If you ever opened a computer science textbook, it probably introduced *computational complexity* somewhere in the very beginning. Simply put, it is the total count of *elementary operations* (additions, multiplications, reads, writes…) that are executed during a computation, optionally weighted by their *costs*.

Complexity is a very old concept. It was systematically formulated in the early 1960s, and since then it has been universally used as the cost function for designing algorithms. The reason this model was so quickly adopted is because it actually was a very good approximation of how computers worked at the time.

## Classical Complexity Theory

The “elementary operations” of a CPU are called *instructions*, and their “costs” are called *latencies*. Instructions are stored in memory and executed one by one by the processor, which has some internal *state* stored in a number of *registers*, one which is the *instruction pointer* that indicates the address of the next instruction to read and execute. Each instruction changes the state of the processor in a certain way (including moving the instruction pointer), possibly modifies the main memory and takes different amount of *CPU cycles* to complete before the next one can be started.

To estimate the real running time of a program, you need to sum all latencies for its instructions and multiply by *clock frequency*, that is, the number of cycles a particular CPU does per second.

The clock frequency is a volatile and often unknown variable that depends on the CPU model, operating system settings, microchip temperature, power usage of other components and quite a few other things. In contrast, instruction latencies are static and even somewhat consistent across different CPUs when expressed in clock cycles, and so counting them instead is much more useful for analytical purposes.

For example, matrix multiplication requires the total of $n^2 \cdot (n + n - 1)$ arithmetic operations: specifically, $n^3$ multiplications and $n^2 \cdot (n - 1)$ additions. If we look up the latencies for these instructions (in special documents called *instruction tables*, like this one), we can find that multiplication takes e. g. 3 cycles, while addition takes 1, so we need a total of $3 \cdot n^3 + n^2 \cdot (n - 1) = 4 \cdot n^3 - n^2$ clock cycles for the entire computation (bluntly ignoring everything else that needs to be done in order to feed these instructions with the right data).

Similar to how the sum of instruction latencies can be used as a clock-independent proxy for total execution time, computational complexity can be used to quantify the *intrinsic time requirements* of an abstract algorithm, without relying on the choice of a specific computer.

### Asymptotic Complexity

The idea to express execution time as a function of input size seems obvious now, but it wasn’t so in the 1960s. Back then, typical computers cost millions of dollars, were so large they required a separate room and had clock rates measured in kilohertz. They were used for practical tasks at hand, like predicting weather, sending rockets into space, or figuring out how far a Soviet nuclear missile can fly from the coast of Cuba — all of which are finite-length problems. People were mainly concerned with how to multiply $3 \times 3$ matrices rather than $n \times n$ ones.

What caused the shift in approach was the acquired confidence among computer scientists that computers will continue to become faster — which they did. Over time, people stopped counting time, then stopped counting cycles, then even stopped counting operations exactly, replacing it with an estimate that, that, on sufficiently large inputs, is only off by no more than a constant factor — called *asymptotic complexity* — which turns verbose “$4 \cdot n^3 - n^2$ operations” into plain “$\Theta(n^3)$”, hiding the initial “costs” and counts of individual operations along with all the other intricacies of the hardware in the “Big O”.

The reason we use asymptotic complexity is because it provides simplicity while still being just precise enough to yield useful results about relative algorithm performance on large datasets. Under the promise that computers will eventually become fast enough to handle any “sufficiently large” input in a reasonable amount of time, asymptotically faster algorithms will always be faster in real time too, regardless of the hidden constant.

But this promise turned out to be not true, at least not in terms of clock speeds and instruction latencies.

## Microchips

The main disadvantage of the supercomputers of 1960s wasn’t that they were slow — relatively speaking, they weren’t — but that they were giant, complex to use, and so expensive that only the governments of world superpowers could afford them. Their size was in fact the reason they were so expensive: they required a lot of custom components that had to be very carefully assembled in the macro-world by people holding advanced degrees in electrical engineering, in a process that couldn’t be up-scaled for mass production.

The tipping point was the development of *microchips* — single, tiny, complete circuits — which revolutionized the industry and turned out to be probably the most important invention of the 20th century. What was a multimillion-dollar cupboard of computing machinery in 1965 could in 1975 fit on a 4×4 mm slice of silicon^{1} that you can buy for $25. This dramatic improvement in affordability started the home computer revolution during the following decade, with computers like Apple II, Atari 2600, Commodore 64 and IBM PC becoming available to the masses.

