Direct and Indirect Interconnection - Parallel Processing - Lecture Slides, Slides of Parallel Computing and Programming

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Lecture 4: Direct and Indirect Interconnection Networks for Distributed-Memory Multiprocessors

Interconnection Networks

for Parallel Computers

• Interconnection networks carry data between processors and to memory.
• Interconnects are made of switches and links (wires, fiber).
• Interconnects are classified as static or dynamic.
• Static networks consist of point-to-point communication links among processing nodes and are also referred to as direct networks.
• Dynamic networks are built using switches and communication links. Dynamic networks are also referred to as indirect networks.

Interconnection Networks

• Switches map a fixed number of inputs to outputs.
• The total number of ports on a switch is the degree of the switch.
• The cost of a switch grows as the square of the degree of the switch, the peripheral hardware linearly as the degree, and the packaging costs linearly as the number of pins.

Interconnection Networks:

Network Interfaces

• Processors talk to the network via a network interface.
• The network interface may hang off the I/O bus or the memory bus.
• The relative speeds of the I/O and memory buses impact the performance of the network.

Network Topologies: Buses

• Some of the simplest and earliest parallel machines used buses.
• All processors access a common bus for exchanging data.
• The distance between any two nodes is O(1) in a bus. The bus also provides a convenient broadcast media.
• However, the bandwidth of the shared bus is a major bottleneck.
• Typical bus based machines are limited to dozens of nodes. Sun Enterprise servers and Intel Pentium based shared-bus multiprocessors are examples of such architectures.

Network Topologies: Buses

Bus-based interconnects (a) with no local caches; (b) with local memory/caches. Since much of the data accessed by processors is local to the processor, a local memory can improve the performance of bus-based machines.

Network Topologies: Crossbars

• The cost of a crossbar of p processors grows as O(p^2 ).
• This is generally difficult to scale for large values of p.
• Examples of machines that employ crossbars include the Sun Ultra HPC 10000 and the Fujitsu VPP500.

Network Topologies:

Multistage Networks

• Crossbars have excellent performance scalability but poor cost scalability.
• Buses have excellent cost scalability, but poor performance scalability.
• Multistage interconnects strike a compromise between these extremes.

Network Topologies: Multistage Omega Network

• One of the most commonly used multistage interconnects is the Omega network.
• This network consists of log p stages, where p is the number of inputs/outputs.
• At each stage, input i is connected to output j if:

Essentially, j is the rotate-left operation of the bit-repr. of i

Network Topologies: Multistage Omega Network

Each stage of the Omega network implements a perfect shuffle as follows:

A perfect shuffle interconnection for eight inputs and outputs.

Network Topologies: Multistage Omega Network

A complete omega network connecting eight inputs and eight outputs. An omega network has p/2 × log p switching nodes, and the cost of such a network grows as (p log p).

A complete Omega network with the perfect shuffle interconnects and switches can now be illustrated:

Network Topologies:

Multistage Omega Network – Routing

• Let s be the binary representation of the source and d be that of the destination processor.
• The data traverses the link to the first switching node. If the most significant bits of s and d are the same, then the data is routed in pass-through mode by the 1 ’st level switch else, it switches to crossover.
• If the j’th bits (counting from the left) of s and d are the same, then data is routed in pass- through mode by the ((log P)-j)’th level switchDocsity.com

Network Topologies:

Completely Connected Network

• Each processor is connected to every other processor.
• The number of links in the network scales as O(p^2 ).
• While the performance scales very well, the hardware complexity is not realizable for large values of p.
• In this sense, these networks are static counterparts of crossbars.

Network Topologies: Completely Connected and Star Connected Networks

Example of an 8-node completely connected network.

(a) A completely-connected network of eight nodes; (b) a star connected network of nine nodes.