Concurrent Systems: Basics

Concurrent Systems: Basics

2021, Feb 03    
  • Concurrency is about dealing with lots of things at once but Parallelism is about doing lots of things at once.

Difference between Concurrrent and Distributed System

  • Underlying primitives: Shared Memory vs Message Passing
  • Distributed systems experience communications failure
  • Distributed systems may experience unbounded latency
  • Difficulty in syncing distributed time

What is Concurrrency?

  • Computers appear to do many things at once
    • E.g: running multiple programns on a laptop
    • E.g: writing back data buffered in memory to hard disk while the program continue to execute.
  • In the first case, this may actually be an illusion
    • E.g: processes time sharing a single-cored CPU
  • In the second case, there is a true parallelism
    • E.g: Direct Memory Access(DMA) transfers data between memory and I/O devices at the same time as the CPU executes code.
    • E.g: two CPU’s execute code at the same time.

Process

  • Processes are instances of programs in execution
  • OS unit of protection & resource allocation
  • Has a virtual address space; and one or more threads

Threads

  • Threads are entities managed by the scheduler
  • Represents an individual execution context
  • A thread control block (TCB) holds the saved context (registers including stack pointer), scheduler info etc.
  • Threads run in the address spaces of their process
  • Context switches occur when the OS saves the state of one thread and restores the state of another.
  • If a switch is between threads in different processes, then process state is also switched: eg: the address space.

Concurrency with a single CPU

  • With just one CPU, we can think concurrency as interleaving of different executions.
-> Proc(A) -> OS  ->  Proc(B) -> OS ->  Proc(B) -> OS -> Proc(C) -> OS -> Proc(A)
              |                  |                 |                |
time->    timer interrupt     disk interrupt      system call     page fault
  • Concurrency is challenging since where execution will be interrupted and resumed is not known in advance.

Process/OS concurrency

  • Process X runs for a while (until blocks or interrupted)
  • OS runs for a while (e.g. does some TCP processing)
  • Process X resumes where it left off

Concurrency with a multiple CPUs

  • Different threads can be executing on different CPU’s of same process.
  • Ordering of threads execution will be dynamic and are non-deterministic
Twitter Facebook