

    • In this scheme, all copies of a piece of data except one are invalidated before write operation can be performed on it.
    • When a write faults occurs, its fault handler copies the accessed block from one of the block’s to its own node and invalidates all other copies by sending an invalidate message.
    • The write operation is performed on own node.
    • The own node holds the modified version of block and is replicated to other nodes.

Replicated Non-Migrating Blocks (RNB’s)

  • In this strategy, shared-memory blocks may be replicated at multiples node of the system.
  • The location of replica is fixed.
  • A read or write access to a memory address is carried out by sending the access request to one of the nodes having a replica of the block containing the memory address.
  • Write-update protocol is used.
  • Sequential consistency is ensured by using a global-sequencer.
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