Problem Detail: I am having difficult time understanding the difference between weakly fair and strongly fair schedulers. Can someone provide an example and explain how they are different? for reference, here are the definitions I have of each: weakly fair: A condition becomes true and remains true at least until after the conditional atomic action has been executed. strongly fair: If the condition is infinitely often true, a process will eventually see it as true and be able to proceed. So I think I understand weakly fair, a conditional atomic action will execute when its condition is true, and the condition will remain true until the atomic action has ended. But I don’t see how strongly fair is different from this. for a conditional atomic action, once its condition is true it executes, and because it is atomic, the condition should remain true until it ends, right? this is the same as weakly fair isn’t it?
Asked By : JDOdle
Answered By : Shaull
If you are familiar with temporal logic, the difference is quite easy to demonstrate: Weak fairness is $FGpto Fq$. That is, if $p$ holds from some point and on, then $q$ will hold eventually. Strong fairness is $GFpto Fq$ (or sometimes $GFpto GFq$). That is, if $p$ holds infinitely often, then eventually $q$ will hold (or $q$ will hold infinitely often, in the second version). Your description is slightly different from the above, in that your weak fairness has the “until” flavour, but it’s the same idea. To demonstrate the difference in the approaches, think of yourself as a process trying to gain access to some critical section. If you know that the system is weakly fair, then you need to raise a flag saying that you want access, and you have to keep this flag up all the time, until you get access. If the system has strong fairness, it is enough that you raise the flag every now and then, and you know that eventually you’ll get access. As an alternative example, children who want attention often go “mom” repeatedly until they are granted attention. This is similar to weak-fairness (of the mother), and as you can see, it prevents the child from doing anything else until granted attention. With a strongly-fair mother, it suffices for the child to say “mom” every few minutes, and go about doing other things in the mean time. By the way, this question has an excellent answer by Cynthia Disenfeld at quora.
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