Chapter 8-2 : Multicomputers

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Chapter 8-2 Multicomputers

• Multiprocessors vs multicomputers
• Interconnection topologies
• Switching schemes
• Communication with messages
• Remote procedure calls
• Distributed shared memory
• Multicomputer scheduling
• Virtualization

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Multicomputers

• Multiprocessors share the common memory
• Multicomputers are tightly coupled CPUs with
  memories of their own and do not share them
• These systems are also known as cluster computers
  and COWS (Clusters of Workstations)
• The secret of high performance is the
  interconnection network

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Multiprocessor vs Multicomputer

• Figure 8-29. Comparison of three kinds of
  multiple CPU systems.

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Interconnection Topologies

• Figure 8-16. Various interconnect topologies.
  (a) A single switch (b) A ring (c) A grid or
  mesh (d) A double torus (e) A cube (f) A 4D
  hypercube
• Note A diameter is defined as the longest path
  between any two nodes

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Switching Schemes

• Store-and-forward packet switching messages are
  broken into packets. Packets move from one switch
  to another
• This scheme has increasing latency (delay)
  problem because of store-and-forwarding in
  intermediate switches
• Circuit switching a path is established from
  the source to the destination. Once this path is
  setup bits are pumped from source to destination
  with no buffering in the intermediate switches
• Circuit setup may need some time but then the
  transfer is fast

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User Level Communication Software

• Multicomputers communicate through messages
• Send(dest, mptr) send a message pointed by
  mptr to a process identified by dest
• Receive(addr, mptr) addr is usually CPU number
  and a process or port number
• Send calls can be blocked or nonblocked
• Receive calls are always blocked. That is, the
  receiving process has to wait if the message has
  not been sent before

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Blocking Calls

• Figure 8-19. (a) A blocking send call.

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Nonblocking Calls

• Figure 8-19. (b) A nonblocking send call.

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Remote Procedure Call

• Figure 8-20. Steps in making a remote procedure
  call.
• The stubs are local procedures implementing the
  interface between the client and the server
• Parameter and result passing are done by messages

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Distributed Shared Memory (1)

• Figure 8-21. Various layers where shared memory
  can be implemented (a) The hardware (b) The
  operating system (c) User-level software

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Distributed Shared Memory (2)

• Figure 8-22. (a) Pages of the address space
  distributed among four machines.
• When a CPU references an address that is not
  local, a trap occurs, and the DSM software
  fetches the page containing the address and
  restarts the faulting instruction

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Distributed Shared Memory (3)

• Figure 8-22. (b) Situation after CPU 0
  references page 10 and the page is moved there.

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Distributed Shared Memory (4)

• Figure 8-22. (c) Situation if page 10 is read
  only and replication is used.

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Multicomputer Scheduling

• On a multiprocessor, all processes reside on the
  shared memory
• On a multicomputer each node has its own memory
  and its own set of processes
• Scheduling within a node is easier but allocation
  of processes to nodes is much more important
  (load balancing)
• The algorithms and heuristics for doing this
  assignment are known as processor allocation
  algorithms

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Scheduling Algorithms

• Graph-Theoretic Deterministic Algorithm
• Sender-Initiated Distributed Heuristic Algorithm
• Receiver-Initiated Distributed Heuristic
  Algorithm
• Please read from the book

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Virtualization

• Virtualization is the virtual machine concept
  introduced in Chapter 1 Introduction
• VM technology allows a single computer to host
  multiple virtual machines, each potentially
  running a different operating system
• Advantages
• Failure in one VM does not automatically bring
  down any others since VMs are isolated
• Fewer machines to provide an environment for
  different applications (instead of having one
  server for each of the different apllications, a
  server with VMs can do the job)
• Ability to run different and probably older
  versions of Oss needed by some appications

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Virtualization Methods

• Type 1 hypervisor (virtual machine monitor)
• Type 2 hypervisor