L2 - Factors that affect performance of CPU

it is what it is reallly

how does increasing clock speed improve cpu performance?

how does increasing clock speed improve cpu performance?

• Sends out regular pulses between 1 and 0 which synchronises the carrying out of instructions in the fetch-decode-execute cycle (with each tick of the clock, one core can process one instruction) 

• when clock speed is increased, the clock pulses more times per second, so more instructions are carried out per second by the CPU, so performance increases

• however, every time the clock pulses, it uses power, which generates heat, which can damage the chip, so most clocks are limited at 3.5 GHz

• overclocking= running the CPU faster than its normal design speed so that more instructions can be processed per second. use heat sinks with metal fins to dissipate heat away from the CPU

how does increasing cores improve cpu performance?

• a core= a single unit of a processor including ALU, CU and registers

• the more cores, the more instructions that can be processed simultaneously so multiple applications can be run seamlessly without the CPU slowing down

• Multitasking is when the CPU carries out more than one task simultaneously. If the CPU has two cores, it can run two applications simultaneously, for example a word processor and email management software. 

what is parallel processing?

• Parallel processing is splitting tasks (programs) up into smaller parts so that each part can be processed at the same time. Parallel processing doesn’t always lead to an improvement in CPU performance because many programs cannot be neatly split into two or more parts, as the programs are sequential and the next task depends on the outcome of a previous task. 

how does increased cache improve cpu performance?

• the cache stores frequently used instructions and data that are quicker to access than the RAM, as the RAM is further away from the CPU than cache, so by using cache, the CPU doesn't have to fetch data from the RAM. The cache stores data that is too big to be stored in registers. 

• The more cache a processor has, the more frequently used data and instructions can be stored there so that the CPU has to access the RAM less frequently. The processing time speeds up because less time is spent slowly accessing RAM values because more values are stored in cache. 

• Cache memory is more expensive than RAM because cache consists of high-speed static RAM (SRAM) instead of cheaper, slower dynamic RAM. 

  Cache comes in Level 1, 2 and 3 (descending order of speed, ascending order of distance from CPU, ascending order of size) 

how does increasing ram improve cpu performance?

• RAM is main memory that stores data and programs/instructions that are currently being used by the CPU 

• virtual memory is an area of the hard disk that acts as a very slow form of RAM if there is not enough space in the actual RAM. The OS creates a "swap file" in virtual memory It stores some of the currently running programs and data if RAM is full, and makes the computer appear to run slowly. 

• If the computer doesn't have enough RAM to store the programs and data, the rest of the programs and data will be stored in virtual memory, which causes the computer to run slowly as virtual memory is a slower form of RAM. 

• When the right amount of RAM is added, the computer performs best because all the programs and data currently being used can be stored in the RAM, instead of being put in virtual memory.

• When there is already enough RAM, adding extra RAM won't alter performance because all the programs were in RAM anyway, and virtual memory was not running so wasn't slowing down the computer, so the overall speed won't change. 

how does getting a GPU improve cpu performance?

• An average computer has a basic graphics chip on its motherboard, which is suitable for everyday use but is unsuitable for video games where 3D graphics require lots of processing power.

• Installing a graphics card help because it has its own Graphics Processing Unit which handles graphics much better than the CPU's graphics chip, so the CPU can send any graphics-related tasks to the GPU instead of the CPU doing it itself.

• The GPU can also do Single Instruction Multiple Data (parallel) processing which improves performance because a large amount of data, eg pixel colours, is changed at the same time. 

• Graphics cards are expensive 

• They only improve performance in graphics and video related apps 

• They use a lot of power (expensive again) so need a big power supply (expensive) 

• They need cooling fans (noisy) to cool them down 

how does pipelining improve cpu performance?

• Pipelining is the technique of fetching an instruction whilst the prior one is being decoded and the one before that is being executed

• Without pipelining, the stages of the FDE cycle would have to take place one at a time, which is inefficient because several parts of the CPU would be idle if they are not involved in certain stages. 

• With pipelining, the next instruction can be fetched at the same time that arithmetic operations are being performed in the ALU for the previous instruction, allowing the processor to make efficient use of registers/cache (multiple registers are used at the same time), so multiple instructions can be executed simultaneously. 

• the processor uses “predictive pipelining” to predict which branches of the program will be carried out; if it gets it wrong it has to flush the pipe

• Flushing the pipe= when the processor has to quickly move to another stage of a branched program after having executed a selection statement or similar , it has to "flush the pipe" to remove the processing of unnecessary instructions in between, which degrades performance

• processors can have an arithmetic pipeline and instruction pipeline