A couple years ago I have started to write my second book, whose main point is to teach students elements of computer system design, largely speaking. Besides discussing the notion of systems, complexity and approaches to managing systems' complexity in a meaningful way, I want to give special attention to the programming languages. Source code transformed into a running program is an important part of system, the language design and the software design affect the resulting system in a lot of ways.
The book remains largely unfinished, but as a mean to push myself to complete it and to share the first versions of the chapters I will post them here.
The first chapters introduce the notions of systems and computer systems. Software and hardware engineers can greatly benefit from studying systems: system design techniques can be applied to designing all kinds of software, hardware, hybrid systems, distributed systems, and even applied to developer teams, which are systems themselves. However, we are not focusing on the systems theory itself, and many ideas can be expanded or made more precise.
Systems around us
In our lives we encounter multiple collections of objects that we call systems. System is a collection of components interacting with each other and outside world, giving the system additional properties, that individual components do not possess. Such properties are called systemic or emergent.
This drawing shows a system of three components, denoted A, B, and C. Component B interacts with A and C, while A and C do not interact directly with each other. A big circle is separating the system from the outside world.
The world around us is filled with systems. Here are some examples:
Cars can transport passengers and objects. This ability is a systemic property of cars. No part of a car can transport objects like a whole functioning car.
Moreover, when the car is assembled but not running, it stays still. Only the interaction between the parts of a car allows it to be a transport.
- Living humans are systems. Unlike in a corpse, our cells are interacting. Our ability to walk, think, breathe, being alive and conscious is our systemic property.
- Computers will not perform any work unless powered up and turned on. The ability to compute is their systemic property. It is born from interactions of software and CPU, memory, storage, buses, and other hardware components.
- A team of software, or hardware developers is unable to produce a complex piece of software unless all participants interact with each other, making sure that they are on the same page. The ability to produce an output together through solo work and interactions is a systemic property.
In this part we will continuously revisit these three examples: cars, humans, computers, and teams.
There is a saying: "a system is more than a sum of its parts." Indeed, a system is not only a sum of its parts, but also of interactions between them. Disregarding the interactions takes the emergent properties out of the picture.
Systems emerge in many areas of our lives, e.g., sociology, biology, physics, economics, computer science, and engineering. Systems theory has become an interdisciplinary field studying systems models at a high level of abstraction, disregarding specific domains.
Systems do not exist in the void, they are interacting with the world.
- Cars interact with the driver and the road, the trees around them, other cars.
- Humans interact with other humans, sounds, visuals, the air we breathe, and the food we consume.
- Computers interact with other devices, with humans through screens, keyboards, and other interfaces.
Environment is a collection of objects outside the system with which the system engages in meaningful interactions.
The context of our analysis dictates which objects should be part of the environment. The car might, in some cases, interact with blue whales, sound waves, bacteria, or asteroids, but most analyses can safely ignore them. If the system interacts with an object from an outside world, we include this object in the environment. The interaction should be essential for our analysis.
The environment usually includes an organized part and a chaotic part. Sometimes it may be viewed as a system on its own, consisting of multiple systems and interactions between them. If we design our own system, we often want to protect it from interacting with the chaotic part and set up the correct interactions with the organized part.
Usually only a handful of the system's components are interacting with the environment directly, and we call them the interface of the system. Here are some examples:
- The interface of cars includes its controls and wheels.
- The interface of humans includes their eyes, ears, and skin.
- The interface of computers includes external devices, e.g. keyboard, screens, mice, network adapters.
In other words, interface consists of such components that can be viewed both as parts of the system, or of its environment.
The interface can vary depending on the system's function: if we consider a verbal conversation between two humans, their interface is a spoken word; however, when these humans dance together, the interfacing happens through visual contact and touch.
An interface is like a border between the system and its environment. The border consists of such parts of the system that can be attributed to either the system itself or its environment. It is not always easy to define the border well, it may be blurred.
In computer systems, we peek through the interface of a computer system and observe the processes inside. Then we interpret our observations, and this interpretation is what constitutes computations.
