Stretch tasks CPU Performance

Round 1: Quiz Blitz

1. 3.2 billion cycles every second 1 GHz is one billion cycles per second, so 3.2 GHz is 3.2 billion cycles each second.
2. False Only if the software splits its work between the cores, and the cores also lose time sharing resources.
3. Faster clocks make chips too hot to cool reliably Heat and power use rise sharply with clock speed. Beyond a point the chip cannot be cooled reliably.
4. Clock speed; Number of cores; Cache size The specification names three: clock speed, number of cores and cache size.
5. 1000000 Mega means million, so 1 MHz is one million cycles per second.
6. A complete processing unit with its own ALU, CU and registers Each core can fetch, decode and execute instructions independently of the others.
7. True A single sequence of instructions cannot be split, so the other cores sit idle.
8. B, because rendering can be split across 8 cores Rendering software is multi-threaded, so eight cores working at once beat a faster clock.
9. The most useful data is already in cache, so the gains shrink Extra cache gives smaller and smaller gains, and a bigger cache is also slower to search.
10. 800 MHz → 1.2 GHz → 2,500 MHz → 3.6 GHz Turn them all into MHz: 800, 1,200, 2,500 and 3,600.
11. clock speed Clock speed is measured in hertz: one hertz is one cycle per second.
12. True That is a higher hit rate, so the CPU spends less time waiting for slow RAM.

Round 2: Quick Fire

1. • 3 billion cycles per second
   • Each cycle is a fetch-decode-execute cycle, roughly one instruction
   Model answer

   The clock pulses 3 billion times a second, so the CPU can carry out 3 billion fetch-decode-execute cycles every second, roughly one instruction per cycle.

2. • Not all software is written to use more than one core
   • Cores have to communicate and share resources, which adds overhead
   Model answer

   Not all software is written to split its work between cores, so a single threaded program can only use one core. The cores also have to communicate and share cache and memory, which adds overhead.

3. • A complete processing unit, with its own ALU, control unit and registers
   • It can fetch, decode and execute instructions independently, at the same time as other cores
   Model answer

   A core is a complete processing unit with its own ALU, control unit and registers. Each core can fetch, decode and execute instructions independently, at the same time as the other cores.

4. • The CPU produces more heat
   • It needs more power and cooling, or it may become unreliable
   Model answer

   A higher clock speed makes the CPU produce more heat and use more power. If the chip cannot be cooled well enough it becomes unreliable and may crash or be damaged.

Round 3: Big Think

1. • B's cores let the work be split and processed at the same time
   • Video editing software is multi-threaded, written to use many cores
   • B's bigger cache means fewer slow trips to RAM
   • A's faster clock only helps single-threaded tasks, such as older games
   • A clear conclusion: B is the better choice for video editing
   Model answer

   I would recommend Processor B. Video editing software is multi-threaded, so its work can be split across all 8 cores and processed at the same time. B also has four times as much cache, so more of the data it needs is held close to the CPU, with fewer slow trips to RAM. Processor A has the higher clock speed, so each single thread runs faster, which suits older games, but that matters less here. B is the better choice for video editing.

2. • A higher clock speed means more cycles, so more instructions, per second
   • But a faster clock produces more heat and uses more power
   • More cores let several instructions be processed at the same time
   • But only if the software is written to use them, and cores must share resources
   • More cache means less time waiting for RAM
   • But extra cache gives smaller gains, costs more and is slower to search
   Model answer

   A higher clock speed means more fetch-decode-execute cycles per second, so more instructions are processed, but it also produces more heat and uses more power, and beyond a point the chip cannot be cooled. More cores let several instructions be processed at the same time, but only if the software is written to split its work, and the cores also have to communicate and share resources. A bigger cache holds more frequently used data, so the CPU spends less time waiting for RAM, but the gains get smaller as it grows, and cache is expensive and slower to search.

This or That

1. 3.8 GHz with 2 cores A single thread can only use one core, so the faster clock wins.
2. 2,400 MHz 1 GHz is 1,000 MHz, so 2.4 GHz is 2,400 MHz.
3. A sequence of instructions A thread is a sequence of instructions that the operating system can place on a core.
4. It still runs on one core Its instructions form one sequence, so the extra cores have nothing to do.
5. Hertz Hertz counts cycles per second. Bytes measure data.
6. Reasons for each, then a clear choice Say what each processor is better at, name the factor that causes it, then commit to an answer.
7. Video rendering Rendering splits into many threads, one per core. A single thread can only use one core.
8. Each core runs instructions alongside the others Each core is a complete processing unit, with its own ALU, control unit and registers.

Memory Match

• A 2 Clock speed: Cycles the CPU completes each second
• B 6 1 GHz: One billion cycles per second
• C 5 Hertz: One cycle per second
• D 3 Core: A complete processing unit in the CPU
• E 4 Thread: A sequence of instructions
• F 1 Cache: Fast memory for recently used data

Sort It

1. Number of cores Only helps if software is written to split its work A program must be multi-threaded to spread across cores.
2. Number of cores Lets several instructions be processed at the same time Each core works independently, so four cores can work on four instructions at once.
3. Clock speed Measured in gigahertz Clock speed is cycles per second, measured in hertz, megahertz or gigahertz.
4. Number of cores Each one has its own ALU, control unit and registers A core is a complete processing unit.
5. Cache size Means fewer slow trips out to RAM A bigger cache holds more of what the CPU needs, so it finds more there instead of in RAM.
6. Cache size Holds recently and frequently used instructions That is exactly what cache is for: keeping likely-needed instructions close to the CPU.
7. Number of cores Makes little difference to a single-threaded program One thread runs on one core, however many cores there are.
8. Clock speed Raising it makes the chip much hotter A faster clock raises heat and power use sharply, which limits how high it can go.
9. Cache size Gives smaller gains as it grows, because the useful data is already there Extra cache has diminishing returns, and a larger cache is slower to search.
10. Clock speed Sets how many fetch-decode-execute cycles happen each second Each clock pulse is one cycle, so the clock sets the pace of the whole CPU.