Hardware Concepts

Software Concepts

Program - Process - Threads - Heavyweight／Kernel／OS／Lightweight／Virtual／User-Mode Threads ／ Fibers - Task (Software)

Task Type	Description
program	is a sequence of lines-of-code and/or machine-code
processes	is a program loaded into memory (i.e. in execution) processes are independent sequences of execution a process is an execution of a program processes run on separate memory spaces usually starts with a single thread i.e a primary thread but later down the line of execution it can create multiple threads
threads	threads are independent sequences of execution a thread is an execution of a program driven by the environment of a process threads run in the same memory space of its parent process
virtual thread fiber	a thread is managed and scheduled by the operating system, while a virtual thread is managed and scheduled by a virtual machine
task	either a process or thread or virtual-thread

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Hardware & Software: Putting It Together

Hardware Diagram

Hardware & Software Diagram

Whenever there are more software threads of execution trying to execute at any given time than there are available hardware (simultaneous) threads of execution, then these software threads will be “interleaved” among the available cores. In the case of a “uniprocessor” (one CPU core with no hyper-threading), if you have more than one software thread, they will always be interleaved. If you have a 4-core CPU with hyper-threading, that’s 8 “hardware threads”, meaning the CPU can execute 8 simultaneous threads of execution at the same instant, so if you had 8 software threads trying to run, they could all run at once; but if you had 9 software threads, one of the hardware threads would have to interleave a pair of threads (the exact pair of threads chosen would depend on the operating system’s scheduler implementation).

General Concepts

• synchronous - tasks are executed one after another, each task waits for any previous task to complete and then gets executed
• asynchronous - when one task gets executed, you could switch to a different task without waiting for the previous to get completed
  • asynchronous with outside entity - the dispatching of task(s) to an outside entity, leaving the originating program free to execute other task(s) until it receives a signal from that entity (e.g. Ajax call). That signal can be either: task-finished or error
  • asynchronous without outside entity - similar to multi-tasking
• concurrency - multiple tasks (e.g. processes or threads) in execution, either: simultaneously or not
  • parallel and distributed - multiple tasks in execution simultaneously. It achieves this by distributing tasks to multiple resources (e.g. processors, execution-units)
    • parallel - communicate over a shared memory
    • distributed - communicate over a network
  • multi-tasking - multiple tasks in execution but NOT simultaneously. There is only 1 resource that executes these tasks, therefore at any given moment only 1 task is actually executing code. However, multi-tasking emulates “parallelism” via context switching (i.e. execution time of tasks are interleaved with each other on a single resource)

More Terminology

Synchronous/Asynchronous vs Single/Multi Resource

Resource = Processor or Execution-Unit or Virtual-Unit

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Subpages

• Concurrency Models
• Dish Washing Metaphor - Understanding Async, Non-Blocking, Concurrent, Parallel and More