Mumbai University > Information Technology > Sem6 > Distributed System

Marks: 10M

Year: Dec 2015

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.

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.

• 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.