2.2 Basic Pipeline Hazards and Solutions
6 min read•july 30, 2024
Pipelining boosts processor performance by overlapping instruction execution. But it's not all smooth sailing. Pipeline hazards can throw a wrench in the works, causing slowdowns and wasted cycles. These hiccups come in three flavors: structural, data, and .
Luckily, clever engineers have cooked up ways to tackle these issues. From forwarding data to predicting branches, these techniques help keep the pipeline flowing smoothly. Understanding hazards and their solutions is key to grasping how modern processors squeeze out every bit of performance.
Pipeline Hazards
Types of Pipeline Hazards
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- occur when hardware resources required by the pipeline stages cannot be supplied simultaneously due to resource conflicts
- Example: Two instructions requiring access to the same memory unit at the same time
- arise when instructions have data dependencies between them that prevent parallel execution
- (RAW) dependencies occur when an instruction reads a source before a previous instruction writes to it
- (WAR) dependencies occur when an instruction writes to a destination before a previous instruction reads from it
- (WAW) dependencies occur when two instructions write to the same destination in a different order than intended
- Control hazards, also known as branch hazards, occur when the flow of instruction execution is altered by branch or jump instructions
- Causes subsequent instructions that have been fetched or decoded to be discarded
- Example: A branch instruction causing the pipeline to fetch instructions from a different memory address
Impact of Pipeline Hazards on Performance
- Pipeline hazards can significantly degrade processor performance by causing stalls, bubbles, or flushes
- Stalls occur when the pipeline must wait for a hazard to be resolved before continuing execution
- Bubbles are wasted cycles inserted into the pipeline to delay instruction execution until a hazard is resolved
- Flushes occur when incorrectly fetched or executed instructions must be discarded from the pipeline
- The performance impact of a hazard depends on its frequency and the number of cycles required to resolve it
- Example: Frequent data hazards causing multiple stalls can significantly reduce instruction
Causes and Effects of Pipeline Hazards
Structural Hazards
- Caused by resource limitations when multiple instructions in different pipeline stages simultaneously require use of the same processor component
- Example: Two instructions requiring access to a single ALU at the same time
- Can stall the pipeline as instructions wait for the shared resource to become available
- Reduces instruction throughput and increases execution time
- More likely to occur in processors with limited hardware resources or complex instructions requiring multiple cycles and resources
Data Hazards
- Occur when data dependencies exist between instructions, preventing parallel execution
- An instruction may need to use a value that has not yet been calculated by a previous instruction still in the pipeline
- Can cause pipeline stalls or require the insertion of bubbles (wasted cycles) to resolve
- Stalls delay instruction execution until the required data is available
- Bubbles are inserted to delay instruction execution and align data dependencies
- Read after write (RAW) hazards are the most common type of data hazard
- Occur when an instruction reads a source before a previous instruction writes to it
- Require a stall until the write completes to ensure correct execution
- Write after read (WAR) and write after write (WAW) hazards can occur in systems allowing out-of-order completion
- Instructions may read or write data in a different order than intended, leading to incorrect results
Control Hazards
- Caused by branch or jump instructions altering the sequential flow of execution
- The pipeline may fetch and begin executing instructions from the wrong path before the branch outcome is known
- Can cause pipeline flushes to discard incorrectly fetched instructions and stall the pipeline until the branch target is known
- Flushing the pipeline discards instructions and wastes the work already done on them
- Reduce instruction throughput as cycles are wasted due to pipeline flushes and stalls after branch instructions
- The number of cycles wasted depends on the branch instruction's location in the pipeline when the branch outcome is determined
- Earlier branch resolution reduces the number of wasted cycles
Pipeline Hazard Mitigation Techniques
Resolving Structural Hazards
- Provide more hardware resources to reduce conflicting resource requirements between pipeline stages
- Example: Adding additional memory ports or ALUs to allow simultaneous access
- Optimize instruction scheduling to minimize resource conflicts
- Rearrange instructions to avoid multiple instructions requiring the same resource in the same cycle
- Use to allow instructions to execute in a different order than fetched, reducing resource conflicts
Resolving Data Hazards
- Forwarding () forwards the required data from a later pipeline stage back to an earlier stage when available
- Avoids waiting for the data to pass through pipeline registers
- Implemented using multiplexers that select between register file values and forwarded results based on the hazard type
- the pipeline by inserting bubbles (empty cycles) can resolve data hazards when forwarding is not possible
- Gives instructions enough time to complete and write their results back to the register file
- Compiler optimizations can arrange code to minimize data hazards and stalling
- Out-of-order execution allows instructions to execute in a different order than fetched, reducing data dependencies
- Requires complex hardware to track dependencies and reorder instructions
Resolving Control Hazards
- techniques attempt to predict the outcome of a branch before it is known
- Allows the pipeline to speculatively fetch and execute instructions from the predicted path
- Static branch prediction uses fixed rules based on branch instruction type or direction
- Dynamic branch prediction uses runtime information and adaptive predictors to improve accuracy
