The Main Architectural Classes

Since many years the taxonomy of Flynn [#flynn##1#] has proven to be useful for the classification of high-performance computers. This classification is based on the way of manipulating of instruction- and data streams and comprises four main architectural classes. We will first briefly sketch these classes and afterwards fill in some details when each of the classes are described separately.

• SISD machines: These are the conventional systems that contain one CPU and hence can accommodate one instruction stream that is executed serially. Nowadays many large mainframes may have more than one CPU but each of these execute instruction streams that are unrelated. Therefore, such systems still should be regarded as (a couple of) SISD machines acting on different data spaces. Examples of SISD machines are for instance most workstations like those of DEC, Hewlett-Packard, and Sun Microsystems. The definition of SISD machines is given here for completeness' sake. We will not discuss this type of machines in this report.

• SIMD machines: Such systems often have a large number of processing units, ranging from 1,024 to 16,384 that all may execute the same instruction on different data in lock-step. So, a single instruction manipulates many data items in parallel. Examples of SIMD machines in this class are the CPP DAP Gamma and the MasPar MP-2.

• Another subclass of the SIMD systems are the vectorprocessors. Vectorprocessors act on arrays of similar data rather than on single data items using specially structured CPUs. When data can be manipulated by these vector units, results can be delivered with a rate of one, two and -- in special cases -- of three per clock cycle (a clock cycle being defined as the basic internal unit of time for the system). So, vector processors execute on their data in an almost parallel way but only when executing in vector mode. In this case they are several times faster than when executing in conventional scalar mode. For practical purposes vectorprocessors are therefore mostly regarded as SIMD machines. Examples of such systems are for instance the Convex C410, and the Hitachi S3600.

• MISD machines: Theoretically in these type of machines multiple instructions should act on a single stream of data. As yet no practical machine in this class has been constructed nor are such systems easily to conceive. We will disregard them in the following discussions.

• MIMD machines: These machines execute several instruction streams in parallel on different data. The difference with the multi-processor SISD machines mentioned above lies in the fact that the instructions and data are related because they represent different parts of the same task to be executed. So, MIMD systems may run many sub-tasks in parallel in order to shorten the time-to-solution for the main task to be executed. There is a large variety of MIMD systems and especially in this class the Flynn taxonomy proves to be not fully adequate for the classification of systems. Systems that behave very differently like a four-processor Cray Y-MP T94 and a thousand processor nCUBE 3 fall both in this class. In the following we will make another important distinction between classes of systems and treat them accordingly.
  • Shared memory systems: Shared memory systems have multiple CPUs all of which share the same address space. This means that the knowledge of where data is stored is of no concern to the user as there is only one memory accessed by all CPUs on an equal basis. Shared memory systems can be both SIMD or MIMD. Single-CPU vector processors can be regarded as an example of the former, while the multi-CPU models of these machines are examples of the latter. We will sometimes use the abbreviations SM-SIMD and SM-MIMD for the two subclasses.

  • Distributed memory systems: In this case each CPU has its own associated memory. The CPUs are connected by some network and may exchange data between their respective memories when required. In contrast to shared memory machines the user must be aware of the location of the data in the local memories and will have to move or distribute these data explicitly when needed. Again, distributed memory systems may be either SIMD or MIMD. The first class of SIMD systems mentioned which operate in lock step, all have distributed memories associated to the processors. For the distributed memory MIMD systems again a subdivision is possible: those in which the processors are connected in a fixed topology and those in which the topology is flexible and may vary from task to task. For the distributed memory systems we will sometimes use DM-SIMD and DM-MIMD to indicate the two subclasses.

Although the difference between shared- and distributed memory machines seems clear cut, this is not always entirely the case from user's point of view. For instance, the late Kendall Square Research systems employed the idea of ``virtual shared memory'' on a hardware level. Virtual shared memory can also be simulated at the programming level: The first draft proposal for High Performance Fortran (HPF) was published in November 1992 [#HPFspec##1#] which by means of compiler directives distributes the data over the available processors. The proposal was fixed by May 1993. Therefore, the system on which HPF is implemented will act in this case as a shared memory machine to the user. Other vendors of Massively Parallel Processing systems (the buzz-word MPP systems is fashionable here), like Convex and Cray, also support proprietary virtual shared-memory programming models which means that these physically distributed memory systems, by virtue of the programming model, logically will behave as shared memory systems. In addition, packages like TreadMarks [#tredmarks##1#] provide a virtual shared memory environment for networks of workstations.

Another trend that has came up in the last few years is distributed processing. This takes the DM-MIMD concept one step further: instead of many integrated processors in one or several boxes, workstations, mainframes, etc., are connected by Ethernet, FDDI, or otherwise and set to work concurrently on tasks in the same program. Conceptually, this is not different from DM-MIMD computing, but the communication between processors is often orders of magnitude slower. Many packages to realise distributed computing, commercial, and non-commercial are available. Examples of these are Parasoft's Express (commercial), PVM (standing for Parallel Virtual Machine, non-commercial) citepvm, and MPI (Message Passing Interface, [#mpi##1#] also non-commercial). PVM and MPI have been adopted for instance by Convex, Cray, IBM and Intel for the transition stage between distributed computing and MPP on the clusters of their favorite processors and they are available on a large amount of distributed memory MIMD systems and even on shared memory MIMD systems for compatibility reasons. In addition there is a tendency to cluster shared memory systems, for instance by HIPPI channels, to obtain systems with a very high computational power. E.g., Silicon Graphics is already providing such arrays of systems, the Intel Paragon with the MP (Multi Processor) nodes, and the NEC SX-4 also have this structure. The Convex Exemplar SPP-1200 could be seen as a more integrated example (although the software environment is much more complete and allows shared memory addressing).

• Shared-memory SIMD machines
• Distributed-memory SIMD machines
• Shared-memory MIMD machines
• Distributed-memory MIMD machines