CAP 3 - Instruction-Level Parallelism in Pipelining: Concepts and Techniques

<p>To illustrate the different stages of instruction execution in a pipelined processor</p> Signup and view all the answers

<p>Enhanced performance through overlapping instruction execution</p> Signup and view all the answers

<p>They illustrate the potential for parallel execution of multiple instructions</p> Signup and view all the answers

<p>The pipeline is waiting for the branch condition to be resolved before it can proceed.</p> Signup and view all the answers

<p>Decode the next instruction or take the branch and fetch the target instruction.</p> Signup and view all the answers

<p>It results in lots of idle time and inefficiency, represented by the empty spaces in the pipeline.</p> Signup and view all the answers

<p>Functional dependency</p> Signup and view all the answers

<p>Race condition involving two or more instructions competing for the same resource at the same time.</p> Signup and view all the answers

<p>Data hazards caused by dependencies between instructions.</p> Signup and view all the answers

<p>Read after Write (RAW)</p> Signup and view all the answers

<p>Both WAR and WAW hazards</p> Signup and view all the answers

<p>Stall the instruction that tries to read the register until the previous write is complete</p> Signup and view all the answers

<p>The two instructions are independent as long as there is no out-of-order execution</p> Signup and view all the answers

<p>RAW hazards cannot occur in a simple five-stage pipeline</p> Signup and view all the answers

<p>The second instruction may try to read the value of A before the first instruction has finished writing to it</p> Signup and view all the answers

<p>First half of the WB stage</p> Signup and view all the answers

<p>Maintain the PC value and set control signals to harmless values</p> Signup and view all the answers

<p>Two-stall penalty</p> Signup and view all the answers

<p>Second half of the ID stage</p> Signup and view all the answers

<p>Inserting independent instructions in between</p> Signup and view all the answers

<p>It does not change</p> Signup and view all the answers

<p>Two instructions are attempting to access the same memory at the same time</p> Signup and view all the answers

<p>Stall the pipeline by inserting a bubble</p> Signup and view all the answers

<p>To delay the execution of the conflicting instruction</p> Signup and view all the answers

<p>Separate the instruction cache and data cache</p> Signup and view all the answers

<p>To store frequently used instructions and data for faster access</p> Signup and view all the answers

<p>Both solutions involve modifications to the hardware architecture</p> Signup and view all the answers

<p>Decode I6 as the next instruction or take the branch and fetch I19</p> Signup and view all the answers

<p>Other instructions are blocked until the conditional branch is decoded</p> Signup and view all the answers

<p>It causes lots of idle time and inefficiency</p> Signup and view all the answers

<p>Functional dependency</p> Signup and view all the answers

<p>To improve efficiency by allowing multiple instructions to be processed simultaneously</p> Signup and view all the answers

<p>Structural hazard or functional dependency</p> Signup and view all the answers

<p>The efficiency $\eta$ is related to the throughput W by the equation W = $\frac{n}{p + n - 1}$ T$\eta$, where n is the number of pipeline stages and p is the number of parallel pipeline stages. As the efficiency $\eta$ approaches 1 (maximum efficiency), the throughput W approaches the maximum throughput of $\frac{n}{p + n - 1}$ T, where T is the time per task.</p> Signup and view all the answers

<p>The three main types of hazards are: 1) Structural hazards, which are resource conflicts; 2) Data hazards, which involve RAW (read after write), WAR (write after read), and WAW (write after write) dependencies; and 3) Control hazards, which involve issues like branch prediction and changes in the program counter. These hazards can cause stalls or bubbles in the pipeline, reducing the overall performance and efficiency of the processor.</p> Signup and view all the answers

<p>When a conditional branch instruction (such as I5) is in the pipeline, it can cause a control hazard. The primary cause of the idle stages in the pipeline is the uncertainty about whether the branch will be taken or not. The two possibilities for the pipeline to follow are: 1) If the branch is taken, the pipeline must be flushed and restarted from the branch target; 2) If the branch is not taken, the pipeline can continue executing the instructions in order.</p> Signup and view all the answers

<p>The key concept that enables ILP is pipelining, where multiple instructions can be executed concurrently by dividing the instruction execution into multiple stages. By overlapping the execution of multiple instructions in the pipeline, the processor can achieve a higher throughput and better performance, even though the individual instructions may take the same amount of time to execute.</p> Signup and view all the answers

<p>The 'bubble' inserted in the first solution is a hardware stall, which is used to resolve data hazards. The purpose of the bubble is to insert an empty stage in the pipeline to prevent a structural hazard, where an instruction would attempt to use a resource that is already in use by another instruction. By inserting the bubble, the pipeline can be stalled temporarily to allow the resource to become available, preventing the structural hazard and maintaining the correct execution of instructions.</p> Signup and view all the answers

<p>The main problem caused by the unbalanced length of the pipeline stages is that it can lead to a reduction in the maximum efficiency ($\eta$) of the pipeline. When the pipeline stages have different execution times, it can result in some stages being idle or waiting for the slower stages to complete, which reduces the overall efficiency and throughput of the processor. This unbalanced pipeline can be a key barrier to achieving the maximum speedup, efficiency, and throughput in a pipelined system.</p> Signup and view all the answers

<p>The key insight is that by breaking down instruction execution into sequential stages (e.g., fetch, decode, execute, etc.), different instructions can overlap and execute different stages simultaneously. This allows multiple instructions to be in the pipeline at once, enabling ILP. By overlapping instructions, the effective CPI can be reduced below the total number of stages.</p> Signup and view all the answers