### Fabrication Process

Microchips are “printed” on a slice of crystalline silicon using a process called photolithography, which involves

- growing and slicing a very pure silicon crystal,
- covering it with a layer of substance that dissolves when photons hit it,
- hitting it with photons in a set pattern,
- chemically etching the now exposed parts,
- removing the remaining photoresist,

…and then performing another 40-50 steps over the span of several months to complete the rest of the CPU.

Consider now the “hit it with photons” part. For that, we can use a system of lenses that projects a pattern onto a much smaller area, effectively making a tiny circuit with all the desired properties. This way, the optics of the 1970s were able to fit a few thousand transistors on the size of a fingernail, which gives microchips several key advantages that macro-world computers didn’t have:

- higher clock rates (that were previously limited by the speed of light);
- ability to scale the production;
- much lower material and power usage, translating to much lower cost per unit.

Apart from these immediate benefits, photolithography enabled a clear path to improve performance further: you can just make lenses stronger, which in turn will create identically smaller devices with relatively little effort.

### Dennard Scaling

Consider what happens when we scale a microchip down. A smaller, but nearly identical circuit requires proportionally less materials, and smaller transistors take less time to switch (along with all other physical processes in the chip), allowing reducing the voltage and increasing the clock rate.

A more detailed observation, known as the *Dennard scaling*, states that reducing transistor dimensions by 30%

- doubles the transistor density ($0.7^2 \approx 0.5$),
- increases the clock speed by 40% ($\frac{1}{0.7} \approx 1.4$),
- and leaves the overall
*power density*the same.

Since the per-unit manufacturing cost is a function of area, and the exploitation cost is mostly the cost of power^{2}, each new “generation” should have roughly the same total cost, but 40% higher clock and twice as many transistors, which can be promptly used, for example, to add new instructions or increase the word size — to keep up with the same miniaturization happening in memory microchips.

Due to the trade-offs between energy and performance you can make during the design, the fidelity of the fabrication process itself, such as “180nm” or “65nm”, directly translating to the density of transistors, became the trademark for CPU efficiency.

Throughout most of the computing history, optical shrinking was the main driving force behind performance improvements. Gordon Moore, the former CEO of Intel, made a prediction in 1975 that the transistor count in microprocessors will double every two years. His prediction held to this day and became known as *Moore’s law*.

Both Dennard scaling and Moore’s law are not actual laws of physics, but just observations made by savvy engineers. They are both destined to stop at some point due to fundamental physical limitations, the ultimate one being the size of silicon atoms. In fact, Dennard scaling already did — due to power issues.

Thermodynamically, a computer is just a very efficient device for converting electrical power into heat. This heat eventually needs to be removed, and there are physical limits to how much power you can dissipate from a millimeter-scale crystal. Computer engineers, aiming to maximize performance, essentially just choose the maximum possible clock rate so that the overall power consumption stays the same. If transistors become smaller, they have less capacity, meaning less required voltage to flip them, which in turn allows increasing the clock rate.

Around 2005–2007, this strategy stopped working because of *leakage* effects: the circuit features became so small that their magnetic fields started to make the electrons in the neighboring circuitry move in directions they are not supposed to, causing unnecessary heating and occasional bit flipping.

The only way to mitigate this is to increase voltage, and to balance off power consumption you need to reduce clock frequency, which makes the whole process progressively less profitable as transistor density increases. At some point, clock rates could no longer be increased by scaling, and then miniaturization trend started to slow down.

### Modern Computing

Dennard scaling ended, but Moore’s law has not died yet.

Clock rates plateaued, but the transistor count is still increasing, allowing for creation of new, *parallel* hardware. Instead of chasing faster cycles, CPU designs started to focus on getting more useful things done in a single cycle. Instead of getting smaller, transistors have been changing shape.

This resulted in increasingly complex architectures capable of doing dozens, hundreds or thousands of different things every cycle. For modern computers, the “let’s count all operations” approach for predicting algorithm performance isn’t just slightly wrong, but is off by several orders of magnitude.

Theoretical Computer Science is now what’s called a dead field: really exciting things stopped happening in the 70s; everything past that are just attempts to replace logarithms in the asymptotic with something slightly less than logarithms. If 50 years ago such algorithms had hope that eventually there will be enough computing power to process the large datasets for which they beat their asymptotically inferior, but practical counterparts, nowadays we know for a fact that they never will.

This is what this book is about: accepting the reality and optimizing for the hardware you have, beyond just asymptotic complexity. You will probably not learn a single asymptotically faster algorithm here, but you will learn how to squeeze performance from all of non-exponentially-increasing transistors you have, which is a far more impactful skill.