GATE - 2019 | Consider the following snapshot of a system running n concurrent processes. Process i

GATE - 2019 [Operating System]

Question:

Consider the following snapshot of a system running n concurrent processes. Process i is holding X_i instances of a resource R, 1 ≤ i ≤ n. Assume that all instances of R are currently in use. Further, for all i, process i can place a request for at most Y_i additional instances of R while holding the X_i instances it already has. Of the n processes, there are exactly two processes p and q such that Y_p = Y_q = 0. Which one of the following conditions guarantees that no other process apart from p and q can complete execution?

A	Min (Xp, Xq) ≥ Min {Yk | 1 ≤ k ≤ n, k ≠ p, k ≠ q}
B	Xp + Xq < Max {Yk | 1 ≤ k ≤ n, k ≠ p, k ≠ q}
C	Min (Xp, Xq) ≤ Max {Yk | 1 ≤ k ≤ n, k ≠ p, k ≠ q}
D	Xp + Xq < Min {Yk | 1 ≤ k ≤ n, k ≠ p, k ≠ q}

 

Solution 1:

{P₁, P₂, ..., P_n}
P_i holds X_i instances.
P_i can request additional Y_i instances.
Given two process p & q such that their additional requests are zero.
Y_p = Y_q = 0
{Y_k | 1 ≤ k ≤ n, k ≠ p, k ≠ q} means that out of 'n' processes, we are left with (n-2) process (except p&q), i.e., Y_k indicates additional request of all the processes (n-2) except p & q.
For p & q to complete first, accordingly
X_p + X_q < Min {Y_k}
Option D is correct.
There are exactly two process p and q which do not need any additional instances of resources.
So, p and q will complete their execution and will release X_p and X_q instances of resources.
Now to guarantee that no other process apart from p and q can complete execution, the no. of instances of resources available must be less than the minimum no. of instances of resources required by any other process, i.e.,
X_p + X_q < Min {Y_k | 1 ≤ k ≤ n, k ≠ p, k ≠ q}

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Solution 2:

 

Xi → Holding resources for process pi,
Yi → Additional resources for process pi. 

As process p and q doesn’t require any additional resources, it completes its execution and available resources are (Xp + Xq)

There are (n – 2) process pi (1< i < n, i ≠ p, q) with their requirements as Yi (1 < i < n, i ≠ p, q).

In order to not execute process pi, no instance of Yi should be satisfied with (Xp + Xq) resources, i.e., minimum of Yi instances should be greater than (Xp + Xq).

 

       

Thank You

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