Asynchronous Model - Keep track of requests waiting to be fulfilled.

Thread Model

Each thread has its own stack

  • Local variables per thread
    • Allocated in their own stack frame
  • Global variables are shared between all threads
    • Allocated in the data section
    • Concurrency control is an issue
  • Dynamically allocated memory (malloc) can be global or local
    • Program defined (pointer can be global or local)

A thread uses less resources than a process.

  • Program Counter
  • Registers
  • Stack
  • State

Example - Multithreaded Web Server

Consider two requests made to a single threaded server.
If the first request takes a long time to fetch a resource, the second requests will never be attended to until the first request is done.

In a multi-threaded server, there is a dispatcher thread which delegates requests to the pool of worker threads.
This creates a degree of parallelism.

Kernel-Level Threads

  • TCB stored in the kernel

  • Pros

    • Preemptive Multithreading
    • Can pause the state of a thread
    • Parallelism
    • Can perform other tasks whilst one thread is blocked from an outstanding syscall
    • Multiprocessor usage

  • Cons

    • Operations are more expensive than with user-level threads
    • Thread operations (creation, destruction, blocking, unblocking) require the processor to enter kernel mode
      • Kernel entry and kernel exit delay

User-Level Threads

  • User-level Thread Control Block (TCB)
  • User-level ready queue
  • User-level blocked queue
  • User-level dispatcher

A developer can implement a scheduler into their own program.
The kernel will only see this program as a single process.

Consider yield in Python

  • Pros
    • Don't need to wait for traps to and from the kernel
    • Fine tuned dispatching algorithm
    • OS independent
  • Cons
    • You probably will screw up writing it
    • Does not utilise multiple CPUs, as the program was only allocated one CPU
    • Consider a virtual machine which has been assigned only one CPU on a device with 8 cores
    • A blocking syscall will block the process, and hence all the threads