Threads

“Lightweight Processes”

Resources

• Every thread belongs to one process
• Resources between threads of a Process
  • Shared
    • Text (Instructions)
    • Data (Constants/Statics)
    • Heap
    • Open Files
    • Signals
    • Pending alarms
  • Separate
    • Registers
    • Stack
    • Program counter
    • Scheduler State

Why

• Usage
  • Avoid wasting memory with repeated information
  • Break up complicated tasks into simpler ones
  • Improve efficiency via parallelism
• Advantages over Processes
  • Lower memory cost (shared segments)
  • Information sharing (shared data)
  • Reduced creation, destruction time (no need to copy memory)
    • fork is an expensive system call
  • Reduced context switch time (when within the same process)

pthread Library

• User space threading library
• pthread_create
  • does not promise immediate execution
• termination
  • the main thread terminating terminates all threads
  • any thread calling exit terminates the whole process
  • pthread_exit only terminates the current thread (even if that thread is the main thread)
• pthread_join
  • blocks until a different thread ends
  • if the thread to wait on has already terminated, it does not block

User-Space vs Kernel-Space

• User-Space
  • 
  • Managed by user library (within a process)
  • OS-Independent
  • Syscalls block all process threads
  • Process threads in one CPU
    • Does not allow for concurrency
  • Managed by procedure calls (cheap)
    • Switching between threads is very fast
  • The advantage is sharing resources
• Kernel-Space
  • 
  • Managed by a system library
  • OS-Specific
  • Syscalls block calling thread
  • Process threads in multiple CPUs
    • Allows for concurrency
  • Managed by system calls (expensive)
    • Switching between threads is slower
  • The advantage is sharing resources and concurrency
• Hybrid
  • 
  • We can mix to try to get the best of both worlds
  • However, this is very complex so best only used if necessary