ISRO-DEC2017-63

Question

Which of the following is false?

• A. User level threads are not scheduled by the kernel.
• B. Context switching between user-level threads is faster than context switching between kernel-level threads.
• C. When a user thread is blocked all other threads of its processes are blocked.
• D. Kernel-level threads cannot utilize multiprocessor systems by splitting threads on different processors or cores.

Comments

option d

Answer 1

There are two approaches to implement a thread package.

(1) User Level Threads

Threads which are created and handled by thread library are called user level threads.

• OS is unaware of user level threads
• Creation and handling is cheaper as no system calls are needed. Only CPU registers need to be stored and reloaded
• But if one thread is blocked, whole process will get blocked.
• Eg: Pthreads, Mach C-threads, and Solaris 2 UI-threads

(2) Kernel Level thread

Threads which are created and handled by OS are called Kernel Level Threads.

• Managing is expensive as some system calls are needed.
• Kernel Level scheduler schedules threads from all processes. Hence it can distribute threads on different processors/cores.
• Even if one thread is blocked, process will be active. Hence useful if process gets blocked frequently
• Eg: Windows , UNIX

D is false!!

Answer 2

option D

http://www.geeksforgeeks.org/gate-gate-cs-2014-set-1-question-30/