Topic Covered:"I2O2 Processors, I2O1 Processors, IO3 Processors, IO2I Processors, Speculation and Branch, Register Renaming Introduction, Register Renaming with Pointers to IQ and ROB, Memory Disambiguation, Limits of Out-of-Order Design Complexity, Introduction to VLIW, VLIW Compiler Optimizations, Classic VLIW Challenges, Introduction to Prediction Scheduling Model , Review of Predication Predication, Implementation Speculation Execution ,Case Study: IA-64/Itanium "
1.I2O2 Processors
- Explanation: "Issue 2, Operate 2" processors can issue and execute two instructions per clock cycle. This is a form of superscalar architecture, where the CPU can process multiple instructions in parallel by having multiple execution units.
- Mechanism: The processor fetches two instructions and sends them to different functional units (like ALU, floating point unit, etc.) for execution simultaneously. This improves throughput but requires hardware to ensure that the instructions are independent (i.e., no data dependencies).
- Example: If the two instructions are
add r1, r2, r3andsub r4, r5, r6, both can be executed in parallel if they don’t interfere with each other’s registers. - Real-Life Example: Imagine you’re at a fast-food restaurant where two chefs are working in the kitchen. Both can prepare two orders at the same time. The restaurant gets orders done faster because the two chefs work independently, each completing a different dish.
2.I2O1 Processors
- Explanation: "Issue 2, Operate 1" processors fetch two instructions but can only execute one per cycle. This limitation may arise from pipeline stalls due to data hazards or insufficient execution resources.
- Mechanism: The processor will keep issuing instructions, but only one gets processed. The second instruction waits until resources are available, which may lead to underutilization of the CPU's potential.
- Example: If you have a resource-intensive task (like multiplication), it occupies the functional unit, and the second instruction has to wait, leading to partial parallelism.
- Real-Life Example: This is like you starting two assignments but focusing only on one because the other requires information that you don’t have yet. You have to wait for it, so only one task gets done while the other is pending.
3.IO3 Processors
- Explanation: "Issue 0, Operate 3" processors refer to scenarios where no new instructions are issued, but the processor can complete three previously issued operations.
- Mechanism: The processor continues working on operations in the pipeline, finishing them even if the instruction fetch unit is stalled or waiting on a branch prediction.
- Example: Imagine a pipeline where three tasks are halfway through, and the system finishes them without starting anything new. This is a useful approach when resolving dependencies or waiting on memory fetches.
- Real-Life Example: Think of a warehouse where no new deliveries are coming in, but the workers continue sorting and packaging the products they already have. They're still busy even though no new shipments are arriving.
4.IO2I Processors
- Explanation: This processor alternates between different behaviors: it can issue zero new instructions in one cycle, complete two operations from earlier, and issue one new instruction in another cycle.
- Mechanism: This is often used in dynamic scheduling, where the processor’s ability to issue instructions depends on available resources, such as ready functional units and data dependencies.
- Example: It's like multitasking: on some days, you focus on pending work (complete old tasks), while on others, you also take on new work (issue new instructions).
- Real-Life Example: In a factory, some days the team might focus only on completing tasks already started (processing old work), and other days they may start new tasks while also finishing older ones.
Comparison Table
| Processor Type | Issue Order | Completion Order | Performance | Complexity |
|---|---|---|---|---|
| I2O2 | In-order | In-order | Low (pipeline stalls) | Simple |
| I2O1 | In-order | Out-of-order | Moderate | Moderate |
| IO3 | Out-of-order | Out-of-order | High | Complex |
| IO2I | Out-of-order | In-order | Moderate to High | Moderate to Complex |
5.Speculation and Branch Prediction
- Explanation: Speculative execution is a technique where the CPU predicts the outcome of conditional branches (such as if-else or loops) and continues executing instructions even before the branch is resolved. If the prediction is correct, the processor avoids stalls. If not, the speculative work is discarded, and execution is rolled back to the correct path.
- Branch Prediction Algorithms:
- Static Prediction: Always predicts the same outcome, such as always taking the branch.
- Dynamic Prediction: Tracks previous behavior of branches to predict future outcomes (e.g., Two-level Adaptive Predictor).
- Example: In an if-else statement, if the processor predicts that the "if" condition is true, it begins executing the instructions inside the "if" block even before knowing the result. If the condition turns out false, it undoes the work and executes the "else" block.
6.Register Renaming
- Explanation: Register renaming resolves name dependencies (also called false dependencies) that occur when instructions use the same registers for different data. By renaming registers, the CPU allows independent instructions to execute in parallel without waiting for earlier ones to finish.
- Mechanism: Instead of using the same physical register, the processor dynamically assigns a new physical register to each instruction needing a logical register (like R1).
