Pipeline Scheduling

In Computer Organisation and Architecture (COA), pipelining is a technique used to increase the instruction throughput of a processor. It allows multiple instructions to be executed simultaneously by dividing the execution process into several stages, where each stage performs a part of the instruction.

However, when multiple instructions overlap, conflicts or hazards can occur. To manage this effectively, pipeline scheduling is used — it decides when and how each instruction enters the pipeline to ensure smooth and efficient execution.

Key Points:

• Pipeline scheduling determines the order and timing of instruction execution in a pipeline.
• The goal is to minimise stalls and delays caused by instruction dependencies or resource conflicts.
• It helps achieve maximum pipeline utilisation and reduces idle stages.
• Scheduling ensures that data hazards (like read after write) are avoided or resolved properly.
• Techniques like forwarding, stalling, and reordering are used for effective pipeline scheduling.

Types of Pipeline Scheduling

Static Scheduling

• Done at compile time by the compiler.
• The compiler rearranges instructions to avoid pipeline conflicts.
• No hardware intervention during execution.
• Example: Loop unrolling in compilers.

Dynamic Scheduling

• Done at runtime by hardware using mechanisms like Tomasulo’s algorithm.
• Allows out-of-order execution to avoid stalls.
• Helps when instruction dependencies are unknown at compile time.

Example:

Let’s say we have three instructions:

I1: R1 = R2 + R3  
I2: R4 = R1 + R5  I3: R6 = R7 + R8

Now, I₁ executes while I₂ waits — no idle pipeline stage occurs, and performance improves.

Challenges in Pipeline Scheduling

• Data Hazards: When one instruction depends on the result of another.
• Structural Hazards: When hardware resources are shared or limited.
• Control Hazards: Caused by branch or jump instructions.
• Complex Hardware: Dynamic scheduling adds design complexity