GATE CSE 2004 | Question: 11

Question

Consider the following statements with respect to user-level threads and kernel-supported threads

1. context switch is faster with kernel-supported threads

2. for user-level threads, a system call can block the entire process

3. Kernel supported threads can be scheduled independently

4. User level threads are transparent to the kernel

Which of the above statements are true?

• A. (II), (III) and (IV) only
• B. (II) and (III) only
• C. (I) and (III) only
• D. (I) and (II) only

Comments

4th statement supported by these lines of stackoverflow:

User threads and Kernel threads are exactly the same. (You can see by looking in /proc/ and see that the kernel threads are there too.)

A User thread is one that executes user-space code. But it can call into kernel space at any time. It's still considered a "User" thread, even though it's executing kernel code at elevated security levels.

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@Vikrant Singh

Transparency in Computer Sceince means being unaware of something. (Transparent => can't see it)

4th statement says that Kernel is unaware of user-level threads.

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thanks, was unable to get the meaning of the 4^th statement. You made it clear

Answer 1

Answer: (A)

1. User level thread switching is faster than kernel level switching. So, (I) is false.
2. is true.
3. is true.
4. User level threads are transparent to the kernel
   In case of Computing transparent means functioning without being aware. In our case user level threads are functioning without kernel being aware about them. So (IV) is actually correct. 

Comments

@akash.dinkar12

but user level threads can't scheduled independently.

E.g A user process threads a,b,c if b get block then a,c will also be blocked hence process get blocked

am i right?

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@Abhisek Tiwari 4

right

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so is the meaning of ‘independent’ and ‘transparent’ the same in this question’s context?

Answer 2

User level threads can switch almost as fast as a procedure call. Kernel supported threads switch much slower. So, I is false. 

II, III and IV are TRUE. So A. 

"The kernel knows nothing about user-level threads and manages them as if they were single-threaded processes"

Ref: http://stackoverflow.com/questions/15983872/difference-between-user-level-and-kernel-supported-threads

Comments

okay sir

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Very much contrary to normal english

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Sorry sir but transparent means clear to see through opaque is the the word used to  say for something we can't see through.

Answer 3

Threads are independent subsets of processes.

There can be two types of threads:-

1. User-level threads
2. Kernel-level threads

When the threading is done at user-level, kernel isn't aware of it. Because Kernel just has the process table. It knows what the processes are in Main Memory; it has no thread table. The thread table is managed independently by the process. (Statement IV is True)

User-level threading can be done even when the OS doesn't support it, because OS has practically no role to play.

Since the thread table is managed independently by its process, context-switching is faster in user-level threads, as we don't have to inform the kernel. (Statement I is False)

Now since kernel is unaware of threading at the user-level, when a thread makes a system call, kernel sees it as the process making a system call. So, the whole process gets blocked, and not just the thread. (Statement II is True)

 

When the threading is done at kernel level, Kernel has the thread table. So, threads can be scheduled independently by the kernel. (Statement III is True)

Moreover, since each thread has to be registered in the kernel's thread table, context switching of threads in slower in kernel-level threads. But the plus point is that they can be scheduled independently, and when a kernel-level thread makes a system call, just the thread is blocked, and not the whole process.

Comments

I found this answer to be the best. It very well explains the entire concept.

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best answer for someone who is new to threads

Answer 4

The Option (IV)
Transparency can never mean hidden, laws of our constitution is transparent, we need to maintain transparency in the workplace, these all indicates being open to (visible) Someone being transparent means having no secrets, it’s basic English. Either option (IV) is wrong or they may mean it this way:

“User level threads are transparent to the kernel initially” – False

“User level threads are transparent to the kernel once they get into execution” – True
On a long run statement is true, if a thread comes to execution it will no longer be invisible to kernel, but it need not be true always, tough.