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The Cross Bar Switch

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• Context of the discussion
  • Recall the Shared-Memory MIMD is connected as follows:

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  • The memory is shared and is fragmented into N parts

    Example:

    8 G Bytes of memory, fragmented into 8 modules (banks): Bank 0: addresses 0 - (220 - 1) Bank 1: addresses 220 - (2 * 220 - 1) Bank 2: addresses 2*220 - (3 * 220 - 1) Bank 3: addresses 3*220 - (4 * 220 - 1) Bank 4: addresses 4*220 - (5 * 220 - 1) Bank 5: addresses 5*220 - (6 * 220 - 1) Bank 6: addresses 6*220 - (7 * 220 - 1) Bank 7: addresses 7*220 - (8 * 220 - 1)

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• The Crossbar Switch
  • A crossbar switch is a switch connecting multiple inputs to multiple outputs in a matrix manner.

  • Architecture of a uni-directional crossbar switch:
    1. The crossbar switch can switch inputs (yellow color squares) to the outputs (cyan color squares) - imagine each box is a CPU or a memory module.

    2. The setting of the switching fabric is under the control of a switching logic circuit

    3. The sources (inputs) first send request signals to the switching logic circuit.

       The switching logic circuit will set the switching fabric to route the requests from the input to their destinations.

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  • Example:
    • CPU 0 wants to access memory module 3 (needs to send address value to memory module 3)
    • CPU 1 wants to access memory module 1 (needs to send address value to memory module 1)
    • CPU 2 wants to access memory module 0 (needs to send address value to memory module 0)
    • CPU 3 wants to access memory module 2 (needs to send address value to memory module 2)

    Setting of the switching control to enable the transmission of the memory addresses from CPU to the memory modules:

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• Conflicting requests
  • Different inputs can request the same output.

  • Example:
    • CPU 0 wants to access memory module 3 (needs to send address value to memory module 3)
    • CPU 1 wants to access memory module 3 (needs to send address value to memory module 3)
    • CPU 2 wants to access memory module 0 (needs to send address value to memory module 0)
    • CPU 3 wants to access memory module 2 (needs to send address value to memory module 2)

  • The switching logic must resolve conflicts and determine which request will be forwarded (e.g., lowest CPU id will get through):

    The "denied" CPU will get a WAIT signal and will try its request again.

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• The Non-Blocking property
  • A switching fabric is called non-blocking if multiple simultaneous requests destined for different outputs/destinations can always be forwarded.

  • Clearly, the Cross Bar switching fabric is non-blocking in terms of the number of inputs (that's too fast...)

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• Advantages and disadvantages of the cross bar switch
  • Advantage: maximum amount of data transfered in each transfer operation

  • Disvantage: size switching fabric grows quadratically in terms of the number of inputs (that's too fast...)

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• Bi-directional transfer
  • Bi-directional transfer is accomplished by:

    use the first cross bar switch to connect from the CPUs to the memory modules use a second cross bar switch connecting from memory modules to the CPUs