What is a “tight loop”?

Question

I\'ve heard that phrase a lot. What does it mean?

An example would help.

Answer 1

Many programming environments nowadays don't expose the programmer to the conditions that need a tight-loop e.g. Java web-services run in a container that calls your code and you need to minimise/eliminate loops within a servlet implementation. Systems like Node.js handle the tight-loop and again you should minimise/eliminate loops in your own code. They're used in cases where you have complete control of program execution e.g. OS or real-time/embedded environments. In the case of an OS you can think of the CPU being in an idle state as the amount of time it spends in the tight-loop because the tight-loop is where it performs checks to see if other processes need to run or if there are queues that need serviced so if there are no processes that need to run and queues are empty then the CPU is just whizzing round the tight-loop and this produces a notional indication of how 'not busy' the CPU is. A tight-loop should be designed as just performing checks so it just becomes like a big list of if .. then statements so in Assembly it will boil down to a COMPARE operand and then a branch so it is very efficient. When all the checks result in not branching, the tight-loop can execute millions of times per second. Operating/embedded Systems usually have some detection for CPU hogging to handle cases where some process has not relinquished control of the CPU - checks for this type of occurrence could be performed in the tight-loop. Ultimately you need to understand that a program needs to have a loop at some point otherwise you couldn't do anything useful with a CPU so if you never see the need for a loop it's because your environment handles all that for you. A CPU keeps executing instructions until there's nothing left to execute or you get a crash so a program like an OS must have a tight-loop to actually function otherwise it would run for just microseconds.