“All my friends drive a low rider And a low rider is a little lower… “ American funk band War

Notes on low level things

This post is about lower level concepts that are useful in understanding the higher level concepts. They are in no particular order other than possibly Google Search order. There are definitely more within the context of NodeJS which seems to involve working and understanding on this lower level, but in the final analysis applies to all languages. All of this junk basically is to pull together a foundation for programming in NODE.JS

Threads vs Processes

• a thread of execution is the smallest sequence of programmed instructions that can be managed by the operating system
• implementations of threads varies across O/S’s but for the most part threads are usually a component of a process.
• a PROCESS is an instance of a program, which could contain one or more threads.
• processes are typically independent, while threads exist as subsets of a process
• processes carry considerably more state information than threads, whereas multiple threads within a process share process state as well as memory and other resources
• processes have separate address spaces, whereas threads share their address space
• processes interact only through system-provided inter-process communication mechanisms
• context switching between threads in the same process is typically faster than context switching between processes.
• threads running in the same process share the same address space, which allows them to share data faster.
• user thread implementations typically exist in userspace or “userland” which is seperate memory from where the OS kernel processes run.

Single-Threading vs Multi-Threading

• Note: systems with a single processor implement multi-threading by time-slicing where the CPU switches between different threads.
• On a multi-processor or multi-core system multiple threads can execute in parallel.
• Single threading means processing one command at a time.
• Multi-threading found in multi-tasking systems allows multiple threads to exist within the context of one process, where they share the processes resources, but are able to execute independently.
• Advantages of Multi-threading
  • responsiveness of application(no blocking of main thread execution)
  • faster execution, lower resource use, running in parallel.
• Negatives
  • synchronization/complexity is the main drawback.

non-blocking I/O

• An alternative to multi-threading to keep an application responsive.
• also called asynchronous I/O, the application can keep processing before the async task has completed.
• examples of I/O are hardware devices like keyboards, drives, etc., but also any transfer of information between the CPU and memory is I/O.
• asynchronous I/O is “built in” to modern operating systems.
• Interesting note: the method of implementation can vary among operating systems, or their may be many methods available on the same O/S.
• Forms of I/O
  • Process : see multitasking.
  • Polling : continually checks if something can be done. Single threaded.
  • Select Poll/Loops : a variation of polling, uses a loop(select system call is used to sleep until a change occurs with the file descriptor, timeout occurs or signal received) which finds out which file selector changed and execute the associated code.
  • the “select loop” is often implemented as an EVENT LOOP, often using callback functions.
  • it assumes via Unix that everything is a file(file descriptor), and anything that is not can block the process.