Computer Architecture | Flynn’s taxonomy

Parallel computing is computing where the jobs are broken into discrete parts that can be executed concurrently. Each part is further broken down into a series of instructions. Instructions from each piece execute simultaneously on different CPUs. The breaking up of different parts of a task among multiple processors will help to reduce the amount of time to run a program. Parallel systems deal with the simultaneous use of multiple computer resources that can include a single computer with multiple processors, a number of computers connected by a network to form a parallel processing cluster, or a combination of both. Parallel systems are more difficult to program than computers with a single processor because the architecture of parallel computers varies accordingly and the processes of multiple CPUs must be coordinated and synchronized. The difficult problem of parallel processing is portability.

An Instruction Stream is a sequence of instructions that are read from memory. Data Stream is the operations performed on the data in the processor.

Flynn’s taxonomy is a classification scheme for computer architectures proposed by Michael Flynn in 1966. The taxonomy is based on the number of instruction streams and data streams that can be processed simultaneously by a computer architecture.

There are four categories in Flynn’s taxonomy:

1. Single Instruction Single Data (SISD): In a SISD architecture, there is a single processor that executes a single instruction stream and operates on a single data stream. This is the simplest type of computer architecture and is used in most traditional computers.
2. Single Instruction Multiple Data (SIMD): In a SIMD architecture, there is a single processor that executes the same instruction on multiple data streams in parallel. This type of architecture is used in applications such as image and signal processing.
3. Multiple Instruction Single Data (MISD): In a MISD architecture, multiple processors execute different instructions on the same data stream. This type of architecture is not commonly used in practice, as it is difficult to find applications that can be decomposed into independent instruction streams.
4. Multiple Instruction Multiple Data (MIMD): In a MIMD architecture, multiple processors execute different instructions on different data streams. This type of architecture is used in distributed computing, parallel processing, and other high-performance computing applications.

Flynn’s taxonomy is a useful tool for understanding different types of computer architectures and their strengths and weaknesses. The taxonomy highlights the importance of parallelism in modern computing and shows how different types of parallelism can be exploited to improve performance.

 systems are classified into four major categories: Flynn’s classification –

1. Single-instruction, single-data (SISD) systems – An SISD computing system is a uniprocessor machine that is capable of executing a single instruction, operating on a single data stream. In SISD, machine instructions are processed in a sequential manner and computers adopting this model are popularly called sequential computers. Most conventional computers have SISD architecture. All the instructions and data to be processed have to be stored in primary memory. The speed of the processing element in the SISD model is limited(dependent) by the rate at which the computer can transfer information internally. Dominant representative SISD systems are IBM PC, and workstations.
2. Single-instruction, multiple-data (SIMD) systems – An SIMD system is a multiprocessor machine capable of executing the same instruction on all the CPUs but operating on different data streams. Machines based on a SIMD model are well suited to scientific computing since they involve lots of vector and matrix operations. So that the information can be passed to all the processing elements (PEs) organized data elements of vectors can be divided into multiple sets(N-sets for N PE systems) and each PE can process one data set. Dominant representative SIMD systems are Cray’s vector processing machines.
3. Multiple-instruction, single-data (MISD) systems – An MISD computing system is a multiprocessor machine capable of executing different instructions on different PEs but all of them operate on the same dataset. Example Z = sin(x)+cos(x)+tan(x) The system performs different operations on the same data set. Machines built using the MISD model are not useful in most applications, a few machines are built, but none of them are available commercially.
4. Multiple-instruction, multiple-data (MIMD) systems – An MIMD system is a multiprocessor machine that is capable of executing multiple instructions on multiple data sets. Each PE in the MIMD model has separate instruction and data streams; therefore machines built using this model are capable of any application. Unlike SIMD and MISD machines, PEs in MIMD machines work asynchronously. MIMD machines are broadly categorized into shared-memory MIMD and distributed-memory MIMD based on the way PEs are coupled to the main memory. In the shared memory MIMD model (tightly coupled multiprocessor systems), all the PEs are connected to a single global memory and they all have access to it. The communication between PEs in this model takes place through the shared memory, modification of the data stored in the global memory by one PE is visible to all other PEs. The dominant representative shared memory MIMD systems are Silicon Graphics machines and Sun/IBM’s SMP (Symmetric Multi-Processing). In Distributed memory MIMD machines (loosely coupled multiprocessor systems) all PEs have a local memory. The communication between PEs in this model takes place through the interconnection network (the inter-process communication channel, or IPC). The network connecting PEs can be configured to tree, mesh, or in accordance with the requirement. The shared-memory MIMD architecture is easier to program but is less tolerant to failures and harder to extend with respect to the distributed memory MIMD model. Failures in a shared-memory MIMD affect the entire system, whereas this is not the case in the distributed model, in which each of the PEs can be easily isolated. Moreover, shared memory MIMD architectures are less likely to scale because the addition of more PEs leads to memory contention. This is a situation that does not happen in the case of distributed memory, in which each PE has its own memory. As a result of practical outcomes and user requirements, distributed memory MIMD architecture is superior to the other existing models.

Flynn’s taxonomy itself does not have any inherent advantages or disadvantages. It is simply a classification scheme for computer architectures based on the number of instruction streams and data streams that can be processed simultaneously.

However, the different types of computer architectures that fall under Flynn’s taxonomy have their own advantages and disadvantages. Here are some examples:

1. SISD architecture: This is the simplest and most common type of computer architecture. It is easy to program and debug and can handle a wide range of applications. However, it does not offer significant performance gains over traditional computing systems.
2. SIMD architecture: This type of architecture is highly parallel and can offer significant performance gains for applications that can be parallelized. However, it requires specialized hardware and software and is not well-suited for applications that cannot be parallelized.
3. MISD architecture: This type of architecture is not commonly used in practice, as it is difficult to find applications that can be decomposed into independent instruction streams.
4. MIMD architecture: This type of architecture is highly parallel and can offer significant performance gains for applications that can be parallelized. It is well-suited for distributed computing, parallel processing, and other high-performance computing applications. However, it requires specialized hardware and software and can be challenging to program and debug.

Overall, the advantages and disadvantages of different types of computer architectures depend on the specific application and the level of parallelism that can be exploited. Flynn’s taxonomy is a useful tool for understanding the different types of computer architectures and their potential uses, but ultimately the choice of architecture depends on the specific needs of the application.

Some additional features of Flynn’s taxonomy include:

Concurrency: Flynn’s taxonomy provides a way to classify computer architectures based on their concurrency, which refers to the number of tasks that can be executed simultaneously.

Performance: Different types of architectures have different performance characteristics, and Flynn’s taxonomy provides a way to compare their performance based on the number of concurrent instructions and data streams.

Parallelism: Flynn’s taxonomy highlights the importance of parallelism in computer architecture and provides a framework for designing and analyzing parallel processing systems.

Reference – Flynn’s taxonomy Mastering Cloud Computing: Foundations and Applications Programming