GATE CSE 2011 | Question: 16, UGCNET-June2013-III: 65

Question

A thread is usually defined as a "light weight process" because an Operating System (OS) maintains smaller data structure for a thread than for a process. In relation to this, which of the following statement is TRUE?

• A. On per- thread basis , the OS maintains only CPU register state . 
• B. The OS does not maintain a separate stack for each thread .
• C. On per- thread basis , the OS does not maintain virtual memory state .
• D. On per- thread basis , the OS maintains only scheduling and accounting information .

Comments

WARNING:

Read the question from https://www.geeksforgeeks.org/gate-gate-cs-2011-question-16/  here.

And the answer is below. Someone juristic person changed the question here. 

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@Lakshman Patel RJIT Sir someone has changed the question  acording to @Akash Papnai Please look into this.

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Yes someone has changed it. Check https://www.iitr.ac.in/gate/gate11.html

Answer 1

Answer to this question is (C).

Many of you would not agree at first So here I explain it how.

OS , on per thread basis, maintains ONLY TWO things : CPU Register state and Stack space. It does not maintain anything else for individual thread. Code segment and Global variables are shared. Even TLB and Page Tables are also shared since they belong to same process.

• A. option (A) would have been correct if 'ONLY' word were not there. It NOT only maintains register state BUT stack space also.
• B. is obviously FALSE
• C. is TRUE as it says that OS does not maintain VIRTUAL Memory state for individual thread which isTRUE
• D. This is also FALSE.

Comments

But question still remains that if its a kernel level thread, each thread can be scheduled independently, why? because compared to user threads, kernel threads have fair scheduling and accordingly each thread can have its own scheduled execution time, then why and how the scheduling & accounting information will be the same?

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great explanation man

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thanks man @Manohar135

Answer 2

From Tanenbaum,

 

The items in the first column are process properties, not thread properties. For example, if one thread opens a file, that file is visible to the other threads in the process and they can read and write it. This is logical, since the process is the unit of resource management, not the thread. If each thread had its own address space, open files, pending alarms, and so on, it would be a separate process. What we are trying to achieve with the thread concept is the ability for multiple threads of execution to share a set of resources so that they can work together closely to perform some task.

Comments

justify option-A/ option-C

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Option A says: 

OS maintains only scheduling and accounting information for each thread

this is incorrect because we know OS also maintains register state, program counter, stack for each thread.

Option C says:

OS does not maintain virtual memory state for each thread

this is correct, OS does not maintain virtual memory info for each thread, threads share the same address space of the process(Remember their code/text section is the same) hence virtual memory state is maintained per process and not per thread.

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@Deepak Poonia sir OS maintain scheduling information for each thread?

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@samarpita Kernal is aware of  KLT (Kernal level threads) of each process hence,they can/should be taken into accounting during scheduling process.

Answer 3

this might help

Answer 4

 

We can come to the conclusion that is (C) is the correct answer 

Answer 5

• On per-thread basis, the OS does not maintain virtual memory state. 

Why this is TRUE:

All threads belonging to the same process share the exact same address space and virtual memory state (such as the page table, code section, data section, and shared heap memory). Because the memory environment is shared globally among peer threads, the Operating System does not duplicate or maintain separate virtual memory states for each individual thread. This is the primary reason why threads have lower overhead and are considered "lightweight."

Why the other statements are FALSE:

•  "On per-thread basis, the OS maintains only CPU register state."

False. In addition to CPU registers (like the program counter), the OS must track other thread-specific components, most notably the thread stack and scheduling parameters (like thread ID and execution state).

•  "The OS does not maintain a separate stack for each thread."

False. Every thread requires its own execution history to handle local variables and function call tracking. Therefore, the OS does maintain a separate stack for each individual thread.

• "On per-thread basis, the OS maintains only scheduling and accounting information."

False. As stated above, it must also actively preserve the private CPU register contexts and stack pointers for each thread during execution.