- Delayed branching reduces control hazard penalties by rearranging instructions to fill delay slots after a branch
- Allows useful work to be done while the branch is resolved
- Places the burden on the compiler to correctly fill delay slots
- Branch target buffers store the target addresses of previously executed branches to reduce the cycles needed to calculate the target
Effectiveness of Hazard Resolution Techniques
Evaluating Forwarding and Stalling
- Forwarding is an effective technique for resolving data hazards that can significantly improve pipeline performance
- Avoids stalls and reduces wasted cycles by providing data as soon as it is available
- Can increase processor design complexity and power consumption due to additional forwarding logic
- Stalling is a simple method for resolving data hazards but can significantly reduce pipeline performance if frequent stalls are required
- Effectiveness depends on the frequency of data hazards and the ability of the compiler to arrange code to minimize them
Evaluating Branch Prediction
- Branch prediction is an effective technique for mitigating control hazards, with more advanced dynamic predictors able to achieve high prediction accuracies
- Allows the pipeline to speculatively execute instructions, reducing the impact of control hazards
- Increases processor complexity and power consumption due to the additional hardware required
- Static branch prediction is simple but less accurate, while dynamic prediction adapts to changing program behavior but requires more hardware resources
- The performance impact of branch prediction depends on the frequency and predictability of branches in the code being executed
- Highly predictable branches benefit more from branch prediction than unpredictable ones
Evaluating Delayed Branching
- Delayed branching can be an effective technique for reducing control hazard penalties in simpler pipelines
- Allows useful work to be done while the branch is resolved, reducing wasted cycles
- Effectiveness is limited in deeper pipelines, as the number of delay slots increases and it becomes harder to find useful instructions to fill them
- Places the burden on the compiler to correctly fill delay slots, which can be complex and limit code optimization opportunities
Combining Hazard Resolution Techniques
- The effectiveness of hazard resolution techniques depends on the specific processor implementation and the characteristics of the workload being executed
- A combination of techniques is often used to achieve the best performance trade-offs
- Example: Using both forwarding and stalling to resolve data hazards, while employing branch prediction to mitigate control hazards
- Processor designers must balance the performance benefits of hazard resolution techniques with their impact on processor complexity, power consumption, and area
Key Terms to Review (22)
Branch Prediction: Branch prediction is a technique used in computer architecture to improve the flow of instruction execution by guessing the outcome of a conditional branch instruction before it is known. By predicting whether a branch will be taken or not, processors can pre-fetch and execute instructions ahead of time, reducing stalls and increasing overall performance.
Branch Target Buffer: A branch target buffer (BTB) is a specialized cache used in processors to improve the efficiency of branch prediction by storing the destination addresses of previously executed branch instructions. It helps mitigate pipeline hazards caused by branches by predicting which instruction will be executed next, thereby reducing stalls in the pipeline. This buffer allows the processor to continue fetching instructions without waiting for branch resolutions, thus enhancing overall performance.
Bypassing: Bypassing refers to a technique used in computer architecture to circumvent data hazards in pipelined processors, enabling more efficient execution of instructions without waiting for prior instructions to complete. This method allows for data to be used directly from a preceding stage of the pipeline instead of relying on the typical write-back stage, which minimizes stalls and increases throughput. Bypassing is closely linked to forwarding mechanisms and plays a crucial role in optimizing out-of-order execution strategies.
Control hazards: Control hazards are situations that occur in pipelined processors when the control flow of a program changes unexpectedly, often due to branch instructions. This unpredictability can disrupt the smooth execution of instructions and lead to performance penalties, as the processor must wait to determine the correct path to follow. Effective management of control hazards is crucial in enhancing performance, especially in advanced architectures like superscalar processors, which aim to execute multiple instructions simultaneously.
Data hazards: Data hazards occur in pipelined computer architectures when instructions that depend on the results of previous instructions are executed out of order, potentially leading to incorrect data being used in computations. These hazards are critical to manage as they can cause stalls in the pipeline and impact overall performance, especially in complex designs that leverage features like superscalar execution and dynamic scheduling.
Dynamic Scheduling: Dynamic scheduling is a technique used in computer architecture that allows instructions to be executed out of order while still maintaining the program's logical correctness. This approach helps to optimize resource utilization and improve performance by allowing the processor to make decisions at runtime based on the availability of resources and the status of executing instructions, rather than strictly adhering to the original instruction sequence.
Flush: In the context of computer architecture, a flush refers to the process of clearing or invalidating the contents of a pipeline, effectively removing instructions or data that are no longer valid due to hazards such as data dependencies, control flow changes, or structural conflicts. This action ensures that the pipeline can continue processing without errors, but it can also introduce performance penalties as instructions must be fetched again after the flush.
