1.1.2 CPU Performance

In this lesson, we will explore three essential characteristics: clock speed, cacheA small, high-speed memory inside the CPU that stores frequently used data for quick access, reducing delays from accessing RAM. size, and number of cores. Understanding the impact of these characteristics is crucial for designing and selecting CPUs that meet the specific needs of different computing tasks.

Clock Speed

The clock speed, measured in Hertz (Hz), represents the number of cycles a CPU can executeThe stage where the CPU carries out the operation specified by the instruction. per second. Each cycle is a basic unit of work carried out by the CPU. A higher clock speed means the CPU can perform more cycles in a given time, which typically leads to faster processing.

Effects of Clock Speed on Performance:

• Single-Core Performance: Clock speed has a significant impact on the performance of single-core tasks. Higher clock speeds allow a CPU to complete individual instructions faster, resulting in improved single-core performance.
• Multi-Core Performance: In multi-core tasks, where multiple cores can be executed simultaneously, the clock speed still plays a vital role. Each core executed on a separate core benefits from higher clock speeds, resulting in faster overall performance.
• Heat and Power Consumption: As clock speeds increase, so does power consumption and heat generation. CPUs with high clock speeds often require more advanced cooling solutions to prevent overheating.
• Limitations: There are physical limitations to increasing clock speeds, such as heat dissipation and power consumption. Manufacturers often reach a point where further increases in clock speed become impractical.

Cache Size

Cache is a small, high-speed memoryA unified system that stores both data and program instructions in the same location. located on the CPU chip itself. It serves as a buffer between the CPU and the main memory (Random Access Memory, or RAM). Caches are faster than RAM, allowing the CPU to access frequently used data more quickly.

Effects of Cache Size on Performance:

• Data Access Speed: A larger cache size means the CPU can storeThe stage where the CPU saves the result of the execution back into memory or registers. more data that it frequently accesses, reducing the time needed to fetchThe stage where the CPU retrieves an instruction from memory. data from slower main memory. This results in faster overall performance, especially for repetitive tasks.
• Cache Hit Rate: The cache hit rate refers to the percentage of times the CPU finds the required data in the cache without needing to access the slower main memory. A larger cache improves the cache hit rate, resulting in better performance.
• Cache Levels: Modern CPUs often have multiple cache levels (L1, L2, L3), each with different sizes. Smaller, faster caches (L1 and L2) hold the most critical data, while the larger L3 cache stores additional data shared across cores.
• Cost and Complexity: Increasing cache size comes at a cost, both in terms of chip area and manufacturing complexity. Larger caches can make CPUs more expensive to produce.

Number of Cores

The number of cores in a CPU refers to the number of independent processing units on the chip. Each core can handle its own tasks, allowing for parallel processing of multiple tasks simultaneously.

Effects of Number of Cores on Performance:

• Parallelism: CPUs with more cores can execute multiple tasks in parallel, significantly improving the performance of multi-core applications. Tasks that can be split into independent cores benefit the most from a higher core count.
• Single-Core Performance: While multi-core CPUs excel in parallel tasks, their single-core performance may not always match that of CPUs with fewer but higher-clocked cores.
• Core Management: The operating system must efficiently manage and distribute tasks across multiple cores to fully leverage their potential. Poorly threaded applications may not fully benefit from a higher core count.
• Power Consumption: CPUs with more cores generally consume more power, especially under heavy loads when all cores are utilised.

Combining Characteristics

The performance impact of these characteristics can be increased or reduced when combined:

• Balancing Clock Speed and Cores: A balance between high clock speeds and a sufficient number of cores is essential. A CPU with high clock speed and multiple cores can offer excellent performance for both single-core and multi-core tasks.
• Cache and Multi-Core Processing: A larger cache size is beneficial for multi-core tasks, as it allows each core to access frequently used data quickly. This reduces contention and bottlenecks when multiple cores access the cache simultaneously.
• Workload Dependency: The impact of these characteristics depends on the workload. Different tasks may benefit more from higher clock speeds, larger caches, or a greater number of cores.