How do you evaluate the impact of pipeline hazards on the overall CPU performance and efficiency?

1 Data hazards

Data hazards occur when an instruction depends on the result of a previous instruction that has not yet completed its execution. For example, if instruction A writes to a register that instruction B reads from, and instruction B is in an earlier stage than instruction A, then instruction B will read a wrong value. This can cause incorrect results or program errors. To evaluate the impact of data hazards, you need to identify the data dependencies among the instructions, and measure the number and type of stalls or bubbles that are inserted into the pipeline to resolve them. A stall or bubble is a cycle where no instruction progresses, and it reduces the pipeline efficiency and performance.

2 Control hazards

Control hazards occur when an instruction changes the flow of control, such as a branch or a jump, and the target address or the condition is not known in advance. For example, if instruction A is a conditional branch that depends on the result of instruction B, and instruction B is in a later stage than instruction A, then instruction A cannot decide whether to take the branch or not until instruction B finishes. This can cause the pipeline to fetch and execute wrong instructions, wasting cycles and resources. To evaluate the impact of control hazards, you need to identify the control instructions and their outcomes, and measure the number and type of flushes or predictions that are used to handle them. A flush is a cycle where the pipeline discards the wrong instructions, and a prediction is a technique that guesses the outcome of a branch and executes accordingly.

3 Structural hazards

Structural hazards occur when two or more instructions compete for the same hardware resource, such as a memory unit or a functional unit, and the resource cannot be accessed by more than one instruction at a time. For example, if instruction A and instruction B both need to access the same memory unit in the same cycle, then one of them has to wait until the other finishes. This can cause the pipeline to stall or slow down, decreasing the throughput and performance. To evaluate the impact of structural hazards, you need to identify the hardware resources and their availability, and measure the number and type of stalls or splits that are caused by them. A split is a technique that divides a resource into smaller units or accesses it in different cycles.

4 Pipeline metrics

To quantify the impact of pipeline hazards on the overall CPU performance and efficiency, you need to use some pipeline metrics, such as the CPI, the speedup, and the efficiency. The CPI (cycles per instruction) is the average number of cycles required to execute one instruction. The lower the CPI, the better the performance. The speedup is the ratio of the performance of a pipelined CPU to that of a non-pipelined CPU. The higher the speedup, the better the performance improvement. The efficiency is the ratio of the useful work done by the pipeline to the total work done by the pipeline. The higher the efficiency, the better the utilization of the pipeline resources.

5 Hazard mitigation

To reduce the impact of pipeline hazards on the overall CPU performance and efficiency, you need to use some hazard mitigation strategies, such as forwarding, branch prediction, and pipeline balancing. Forwarding is a technique that bypasses the data from one stage to another without waiting for it to be written to a register or memory. This can eliminate or reduce data hazards and stalls. Branch prediction is a technique that guesses the outcome of a branch instruction and fetches and executes the corresponding instructions. This can eliminate or reduce control hazards and flushes. Pipeline balancing is a technique that adjusts the number and size of the pipeline stages and the hardware resources to match the workload and avoid structural hazards and stalls.

6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?