Examples of computer systems in nature
To demonstrate the importance of our interpretation, let us study some systems where computations happen in unorthodox ways.
Computers are deterministic, so they do not have components to generate random numbers. Unless they incorporate a specific hardware component to generate randomness, they cannot achieve true non-determinism. Computers can imitate randomness through clever mathematical trickery, but such numbers, although they seem random, are predictable. However, actual random numbers have numerous uses, especially in cryptography.
One good way of generating actual random numbers is an online service located at
www.random.org. It uses electromagnetic noise from the atmosphere of Earth as a seed for the random number generator. Using such numbers for cryptography is much more adequate than pseudo-random numbers generated by a traditional computer.
We can say that the part of
random.orgthat performs computations is the atmosphere of Earth itself. It may seem strange because computation results come from observing nature, not through computations on a human-made computer. However, observing atmospheric noise and interpreting our observations, then plugging it into a more conventional computer system, provides us with numbers with very particular properties. What is it, if not a computing process?
- Another example is pulsars. Pulsars are stars at the late stages of their life, emitting bursts of light with high and very stable frequency. Therefore, they can be used as a clock: we count the bursts and know how many milliseconds have passed. Counting time is also computation.
The third example is ant colonies. Ants are searching for food in a process that can be described as a random walk. As they move, they mark their steps with a scent of pheromones.
When an ant worker reaches food or another resource, it goes back to the ant colony, tracing back its steps. Thanks to the smelly footprints, it can go back following precisely the same route from the ant colony to food.
However, its initial path can be far from optimal. For example, the ant may walk a full circle, so its patch will contain loops. Alternatively, it can go uphill instead of going around the hill.
Nature seeks efficiency, and ants use their capacities to recognize stronger and weaker smells to optimize their paths. The smell gradually evaporates, so the path to the ant colony has a weaker smell. Similarly, the path to the food was taken more recently and thus has a stronger smell. Turns out, this information is sufficient to shorten paths substantially.
So, if we put a piece of food in the proximity of an ant colony, we will find an optimal path from this colony to food. All of it — by observing ants, without any circuitry. This is a kind of computation that does not look different from something computed on your laptop. Actually, ant colonies have inspired some path search algorithms.
To sum it up, computations can be performed in a lot of ways, and interpretation plays a huge role in it. Sometimes nature performs computations without computers, and we interpret natural processes as computations.
Programs as systems
Writing programs is of particular interest to us. What is the relationship between programming and computer systems?
When we program, the outcome of our work is the source code of a program. It may be executed through interpretation, it may be compiled and then executed in a different form.
Systems are alive, their parts are interacting, and new properties emerge from these interactions.
The source code is dead. It is akin to a manual on how to assemble a piece of furniture, and someone or something else has to do the actual work.
However, a running program becomes a part of a hybrid hardware-software computer system. It is impossible to understand its behaviour without thinking about hardware, software, and their interaction. By reducing the system to just software or hardware we lose the other part from the sight, and we also lose their interactions.
The source code relates to the running program as a manual on how to assemble a piece of furniture relates to a system of furniture assembler, instruction manual, and furniture parts. This system is performing work on assembling the furniture.
The blueprint of an airplane is also like the program source code. If we load the program in memory and prepare for its execution, it will be an actual plane standing on the ground. A running program will be a flying airplane.
|furniture assembly manual
|working furniture assembler
Functional and structural decomposition
The system of interest may be very complex. It may be difficult to decompose, i.e., identify its parts. There are several distinct ways of doing it.
The first way is to decompose the system structurally. It is useful for the systems assembled from isolated parts, like a car or a house.
- Wheels, chairs, gearbox are all parts of a car.
- Bones, muscles, internal organs are parts of a human.
- Processor, motherboard, memory chips are parts of a computer.
The second, more useful way, is to decompose the system functionally. Each part is identified by its role: what is its function in a system? This is not the same as structurally decomposing system, because we only care about the functionality.
One structural part may participate in multiple interactions and play different roles.
