⭐ Multithreading (MT)

1. Definition

Multithreading is a processor technique that allows a single CPU or core to execute multiple threads concurrently, improving CPU utilization and throughput.

A thread is the smallest unit of execution within a program, with its own program counter, registers, and stack, but sharing the process’s memory.

2. Purpose

1. Hide long-latency operations: e.g., memory access or I/O waits.
2. Improve CPU utilization: When one thread stalls, another can execute.
3. Increase throughput: Multiple threads progress in parallel.

3. Types of Multithreading

A) Fine-Grained Multithreading

• Switches between threads every CPU cycle.
• Advantage: Hides latency effectively.
• Disadvantage: Single thread executes slower, as CPU is shared every cycle.

B) Coarse-Grained Multithreading

• Switches threads only on long-latency events (e.g., cache misses).
• Advantage: Less context-switching overhead.
• Disadvantage: CPU may be idle if no thread stalls.

C) Simultaneous Multithreading (SMT)

• Multiple threads issue instructions in the same cycle using the same pipeline.
• Example: Intel Hyper-Threading Technology.
• Advantage: Maximizes resource utilization in superscalar pipelines.

4. How Multithreading Works

1. Each thread has its own program counter and registers.
2. Threads share execution units, caches, and memory.
3. CPU switches between threads rapidly (fine-grained) or on stalls (coarse-grained).
4. In SMT, instructions from multiple threads can be issued simultaneously to different functional units.

5. Benefits of Multithreading

1. Better CPU utilization: Keeps pipelines busy when one thread stalls.
2. Higher throughput: More instructions executed per unit time.
3. Reduced branch and memory stall penalties: Another thread can execute during a stall.
4. Cost-effective: Multiple threads share a single core, unlike multi-core designs.

6. Challenges / Limitations

1. Resource contention: Threads compete for functional units, caches, or memory bandwidth.
2. Complex scheduling: Requires hardware or OS support to switch threads efficiently.
3. Security risks: Shared caches and resources can lead to side-channel attacks (e.g., Spectre, Meltdown).
4. Not all workloads benefit: Single-thread heavy workloads may not see improvement.

7. Relation to Other Concepts

8. Example

Assume two threads, T1 and T2:

• Fine-grained: Switch threads every cycle → hides stalls.
• Coarse-grained: Switch thread only when one stalls → less frequent context switching.

9. Exam-Friendly Summary

• Multithreading: Concurrent execution of multiple threads on a single CPU/core.
• Types: Fine-grained, Coarse-grained, Simultaneous (SMT).
• Goal: Improve CPU utilization, hide stalls, increase throughput.
• Limitations: Resource contention, complexity, limited benefit for single-threaded workloads.