1.1.2 CPU performance

What is clock speed?

• the CPU is coordinated by the pulse of an internal clock

• number of FDE cycles run per second

• measure of how quickly a CPU can process instructions

• default clock speed is 3.5GHz - can process 3.5 billion instructions per second

How does clock speed affect CPU performance?

• the faster the clock speed, the more FDE cycles are completed so more instructions are processed per second and a better performance

• overclocking - increasing the clock speed higher than the recommended rate. faster performance but also overheating and damage to the machine

• underclocking - decreasing the clock speed lower than the recommended rate. slower performance but will increase the lifespan of the machine.

What is a core?

• a processing unit within CPU

• each core can execute instructions independently

• multicore CPU can process multiple instructions simultaneously

How does the number of cores affect CPU performance?

• one cycle per second = 1Hz = 1 instruction per second

• CPU can only execute 1 instruction at a time

• in theory, a dual core processor has the potential to execute 2 instructions at a time and a quad could execute 4

• only under ideal conditions such as a program that has been designed to work with multi core processors

• if one core is waiting for another core to finish processing or the software used is not written to make use of multi cores, the performance may not increase

What is cache memory?

• small amounts off very fast memory in the CPU

• used for frequently accessed data

How does cache memory affect CPU performance?

• closer to the CPU than RAM so data transferred faster between CPU and cache than between CPU and RAM

• useful if there is a while loop in a program - storing all the instructions in cache speeds up execution

• CPU with more cache has a faster performance - frequently used instructions can be accessed quicker - don’t have to be repeatedly fetched from RAM

• costly so most computers only have a small amount

• in most systems, level 1 is used 50% of the time and level 2 is used 90% of the time - reduces the time the CPU has to wait

What are the features of level 1 cache?

• extremely fast but small

• 2-256KB

• located on the CPU

• each core will have its own level 1 cache

What are the features of level 2 cache?

• very fast but slower and bigger than level 1

• 256KB-8MB

• located on the CPU

• usually also given to each core

What are the features of level 3 cache?

• slowest and biggest cache but still faster and smaller than RAM

• 4MB-50MB

• located on CPU and shared between all cores on the processor.