Fig. 1-2. Advantages of distributed systems over isolated (personal) computers.
1.2.3. Disadvantages of Distributed Systems
Although distributed systems have their strengths, they also have their weaknesses. In this section, we will point out a few of them. We have already hinted at the worst problem: software. With the current state-of-the-art, we do not have much experience in designing, implementing, and using distributed software. What kinds of operating systems, programming languages, and applications are appropriate for these systems? How much should the users know about the distribution? How much should the system do and how much should the users do? The experts differ (not that this is unusual with experts, but when it comes to distributed systems, they are barely on speaking terms). As more research is done, this problem will diminish, but for the moment it should not be underestimated.
A second potential problem is due to the communication network. It can lose messages, which requires special software to be able to recover, and it can become overloaded. When the network saturates, it must either be replaced or a second one must be added. In both cases, some portion of one or more buildings may have to be rewired at great expense, or network interface boards may have to be replaced (e.g., by fiber optics). Once the system comes to depend on the network, its loss or saturation can negate most of the advantages the distributed system was built to achieve.
Finally, the easy sharing of data, which we described above as an advantage, may turn out to be a two-edged sword. If people can conveniently access data all over the system, they may equally be able to conveniently access data that they have no business looking at. In other words, security is often a problem. For data that must be kept secret at all costs, it is often preferable to have a dedicated, isolated personal computer that has no network connections to any other machines, and is kept in a locked room with a secure safe in which all the floppy disks are stored. The disadvantages of distributed systems are summarized in Fig. 1-3.
Despite these potential problems, many people feel that the advantages outweigh the disadvantages, and it is expected that distributed systems will become increasingly important in the coming years. In fact, it is likely that within a few years, most organizations will connect most of their computers into large distributed systems to provide better, cheaper, and more convenient service for the users. An isolated computer in a medium-sized or large business or other organization will probably not even exist in ten years.
| Item | Description |
|---|---|
| Software | Little software exists at present for distributed systems |
| Networking | The network can saturate or cause other problems |
| Security | Easy access also applies to secret data |
Fig. 1-3. Disadvantages of distributed systems.
1.3. HARDWARE CONCEPTS
Even though all distributed systems consist of multiple CPUs, there are several different ways the hardware can be organized, especially in terms of how they are interconnected and how they communicate. In this section we will take a brief look at distributed system hardware, in particular, how the machines are connected together. In the next section we will examine some of the software issues related to distributed systems.
Various classification schemes for multiple CPU computer systems have been proposed over the years, but none of them have really caught on and been widely adopted. Probably the most frequently cited taxonomy is Flynn's (1972), although it is fairly rudimentary. Flynn picked two characteristics that he considered essentiaclass="underline" the number of instruction streams and the number of data streams. A computer with a single instruction stream and a single data stream is called SISD. All traditional uniprocessor computers (i.e., those having only one CPU) fall in this category, from personal computers to large mainframes.
The next category is SIMD, single instruction stream, multiple data stream. This type refers to array processors with one instruction unit that fetches an instruction, and then commands many data units to carry it out in parallel, each with its own data. These machines are useful for computations that repeat the same calculation on many sets of data, for example, adding up all the elements of 64 independent vectors. Some supercomputers are SIMD.
The next category is MISD, multiple instruction stream, single data stream. No known computers fit this model. Finally, comes MIMD, which essentially means a group of independent computers, each with its own program counter, program, and data. All distributed systems are MIMD, so this classification system is not tremendously useful for our purposes.
Although Flynn stopped here, we will go further. In Fig. 1-4, we divide all MIMD computers into two groups: those that have shared memory, usually called multiprocessors, and those that do not, sometimes called multicomputers. The essential difference is this: in a multiprocessor, there is a single virtual address space that is shared by all CPUs. If any CPU writes, for example, the value 44 to address 1000, any other CPU subsequently reading from its address 1000 will get the value 44. All the machines share the same memory.
Fig. 1-4. A taxonomy of parallel and distributed computer systems.
In contrast, in a multicomputer, every machine has its own private memory. If one CPU writes the value 44 to address 1000, when another CPU reads address 1000 it will get whatever value was there before. The write of 44 does not affect its memory at all. A common example of a multicomputer is a collection of personal computers connected by a network.
Each of these categories can be further divided based on the architecture of the interconnection network. In Fig. 1-4 we describe these two categories as bus and switched. By bus we mean that there is a single network, backplane, bus, cable, or other medium that connects all the machines. Cable television uses a scheme like this: the cable company runs a wire down the street, and all the subscribers have taps running to it from their television sets.
Switched systems do not have a single backbone like cable television. Instead, there are individual wires from machine to machine, with many different wiring patterns in use. Messages move along the wires, with an explicit switching decision made at each step to route the message along one of the outgoing wires. The worldwide public telephone system is organized in this way.
Another dimension to our taxonomy is that in some systems the machines are tightly coupled and in others they are loosely coupled. In a tightly-coupled system, the delay experienced when a message is sent from one computer to another is short, and the data rate is high; that is, the number of bits per second that can be transferred is large. In a loosely-coupled system, the opposite is true: the intermachine message delay is large and the data rate is low. For example, two CPU chips on the same printed circuit board and connected by wires etched onto the board are likely to be tightly coupled, whereas two computers connected by a 2400 bit/sec modem over the telephone system are certain to be loosely coupled.