Higher Computing

1. Higher Computing Topic 3 Computer Performance Updated 16-6-11

2. Measuring Performance Clock Speed Every processor has an internal crystal-controlled clock which generates regular pulses used to synchronise the processor while carrying out the fetch-execute cycle. The clock speed is an important factor in determining the performance of a processor. A processor running at 200MHz is likely to execute instructions faster than one that runs at 100MHz. However, when it comes to judging performance between competing processors, clock speed may not always be a reliable measure of relative performance. (Processors must be the same if a true comparison is to be made.

3. Measuring Performance Clock Speed Increasing the clock speed means the fetch-execute cycle will happen faster so will increase system performance. Clock speed indicates the speed of the processors internal clock but does not necessarily represent the number of instructions that the processor can complete (does not indicate the amount of data throughput).

4. Measuring Performance MIPS – Millions of Instructions per Second A clock speed of 200 MHz does not mean that 200 million instructions are executed per second. It can take at least five clock pulses to execute an instruction – depending on the instructions that the processor uses. The MIPS rate does not take into account the complexity of instructions. It may be the case that two processors have the same clock speed but different MIP rates. Therefore, the number of millions of instructions per second is used as an alternative measure to clock speed.

5. Measuring Performance MIPS – Millions of Instructions per Second The MIPS rate does not take into account the complexity of instructions or the number of instructions required to carry out certain tasks and this can vary between computers. Manufacturers of computers often use MIPS to measure the performance of their computer, however, this does not give an accurate measure of performance as the short/fast instructions can be used, thereby making the processor seem faster. EG measuring only internal transfers on the processor eg TAX, TAY instructions.

6. Measuring Performance FLOPS – Floating Point Operations per Second You should be aware that using MIP rate as a comparison factor also has problems. It does not take into account the complexity of instructions. A better measure of performance is: FLOPS (floating point operations per second). The procedures involved in doing a floating-point multiplication are basically the same for every processor.

7. Measuring Performance Benchmark Testing A benchmark is a well defined standardised routine used to test the performance of computer systems. Use the internet to find out about a benchmark test (this was in the 06 CW task!)

8. Measuring Performance Application Based Tests The user sits at each computer, runs an application and so uses this real-life example to determine which computer is fastest, it measures actual time taken and is a measure of the performance of the whole computer system. Eg in a Database the user sorts 20,000 records and times it, the computer which does it fastest is considered fastest in this real-life test.

9. Performance Factors – Clock Speed A computer’s clock can be set to run faster (overclocking), this makes the processor run faster and so the fetch-execute cycle is also faster. However, this can wear the processor out more quickly as the technical limitation of the processor is exceeded due to voltages.

10. Performance Factors – Type of Processors RISC – Reduced Instruction Set Computer. This also increases the number of registers on the processor. Some processors have a reduced list of instructions, these have simpler instructions which can operate faster. This frees up space on the processor and so allows more registers on the processor which are very fast to access. Therefore a RISC processor with simpler instructions and more registers can operate faster than one with fewer registers.

11. Performance Factors – Type of Processors CISC – Complex Instruction Set Computer – processors are designed with more complex instructions which require less accesses to memory and resulting in smaller program size and faster execution.

12. Performance Factors – No of Processors Having a computer with a dual core or quad processors makes the computer run faster. This is because the computer has more than one processor to process instructions.

13. Performance Factors - Data Bus Width A computer is described in terms of its word size. Word – The number of bits a processor can store and process in a single operation! Thus a 32-bit processor can handle 32 bits in a single operation. If the word size of the computer and the data bus width are the same, this allows data transfers to and from main memory to be carried out in a single operation. This means that there are less visits to memory to access the data and we can say that a wider bus width will produce increased performance.

14. Performance Factors – Rate of Data Transfer to and from Peripherals Each PD has an operational speed, uses its own language and deals with different amounts of data at a time. An interface is a unit that sits between the processor and a peripheral device and compensates for the differences in speed, codes etc. to ensure compatibility. An interface can store data from a slow peripheral to allow the processor to go and complete other tasks. The processor can visit the interface and take the data that it requires once the interface is full.

15. Performance Factors – Rate of Data Transfer to and from Peripherals When data transfer does occur, parallel transfer can be faster than serial transfer. This is also a factor that affects system performance. Parallel Transfer All bits travel at same time along a different wire Serial Transfer All bits travel one after the other along the same wire

16. Performance Factors – Rate of Data Transfer to and from Peripherals Parallel transfer can be faster over short distances. Serial transfer is preferred as parallel transfer suffers from skew as all data bits do not arrive at the same time, this worsens as the distance travelled increases. Serial data transfers can take place over greater distances.

17. Performance Factors – Main Memory Amount of Memory If your computer is struggling to run some software or you cannot load all the software you want at the one time, then adding extra memory will improve your system’s performance. This is not a direct improvement in Systems Performance. For example, if you are using an application that needs to manipulate large graphic files, video or sound then you should be thinking of upgrading RAM to the computer’s maximum capability.

18. Performance Factors – Cache Memory is very fast SRAM that can reside inside the processor or sit between the processor and RAM. It’s faster for processor to access it rather than going out to RAM. When writing to main memory the processor uses the cache to deposit data and then resumes its operations immediately. The data is transferred to main memory by the cache controller circuitry.

19. Performance Factors – Cache Memory When reading the processor first checks if info is available in cache and transfers it at high speed to processor, otherwise it accesses RAM.