- Example: Imagine two chefs needing the same ingredient at different times. If they share the same jar, they will have to wait for each other. With register renaming, each chef gets a separate jar, so both can work without waiting.
7.Register Renaming with Pointers to IQ and ROB
- Explanation: Here, the Instruction Queue (IQ) holds instructions waiting to execute, and the Reorder Buffer (ROB) ensures instructions are committed in order, even if executed out of order. The ROB helps handle speculative execution by allowing the processor to rollback in case of mispredictions.
- Mechanism: Pointers to IQ and ROB help track renamed registers, allowing the processor to track instructions and commit results in the correct order while still executing out of order.
- Example: In a restaurant kitchen, each chef (instruction) has a work list (IQ) and their completed dishes (ROB) are served to customers in the correct order, even if they were finished at different times.
8.Memory Disambiguation
- Explanation: Memory disambiguation allows the CPU to decide if a load (reading from memory) can be performed before a store (writing to memory), which normally would need to be serialized. This technique ensures that loads and stores that don’t depend on each other can proceed in parallel, improving performance.
- Mechanism: The processor uses advanced techniques like load/store queues to check for dependencies between loads and stores and reorders them safely.
- Example: Imagine cooking multiple dishes where you can prepare one dish without needing to clean the same pan used for another. Disambiguation helps figure out which tasks can happen in parallel.
9.Limits of Out-of-Order Design Complexity
- Explanation: As processors allow more instructions to execute out-of-order, the complexity of managing dependencies, register renaming, and memory disambiguation increases. This complexity grows exponentially as more instructions are executed in parallel, leading to diminishing returns on performance.
- Example: If you try to manage 10 projects at once, the overhead of tracking what is done, what depends on what, and who is doing what becomes overwhelming, making you less efficient overall.
10.Introduction to VLIW (Very Long Instruction Word)
- Explanation: In VLIW architectures, the responsibility for parallel execution is shifted from the CPU to the compiler. The compiler bundles independent instructions into a single long instruction word, which the processor then executes in parallel.
- Mechanism: The CPU executes multiple instructions in parallel based on the compiler’s scheduling. The processor doesn’t need complex logic to determine instruction dependencies since it assumes the compiler has already resolved them.
- Example: It’s like a team of workers where each one is assigned a specific task that they can do without needing to wait on others. The manager (compiler) plans this ahead of time.
i.VLIW Compiler Optimizations
- Explanation: The compiler plays a critical role in VLIW systems by finding independent instructions that can be packed together into long instruction words. This requires advanced analysis techniques like instruction scheduling, loop unrolling, and software pipelining to maximize parallelism.
- Example: The compiler reorders instructions to minimize idle time, ensuring all functional units in the processor are busy, just like a manager who arranges tasks to keep everyone on the team working.
ii.Classic VLIW Challenges and Predication
- Explanation: VLIW processors struggle with handling unpredictable branches (if-else). Predication helps by turning control dependencies into data dependencies. Instead of branching, the processor computes both outcomes and then selects the correct one.
- Example: In an if-else scenario, predication allows both paths to be computed and discards the one that isn’t needed, ensuring the pipeline doesn’t stall due to branch mispredictions.
Scheduling Model Review
- Explanation: In VLIW and superscalar processors, scheduling determines how instructions are arranged to be executed in parallel. It involves analyzing dependencies between instructions and ensuring that as many can be executed simultaneously as possible without causing conflicts.
- Example: It’s like scheduling a workday where you organize tasks that can happen at the same time to make the most of your day.
Predication Implementation
- Explanation: Implementing predication requires modifying the CPU’s control unit to execute both paths of a conditional statement in parallel, saving the result of the correct path and discarding the other.
- Example: Imagine getting two event invitations and making plans for both. Once you know which one is happening, you cancel the other plan.
- Real-Life Example: Imagine a scenario where you make two meals simultaneously, not knowing which one your guest prefers. Once the guest arrives and makes their choice, you throw away the meal they didn’t choose.
Case Study: IA-64/Itanium
- Explanation: Intel’s IA-64/Itanium architecture was an ambitious implementation of a VLIW-style processor. It aimed to exploit instruction-level parallelism with the help of compilers that could schedule independent instructions. However, its complexity made it difficult for software developers to adapt their programs, limiting its commercial success.
- Challenges: Itanium required extensive compiler optimizations to achieve its potential, but real-world programs often had unpredictable control flows, which made static scheduling (handled by compilers) inefficient.
- Example: It’s like building a car that runs efficiently only if the roads are perfectly straight and smooth, but in reality, roads are often bumpy and winding, making it less effective.
- Real-Life Example: It’s like building a custom car that runs incredibly well on perfectly straight roads but struggles on real-world roads with curves and bumps. The car’s performance is impressive in theory, but it’s not practical for everyday driving.