Steering wheel, knobs, car computer, gear box, steering wheel booster and other parts of a car assist its driver in controlling the car; it is their function. Some of these parts have also other functions, like the car computer, which may regulate the fuel consumption. So, the car can be functionally decomposed in multiple parts, two of which are:
- A part to assist the driver.
- A part to regulate the fuel consumption.
It is impossible to isolate these two parts one from another; therefore it is not a structural decomposition.
- Bones are structural parts of humans, but their roles are many: they support us, they protect bone marrow, they allow an effective functioning of muscles and tendants.
- A hard disk drive may be a structural part of a computer. It has many roles, e.g., it is a data storage hosting partitions with filesystems; it also participates in the functioning of virtual memory, as we will see in Chapter 6.
- Scissors is another good example of a system with different functional and structural decomposition. It is able to cut paper better than a simple knife because of its systemic properties. Structurally scissors are composed of two pieces of metal and a screw that connects them together. Functionally, the same scissors consist of two parts: one is used to hold them, whereas another cuts paper. Not only do scissors have more structural parts than functional, we can not map structural parts to functional parts.
- Consider a team, a system of people working on the same project. Structurally, every person is a part of such system. Functionally, the parts of this system are roles: programmers, designers, managers, testers etc. One of the differences between them is that one person can play several roles (e.g. be a programmer and project manager, or programmer and graphics designer).
Functional view of systems
Functional way of viewing systems is considered more efficient for engineering computer systems, including programs. It emphasizes the important aspect of systems — their functionality. Then this functionality may be implemented in software, in hardware, or in a hybrid form. Later we will study an example of virtual memory; it is a mechanism implemented by an operating system that leverages certain hardware features. Virtual memory is not exclusively hardware or software.
Imagine we are building a system that requires intensive computations, like ray tracing in computer games. Its evolution may look like this:
- The starting point is a simple single-threaded algorithm to perform ray tracing on a single CPU. To speed it up, we provide the fastest CPU available.
- The algorithm is optimized on multiple levels to work as fast as possible.
- The algorithm is parallelized so that ray tracing is performed in multiple threads.
- The computations are moved to a dedicated graphics card. This requires redesigning the algorithm. The graphics card is often a good computation unit to run highly parallelized algorithms.
- If a graphics card is not efficient enough, or if it draws too much power, we may design our own chip. So the whole algorithm will be encoded as a circuit on a FGPA (field-programmable gate array) which will allow to maximize its speed and minimize the energy consumption.
At all these stages of life, the functional decomposition of the system was the same, but its structural decomposition has changed considerably. It is therefore more effective to think about functionality and roles of system parts, than to limit yourself with the fixed implementation straight away. The implementation should follow the functionality and various requirements, not the other way around.
Examples from computer systems
To get accustomed to structural and functional decompositions we will study examples from computer systems and programming.
- The first example is a cursor; we all see it constantly on computer screen as we point at things, drag them or otherwise interact with them. Cursor is a functional component, not a structural one. It is not equivalent to mouse, graphical tablet or any other device, because a program can take control over the cursor and move it around.
A lot of programs are being executed simultaneously on your computer. They coexist and are isolated from one another through virtual memory. The function of virtual memory is to store data. The virtual memory space itself consists of areas of forbidden addresses, files, mapped from hard drive, anonymous pages and so on. The virtual memory is backed by the physical memory, storage (in case of memory swapping or mapping files) and operating system, which ensures transparent operation with it.
It does not seem possible to divide computer system into structural parts in a way that isolates one virtual memory space from all others, so it is not a structural component. Another example is a process — we have all seen a list of processes in a task manager. A similar argument is applicable to it: we can not divide computer on parts structurally to isolate one process. A process contains a virtual address space too, parts of which are shared among processes. So a process is also a functional component.
- The final example is storage — its name alone suggests its function and hence that it is a functional component. It can be implemented as a piece of hardware, but the data can be stored in a distributed filesystem, in a distributed database, in cloud. In cloud storage it is not clear where a specific piece of data is stored due to factors like replication blurring it.
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