CPU Performance
Two computers can look almost identical yet one feels far quicker than the other. The difference usually comes down to the CPU and how fast it can work through the fetch-execute cycle. We call this its performance: roughly, how many instructions it can carry out each second. Three characteristics have the biggest effect on how well a CPU performs — its clock speed, the size of its cache, and its number of cores.
Understanding each one, and how they combine, lets you explain why a gaming desktop outperforms a budget laptop, and why upgrading just one part of a system does not always give the speed boost you might expect.
Clock speed
Inside every CPU is a clock that sends out a steady electrical pulse. Each pulse triggers the processor to carry out one step of its work. Clock speed is the number of these pulses, or cycles, per second. It is measured in hertz (Hz). Because modern processors are so fast, clock speed is usually given in gigahertz (GHz), where one gigahertz equals one billion cycles per second. A CPU running at 3.5 GHz completes 3.5 billion cycles every single second.
As a rough guide, more cycles per second means more instructions carried out per second, so a higher clock speed usually gives better performance. Pushing a CPU beyond its factory clock speed is called overclocking; it can raise speed but produces extra heat, which is why fast computers need effective cooling such as fans or heat sinks.
One gigahertz (GHz) means one billion clock cycles per second, so a 3.0 GHz CPU completes three billion cycles every second. Clock speed is measured in hertz.
Cache size
Fetching data from the computer’s main memory (RAM) takes time, and while the CPU waits it cannot do useful work. To cut this delay, the CPU contains a small amount of extremely fast memory called cache. Cache holds copies of the data and instructions the CPU uses most often, right beside the processor where they can be reached almost instantly.
When the CPU finds what it needs in the cache, this is a cache hit. When it does not, and has to fetch from slower RAM instead, this is a cache miss. A larger cache can hold more data, so more requests become cache hits and fewer slow trips to RAM are needed. That is why increasing the cache size improves performance.
Cache is usually split into levels. L1 cache is the smallest and fastest, built right into the core. L2 cache is larger but slightly slower. L3 cache is the largest and slowest of the three, though still far faster than RAM. The CPU checks L1 first, then L2, then L3, and only goes out to RAM if the data is not in any level of cache.
Cache is small, fast memory inside the CPU that stores frequently used data and instructions. A cache hit avoids a slower trip to RAM; a larger cache produces more hits.
Number of cores
A core is a complete processing unit inside the CPU that can run the fetch-execute cycle by itself. Early CPUs had a single core and could only work on one instruction at a time. Modern CPUs are usually multi-core — dual-core (two cores), quad-core (four) or even octa-core (eight) — with several cores built onto one chip.
Having more cores lets a CPU carry out parallel processing: different cores work on different instructions at the same moment. In theory a quad-core CPU can handle four sets of instructions at once, which is a real advantage when running several programs together, or one program written to share its work across cores.
More cores only help if the software is written to split its work into parts that can run at the same time. Some tasks are sequential — each step depends on the result of the last — and cannot be shared out, so they run no faster on eight cores than on one. Cores also share resources such as memory, so doubling the number of cores rarely doubles real-world performance.
How the three work together
These characteristics affect performance both individually and in combination. A CPU with a very high clock speed but a tiny cache may keep stalling while it waits for data from RAM. Adding more cores helps with several tasks at once but does little for a single, simple job that only uses one core. The strongest processors balance all three: a fast clock, a generous cache and several cores.
There are trade-offs, too. Higher clock speeds and extra cores both draw more electrical power and give off more heat, which matters in laptops and phones where battery life and cooling are limited. Designers pick the balance that suits the device, which is why the CPU in a desktop gaming PC is built very differently from the chip in a smartphone.