Factors affecting computer system performance

Number of processors (cores)

A CPU can contain one or more processing units. Each unit is called a core. A core contains an ALU, control unit and registers. It is common for computers to have two (dual), four (quad) or even more cores. CPUs with multiple cores have more power to run multiple programs at the same time.

However, doubling the number of cores will not simply double a computer's speed. CPU cores have to communicate with each other through channels and this uses up some of the extra speed.

As the number of cores increases, the number of channels increases rapidly

Therefore, if we increase the number of cores in a processor, there will be an increase in system performance.

Width of the data bus

The data bus is a set of parallel wires or connectors that transports data between the processor and main memory.

By increasing the data bus from 32-bit to 64-bit, the computer can transfer twice as much information at one time. Therefore, increasing the size of the data bus improves the system performance of the computer.

Cache memory

Cache is a small amount of memory which is a part of the CPU - closer to the CPU than RAM. It is used to temporarily hold instructions and data that the CPU is likely to reuse.

The CPU control unit automatically checks cache for instructions before requesting data from RAM. This saves fetching the instructions and data repeatedly from RAM – a relatively slow process which might otherwise keep the CPU waiting. Transfers to and from cache take less time than transfers to and from RAM.

The more cache there is, the more data can be stored closer to the CPU.

Cache is graded as Level 1 (L1), Level 2 (L2) and Level 3 (L3):

  • L1 is usually part of the CPU chip itself and is both the smallest and the fastest to access. Its size is often restricted to between 8 KB and 64 KB.
  • L2 and L3 caches are bigger than L1. They are extra caches built between the CPU and the RAM. Sometimes L2 is built into the CPU with L1. L2 and L3 caches take slightly longer to access than L1. The more L2 and L3 memory available, the faster a computer can run.

Not a lot of physical space is allocated for cache. There is more space for RAM, which is usually larger and less expensive.

Clock speed

The clock speed - also known as clock rate - indicates how fast the CPU can run. This is measured in megahertz (MHz) or gigahertz (gHz) and corresponds with how many instruction cycles the CPU can deal with in a second. A 2 gHz CPU performs two billion cycles a second. A faster CPU uses more energy and creates more heat.

A computer will normally have a maximum clock speed set by default, but it is possible to change this speed in the computer BIOS. Some people increase a CPU clock speed to try to make their computer run faster - this is called overclocking.

There are limits to how fast a CPU can run and its circuitry cannot always keep up with an overclocked speed. If the clock tells the CPU to execute instructions too quickly, the processing will not be completed before the next instruction is carried out. If the CPU cannot keep up with the pace of the clock, the data is corrupted. CPUs can also overheat if they are forced to work faster than they were designed to work.

CPU expert Steve Furber explains why modern CPUs have multiple cores

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