Thread

Definition:

• Thread is an execution unit that is part of a Process
• A process can have multiple threads executing at the same time
• Multi-threading

User and Kernel threads:

• User threads - management done by user-level threads library, e.g., Pthreads (Linux), Java threads
• Kernel threads - supported by the Kernel, e.g., Linux, MacOS

Multithreading models:

• Many-to-One
  • Many user-level threads mapped to single kernel thread
  • One thread blocking causes all to block
• One-to-one
  • Each user-level thread maps to kernel thread
    • Creating a user-level thread creates a kernel thread
    • Number of threads per process sometimes restricted due to overhead
• Many-to-Many:
  • Allows many user level threads to be mapped to many kernel threads
  • Allows OSs to create a sufficient number of kernel threads
  • Not very common
• Hybrid (2-level)

Threading issue:

• Semantics of fork() and exec() system calls
  • fork(): duplicate only the calling thread or all threads
• exec(): replace the running process including all threads
• Signal handling: Synchronous and asynchronous
• Thread cancellation of target thread
  • Asynchronous (immediately) or deferred (periodically)
• Thread-local storage: own copy of data
• Scheduler activations: Lightweight process (LWP)