GATE IT 2005 | Question: 62

Question

Two shared resources $R_1$ and $R_2$ are used by processes $P_1$ and $P_2$. Each process has a certain priority for accessing each resource. Let $T_{ij}$ denote the priority of $P_i$ for accessing  $R_j$. A process $P_i$ can snatch a resource $R_k$ from process $P_j$ if $T_{ik}$ is greater than $T_{jk}$.
Given the following :

1. $T_{11} > T_{21}$
2. $T_{12} > T_{22}$
3. $T_{11} < T_{21}$
4. $T_{12} < T_{22}$

Which of the following conditions ensures that $P_1$ and $P_2$ can never deadlock?

• A. (I) and (IV)
• B. (II) and (III)
• C. (I) and (II)
• D. None of the above

Comments

This will make visualization easier.

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why not 3 & 4 also ?

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Yeh que google mei daalne pe aate nhi hai kyu?

Answer 1

If any process has highest priority over all the resources then it can snatch any resource from any other process and so no deadlock can occur with another process as this highest priority process will eventually finish and release all the resources for the other less priority process.

In case of (I) and (II) process $1$ has given highest priority over all the resources and hence deadlock cannot occur.

Similarly, in the case of (III) and (IV) process $2$ has given highest priority over all the resources and hence deadlock cannot occur.

If we consider option (A) (I) and (IV)

•  $T_{11} > T_{21}$     // for resource 1, process 1 has the highest priority
•  $T_{22} > T_{12}$     // for resource 2 , process 2 has highest priority

Let $P_1$ be holding $R_1$ and waiting for $R_2.$

Let $P_2$ be holding $R_2$ and waiting for $R_1.$

This is deadlock as neither is releasing its held resources.

Similarly in option B also deadlock can occur.

Correct answer : C

Comments

Where in the question is written that a process requires both the resources to finish?

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Sir it is not anywhere that i) and ii) together......it is written only i) and ii) if they r taken separately then answer should be D). and how can we make such assumption ? please clearify

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@psrock541 are you really dumb or just acting like one !!

anyways , let me clarify you , If the question wanted to ask what you are saying then they would have mentioned separate options.. Option C is written as I and II which implicitly means that they are to be taken together.

Answer 2

The answer is (C)

If $R_1$ and $R_2$ are allocated to the Process $P_1$ , then it will complete it's job and release it . After that process $P_2$  will get both the resources and complete it's Job .

Comments

Thqq soo much...

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@Digvijay Pandey Thanks for the explanation

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Your comment should be the best answer @Digvijay Sir

Answer 3

Now according to first option::C

T11>T21 Here Process P1 first take resourses and excute

T12>T22 ,Here again process P1 takes resourses and execute in short both of the resourses is taken by process first alone and it complete its execution and after completing it free all its resourses .So it is surely from from all types of deadlock.

Answer 4

Here, we have considered that $R1$ and $R2$ are single instance resources.Hence, option $C$ is correct. 

Hope this helps :)

Comments

thanks man @Franz Kafka

Answer 5

BEST ANSWER

Comments

nice

Answer 6

Option C is right

suppose P2 get the resource R1 and P1 try to get that resource but due to low priority it will get stuck also if P2 get the resource R2 and P1 try to snatch it but due to low priority again it get stuck and in both the cases deadlock will occur.

if the resource allocate to P1 and P2 try to snatch it, it will do it because of higher priority and deadlock would be avoided.