19-05-2012, 09:58 AM
. Interconnection Networks and Clusters
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1. Interconnection Networks Media
There is a hierarchy of media to interconnect computers that varies in cost, performance, and reliability. Network media have another figure of merit, the maximum distance between nodes. This section covers three popular examples, and Figure 8.11 illustrates them.
The frst medium is twisted pairs of copper wires. These are two insulated wires, each about 1 mm thick. They are twisted together to reduce electrical interference, since two parallel lines form an antenna but a twisted pair does not. As they can transfer a few megabits per second over several kilometers without amplification, twisted pair were the mainstay of the telephone system. Telephone companies bundled together (and sheathed) many pairs coming into a building. Twisted pairs can also offer tens of megabits per second of bandwidth over shorter distances, making them plausible for LANs.
The original telephone-line quality was called Level 1. Level 3 was good enough for 10 Mbits/second Ethernet. The desire for even greater bandwidth lead to the Lev-el 5 or Category 5, which is sufficient for 100 Mbits/second Ethernet. By limiting the length to 100 meters, “Cat5” wiring can be used for 1000 Mbits/second Ethernet links today. It uses the RJ-45 connector, which is similar to the connector found on telephone lines.
Coaxial cable was deployed by cable television companies to deliver a higher rate over a few kilometers. To offer high bandwidth and good noise immunity, insulating material surrounds a single stiff copper wire, and then cylindrical conductor surrounds the insulator, often woven as a braided mesh. A 50-ohm baseband coaxial cable delivers 10 megabits per second over a kilometer.
The third transmission media is Fiber optics which transmits digital data as pulses of light. A fiber optic network has three components:
1 the transmission medium, a fiber optic cable;
2 the light source, an LED or laser diode;
3 the light detector, a photodiode.
First, cladding surrounds the glass fiber core to confine the light. A buffer then surrounds the cladding to protect the core and cladding. Note that unlike twisted pairs or coax, fibers are one-way, or simplex, media. A two-way, or full duplex, connection between two nodes requires two fibers.
Since light bends or refracts at interfaces, it can slowly spread as it travels down the cable unless the diameter of the cable is limited to one wavelength of light; then it transfers in a straight line. Thus, fiber optic cables are of two forms:
1. Multimode fiber—It uses inexpensive LEDs as a light source. It is typically much larger than the wavelength of light: typically 62.5 microns in diameter vs. the 1.3-micron wavelength of infrared light. Since it is wider it has more dispersion problems, where some wave frequencies have different propaga-tion velocities. The LEDs and dispersion limit it to up to a few hundred meters at 1000 Mbits/second or a few kilometers at 100 Mbits /second. It is older and less expensive than single mode fiber.
Practical Issues for Commercial Interconnection Networks :
There are practical issues in addition to the technical issues described so far that are important considerations for some interconnection networks: connectivity, standardization, and fault tolerance.
Connectivity
The number of machines that communication affects the complexity of the net-work and its protocols. The protocols must target the largest size of the network, and handle the types of anomalous events that occur. Hundreds of machines communicating are a much easier than millions.
Connecting the Network to the Computer
Computers have a hierarchy of buses with different cost/performance. For example, a personal computer in 2001 has a memory bus, a PCI bus for fast I/O de-vices, and an USB bus for slow I/O devices. I/O buses follow open standards and have less stringent electrical requirements. Memory buses, on the other hand, provide higher bandwidth and lower latency than I/O buses. Where to connect the network to the machine depends on the performance goals, all LANs and WANs plug into the I/O bus.
The location of the network connection significantly affects the software interface to the network as well as the hardware. A memory bus is more likely to be cache-coherent than an I/O bus and therefore more likely to avoid these extra cache flishes. DMA is the best way to send large messages. Whether to use DMA to send small messages depends on the efficiency of the interface to the DMA. The DMA interface is usually memory-mapped, and so each interaction is typically at the speed of main memory rather than of a cache access.
Standardization: Cross-Company Interoperability
LANs and WANs use standards and interoperate effectively. WANs involve many types of companies and must connect to many brands of computers, so it is difficult to imagine a proprietary WAN ever being successful. The ubiquitous nature of the Ethernet shows the popularity of standards for LANs as well as WANs, and it seems unlikely that many customers would tie the viability of their LAN to the stability of a single company.
Message Failure Tolerance
The communication system must have mechanisms for retransmission of a message in case of failure. Often it is handled in higher layers of the software protocol at the end points, requiring retransmission at the source. Given the long time of flight for WANs, often they can retransmit from hop to hop rather relying only on retransmission from the source.
Node Failure Tolerance
The second practical issue refers to whether or not the interconnection relies on all the nodes being operational in order for the interconnection to work properly. Since software failures are generally much more frequent than hardware failures, the question is whether a software crash on a single node can prevent the rest of the nodes from communicating.
Clearly, WANs would be useless if they demanded that thousands of computers spread Clusters
There are many mainframe applications––such as databases, file servers, Web servers, simulations, and multiprogramming/batch processing––amenable to running on more loosely coupled machines than the cache-coherent NUMA machines. These applications often need to be highly available, requiring some form of fault tolerance and repairability. Such applications––plus the similarity of the multiprocessor nodes to desktop computers and the emergence of high-bandwidth, switch-based local area networks—lead to clusters of off-the-shelf, whole computers for large-scale processing.
Performance Challenges of Clusters
One drawback is that clusters are usually connected using the I/O bus of the computer, whereas multiprocessors are usually connected on the memory bus of the computer. The memory bus has higher bandwidth and much lower latency, allowing multiprocessors to drive the network link at higher speed and to have fewer conflicts with I/O traffic on I/O-intensive applications. This connection point also means that clusters generally use software-based communication while multiprocessors use hardware for communication. However, it makes connections non-standard and hence more expensive.
A second weakness is the division of memory: a cluster of N machines has N independent memories and N copies of the operating system, but a shared address multiprocessor allows a single program to use almost all the memory in the computer. Thus, a sequential program in a cluster has 1/Nth the memory available compared to a sequential program in a shared memory multiprocessor. Interestingly, the drop in DRAM prices has made memory costs so low that this multi-processor advantage is much less important in 2001 than it was in 1995. The primary issue in 2001 is whether the maximum memory per cluster node is