Why is gem5 still single threaded?

gem5 simulator is not inherently multithreaded for several reasons:

1. Complexity of Parallelism: Gem5 simulates intricate details of computer architectures, including pipelines, caches, memory hierarchies, and multiple components. Synchronizing and managing these aspects across multiple threads can be extremely challenging and error-prone.
2. Determinism: Many architectural simulations require deterministic behavior to ensure reliable results. Introducing multithreading can introduce non-deterministic behaviors due to race conditions and thread scheduling, making it harder to replicate and verify simulation outcomes.
3. Lack of Multithreading Benefits: In some cases, adding multithreading to a simulator like Gem5 might not lead to substantial performance improvements, as the simulator’s primary bottlenecks are often related to single-threaded execution, memory access patterns, and modeling accuracy.
4. Simulation Overheads: Multithreading can introduce overhead due to the need for synchronization, thread management, and inter-thread communication. These overheads might outweigh any potential performance gains in certain simulation scenarios.
5. Research Focus: Gem5 is primarily designed for research and experimentation, and its primary goal is accuracy and flexibility. Multithreading, while potentially beneficial for speed, could complicate the simulator’s codebase and hinder its flexibility for exploring different architectural models.

It’s important to note that while the core Gem5 simulator is primarily single-threaded, researchers and developers can leverage distributed computing techniques and parallel execution to run multiple Gem5 instances in parallel, simulating multiple cores or systems concurrently. This approach can achieve some level of parallelism while still maintaining the accuracy and determinism required for architectural research.