<p>With perfect pipelining, a new instruction can enter the pipeline every cycle. In a 5-stage pipeline, there can be up to 5 instructions in the pipeline simultaneously. Therefore, the maximum throughput is 5 instructions every 5 cycles, or 1 instruction per cycle. The expression is: Throughput = Number of pipeline stages / Number of cycles to fill the pipeline initially.</p> Signup and view all the answers

<p>The structural hazard arises when multiple instructions need to access the same hardware resource simultaneously, but that resource is not duplicated (e.g., a single memory unit). The first solution duplicates the resource (e.g., separate instruction and data memory units), but increases hardware cost and complexity. The second solution stalls the pipeline when a conflict occurs, reducing performance but requiring less hardware modification.</p> Signup and view all the answers

<p>The RAW hazard occurs when an instruction tries to read a register before a previous instruction has finished writing to it (e.g., <code>add r1, r2, r3</code> followed by <code>sub r4, r1, r5</code>). Hardware stalls insert 'bubbles' to delay instructions until the data is ready, but this reduces performance. Software re-ordering rearranges instructions to avoid the hazard, but requires compiler support and may not always be possible.</p> Signup and view all the answers

<p>Branch prediction is crucial in pipelined processors to avoid stalling the pipeline when a conditional branch is encountered. Static schemes use compiler hints or heuristics, while dynamic schemes use runtime history to predict branches. Static schemes are simple but less accurate, while dynamic schemes are more complex but can adapt to program behavior. Trade-offs include accuracy, hardware complexity, and power consumption.</p> Signup and view all the answers

<p>Out-of-order execution allows instructions to execute as soon as their operands are ready, rather than strictly in program order. This can improve performance by avoiding stalls and better utilizing available execution units. However, it requires complex logic to handle data hazards, control hazards, and exceptions. Trade-offs include potential performance gains versus increased hardware complexity, power consumption, and design challenges.</p> Signup and view all the answers

<p>The 'bubble' instructions are essentially no-op (no operation) instructions that are inserted into the pipeline to create gaps or stalls, allowing the necessary time for the previous instruction to complete and resolve the data hazard.</p> Signup and view all the answers

<p>The hardware stall approach introduces idle cycles or stalls in the pipeline, potentially reducing overall performance, while the software workaround approach attempts to reorder or reschedule independent instructions to avoid stalls, potentially improving performance.</p> Signup and view all the answers

<p>The software workaround could involve rescheduling or reordering independent instructions to be executed between the 'add' and 'sub' instructions, allowing time for the register write to complete before the 'sub' instruction reads it. However, this approach may not always be possible or effective, depending on the instruction dependencies and available independent instructions.</p> Signup and view all the answers

<p>Data hazards can significantly reduce instruction-level parallelism by introducing stalls or bubbles in the pipeline, preventing new instructions from being issued and executed in parallel. Both the hardware stall and software workaround solutions aim to minimize or eliminate these stalls, thereby improving ILP and overall throughput.</p> Signup and view all the answers

<p>A 'false dependency' in the WAW data hazard refers to a situation where two instructions write to the same register, but the second write effectively overwrites the first, making the first write unnecessary. This false dependency can be addressed through register renaming or by reordering the instructions to eliminate the redundant write.</p> Signup and view all the answers

<p>The hardware stall approach introduces additional hardware complexity to detect and handle data hazards, but offloads the responsibility from the compiler. The software workaround approach requires more sophisticated compiler analysis and instruction scheduling, but potentially offers better performance by avoiding unnecessary stalls. The choice depends on the specific system requirements and constraints.</p> Signup and view all the answers

<p>The false dependency mentioned for the WAW data hazard is that as long as there is no out-of-order execution, overwriting the same register (e.g., A = B + C followed by A = D + E) is not an actual problem. This is because in a simple pipeline, instructions are executed in strict program order, so the second write to A will always overwrite the first write.</p> Signup and view all the answers

<p>The potential problem with the RAW data hazard is that an instruction attempts to read a register (e.g., E = A + D) before a previous instruction has finished writing to that register (e.g., A = B + C). The proposed solution is to stall or wait the execution of the instruction that reads the register until the value of that register is ready and available to be used.</p> Signup and view all the answers

<p>Instruction-level parallelism (ILP) is the ability to execute multiple instructions simultaneously or in parallel. Pipelining and out-of-order execution are techniques that enable ILP by allowing instructions to be executed in a different order than they appear in the program, as long as the final result is the same as if they were executed sequentially.</p> Signup and view all the answers

<p>The structural hazard mentioned is related to the unbalanced length of pipeline stages, where some stages may take longer than others. The two proposed solutions are: 1) Adding hardware stalls or 'bubbles' to allow slower stages to catch up, and 2) Modifying the hardware to balance the length of the pipeline stages.</p> Signup and view all the answers

<p>The instruction cache (Im) and data cache (Dm) are used to improve performance by reducing the time required to fetch instructions and data from main memory. In the context of pipelining and out-of-order execution, these caches are essential for providing instructions and data quickly to the pipeline stages, allowing for efficient execution and reordering of instructions.</p> Signup and view all the answers

<p>A possible software workaround for data hazards is to insert additional instructions (e.g., NOP or no-operation instructions) between instructions that may cause hazards, effectively creating a delay or stall in the software. However, this workaround can impact performance negatively by increasing the overall number of instructions and execution time.</p> Signup and view all the answers