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. 

---

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

---

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

yes i got c too

thw word "only" was bugging me

---

can you explain something about scheduling and accounting information..what does it include?

---

@Sandeep_Uniyal

i think you have hiven explanatio for D and C is false

correct me if wrong

---

I think somebody edited the ques plz edit the ans also...its conflicting @Arjun sir

---

What is the meaning of virtual memory state??

---

accounting information is the information related to the amount of cpu time the thread or process has utilised till that instance this is not maintaned per thread basis it is maintained as basis of process per process in PCB

---

But option A is about Scheduling and Accounting information, not about Stack and registers... Please explain about option A

---

The explanation for the first option is not right. Not sure about scheduling, but accounting information is maintained per process (one for all threads, not each thread). I don't know why you compared it with registers and stack. And thus, false.

And also, thread control block not only maintains stack and register pointers but also register state and program counter. And to be more precise, it also contains a PCB pointer. Please don't misguide people.

---

@warlock agreed with you!

---

@warlock

somebody edited the question

the options got shuffled i guess......what is explained as Option A is actually meant for option B i guess

---

I think the virtual memory state is the if a thread wants to access a page of the process it uses the mappings the page tables and the TLB every process has its own page table hence we dont need a seperate page table per thread  correct me if i am wrong @Arjun sir

---

@venkat

threads maintain separately for individual threads only two things _ stack and registers

---

@A_i_$_h  that  is what i was saying threads can share global variables in code(data segment) and the code segment and the heap segment of a process they have their own register file, stack, and Program counter  

---

Small correction here - Each thread will have Program Counter, stack and register seperately.

---

@Lokesh i think what they mean by virtual memory state is the virtual address space. Since each process has it's own Virtual Address Space, all threads of that process will share that space.

---

Consider like this-If each process in the system has 100 threads, if OS maintains per thread virtual memory information, then it will much larger overhead than maintaining virtual mem info for processes.Hence option (C) looks suitable.

---

@Ayush Upadhyaya

@Shaik Masthan

OS maintains per process vitrual memory state  i.e Virtual address space ..All the threads of a process shares that address space..

Per thread OS maintains registers , stack , program counter , pointer to parent PCB , scheduling info(in case of kernel level thread whose scheduling done by kernel) not sure about last one

What is accounting info of process?

---

@jatin khachane 1-Accounting info is like how much cpu time process has used, what I/O devices are held or used by the process all those information that tells how much the process is using what resource of the computer.

---

I think If Its Kernel Level Thread , all threads are scheduled Seperately so i think seperate info per thread is maintained ?

---

In an Operating System, scheduling and accounting information for a thread includes:

1. Scheduling Information: 
   - Priority
   - State
   - Scheduling Queue Pointers
   - CPU Time Usage

2. Accounting Information:
   - Execution Time
   - Resource Usage 
   - Owner or User ID 
   - Start and End Times 

Together, scheduling and accounting information helps the OS manage threads efficiently, allocate CPU resources, and track resource usage for each thread.

---

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?

---

great explanation man

---

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

---

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.

---

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

---

@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