Hazard rate: The hazard rate refers to the frequency at which hazards occur in a pipeline architecture, impacting the overall performance of instruction execution. It is a measure of how likely a particular hazard will disrupt the normal flow of instructions, causing delays or stalls in the pipeline. Understanding the hazard rate is crucial for implementing effective solutions to mitigate these disruptions and maintain efficient processing.
Latency: Latency refers to the delay between the initiation of an action and the moment its effect is observed. In computer architecture, latency plays a critical role in performance, affecting how quickly a system can respond to inputs and process instructions, particularly in high-performance and superscalar systems.
Out-of-order execution: Out-of-order execution is a performance optimization technique used in modern processors that allows instructions to be processed as resources become available rather than strictly following their original sequence. This approach helps improve CPU utilization and throughput by reducing the impact of data hazards and allowing for better instruction-level parallelism.
Pipeline depth: Pipeline depth refers to the number of stages in a processor's instruction pipeline, which affects the throughput and overall performance of the CPU. A deeper pipeline can lead to increased instruction throughput, as multiple instructions can be processed simultaneously at different stages, but it also introduces complexities such as pipeline hazards and recovery mechanisms, particularly when mispredictions occur or side effects arise from certain instructions.
Pipeline efficiency: Pipeline efficiency refers to the effectiveness of a pipelined processor in executing instructions in parallel, minimizing idle time and maximizing throughput. Achieving high pipeline efficiency is crucial as it directly impacts the overall performance of the CPU, allowing multiple instruction stages to be processed simultaneously while addressing potential hazards that can stall the pipeline.
Pipeline stall: A pipeline stall occurs when the next instruction in a CPU pipeline cannot proceed due to various hazards, causing a delay in the execution of instructions. These stalls can arise from data hazards, control hazards, or structural hazards, which interrupt the smooth flow of instruction execution and reduce overall performance. Managing pipeline stalls is essential for optimizing processor efficiency and maintaining high throughput in instruction processing.
Read After Write: Read after write is a type of data hazard that occurs in a pipeline when an instruction tries to read a value that has been written by a previous instruction but has not yet completed. This can lead to incorrect results if the reading instruction executes before the writing instruction finishes updating the data, creating a timing issue in the flow of operations. Understanding this hazard is essential for designing effective solutions to ensure data integrity in pipelined architectures.
Return Address Stack: A return address stack is a specialized hardware structure used in computer architecture to store the return addresses of function calls. This stack helps manage control flow in programs, especially during the execution of nested function calls and when handling interrupts. By keeping track of return addresses, it helps maintain the program's state, allowing for smooth transitions between different levels of execution.
Stalling: Stalling refers to a situation in a pipeline where the progress of instruction execution is temporarily halted due to various hazards, preventing the processor from moving forward efficiently. This can occur because of data dependencies, resource conflicts, or control hazards, and it negatively impacts the overall performance of the system by increasing latency and reducing throughput.
Static scheduling: Static scheduling is a technique used in computer architecture where the order of instruction execution is determined at compile-time rather than at runtime. This approach helps in optimizing the instruction flow, ensuring that dependencies are respected while maximizing resource utilization. By analyzing the code beforehand, static scheduling can minimize hazards and improve performance, especially in systems designed for high instruction-level parallelism.
Structural Hazards: Structural hazards occur in a pipelined architecture when hardware resources are insufficient to support the concurrent execution of instructions. This situation arises when different instructions require the same resource at the same time, leading to conflicts and delays in the pipeline. Structural hazards highlight the limitations of hardware design and emphasize the importance of resource allocation in improving overall processor performance.
Superscalar architecture: Superscalar architecture is a computer design approach that allows multiple instructions to be executed simultaneously in a single clock cycle by using multiple execution units. This approach enhances instruction-level parallelism and improves overall processor performance by allowing more than one instruction to be issued, dispatched, and executed at the same time.
Throughput: Throughput is a measure of how many units of information a system can process in a given amount of time. In computing, it often refers to the number of instructions that a processor can execute within a specific period, making it a critical metric for evaluating performance, especially in the context of parallel execution and resource management.
Write After Read: Write after read is a situation in computer architecture where a write operation follows a read operation for the same data. This can create challenges in pipelined processors, as it can lead to hazards that disrupt the smooth flow of instruction execution. Addressing this issue is essential for maintaining data integrity and ensuring efficient processing in a pipeline architecture.
Write After Write: Write after write is a type of hazard that occurs in pipelined architectures when two write operations are scheduled in such a way that the second write may overwrite the first before it has been read. This situation can lead to data inconsistency and can disrupt the expected behavior of programs if not properly managed. Addressing write after write hazards is critical for maintaining data integrity and ensuring that the pipeline operates efficiently.