Threads

Multithreaded execution: 1 process, N threads of execution

• lightweight processes, allow space and time efficient parallelism
• organised in thread groups, allow simple communication and synchronisation

• threads are in the same address space of a single process
• all info exchange done via data shared between threads
• threads synchronise via simple primitives
• each thread has its own stack, hardware registers, and state
• each thread may call any OS-supported syscall on behalf of the process to which it belongs

User threads - pros and cons

• (+) thread switching time is quicker, as no mode switch
• (+) scalability customizability (since no in-kernel management)
• (-) parallelism (blocking syscalls are problematic)
• (-) transparency (typically needs app cooperation)

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Hybrid implementations of threads:

• thread multiplexing: one kernel thread runs everything
  • virtualises N user threads over 1 kernel thread
  • mitigates transparency and parallelization issues
• scheduler activation: scheduler takes care of blocking/unblocking
  • upcalls notify userland of blocking/unblocking events
  • userland schedules thread accordingly
• pop-up threads: kernel takes care of creating threads
  • kernel spawns new thread in response to event
  • thread may run at kernel or user level

POSIX threads (Pthreads)

pthread is a library interface. names may not always be the same.

Inter-process communication (IPC)

• Processes need some way to communicate - to share data during execution
• No explicit cross-process sharing, data must be normally exchanged between processes
• Processes need a way to synchronise
  • to account for dependencies
  • to avoid them interfering with each other