Week 5: Process: Implementation Unix and WinNT

Question 1

What does the exec system call function do in UNIX?

Answer 1

It replaces the current process with a new process but retains access to open files.

Question 1

What does the fork system call do in UNIX?

Answer 1

It creates an exact duplicate of the original process, including code, data, program counter, and open files.

Question 2

What does the wait system call do?

Answer 2

It causes the parent process to block until a child process terminates, returning the process ID of the terminated child.

Question 3

What happens when a process calls exit?

Answer 3

The process terminates and can return a status value to its parent.

Question 4

What happens if a parent process terminates before its child?

Answer 4

The child becomes an orphan, and the init process adopts it.

Question 5

What happens if a child terminates, but the parent is not waiting

Answer 5

The child becomes a zombie and the parent waits

Question 6

Why are fork and exec separate in UNIX?

Answer 6

• To allow the child process to perform tasks before the exec takes place
• Sometimes they are used alone

Question 7

How does process creation in MS-DOS differ from UNIX?

Answer 7

MS-DOS suspends the parent process while the child runs, and the parent resumes after the child completes.

Question 8

What is the “terminate and stay resident” (TSR) mechanism in MS-DOS?

Answer 8

It’s a way to simulate multitasking by allowing a process to remain in memory while other processes run.

Question 9

What is the basic unit of scheduling in Windows NT?

Answer 9

A thread, which is a lightweight process, sharing the same code and data within a process.

Question 10

What are the main system calls for process and thread creation in Windows NT?

Answer 10

• CreateProcess for processes
• CreateThread for threads.

Question 11

How does Windows NT handle synchronization between parent and child processes?

Answer 11

Through message passing and special kernel routines for inter-process communication.

Question 12

What are the two parts included in a UNIX process structure?

Answer 12

• The user part
• The kernel part

Question 13

What are the two key data structures related to processes the kernel maintains?

Answer 13

• The process table
• The user structure

Question 14

What information does a process table hold?

Answer 14

• Process identification: process ID, parent process ID, user ID and group ID of process owner
• Scheduling information: process status (sleeping, executing, swapped), current priority
• Memory image pointers: pointers to the code (instructions), data and stack segments
• Signal information: showing which signals are trapped, ignored, pending, etc.

Question 15

What information does the user structure hold?

Answer 15

• Execution state information: values of processor registers, current system call (parameters, results, status), kernel stack
• File descriptor table: one entry per open file (holds file modes (rw), status, position), information on where to find the file
• Accounting information: keeps track of total user and system processor time, some implementations have limits to processor, memory, etc.
• Pointer to process table entry

Question 16

How does the UNIX scheduler work?

Answer 16

It uses a two-level system: a low-level (short-term) scheduler for CPU allocation and a high-level (long-term) scheduler for swapping processes.

Question 17

What algorithm does the UNIX scheduler use?

Answer 17

Dynamic Priority Queue

Question 18

What 5 rules does the UNIX scheduler use?

Answer 18

• The lower the priority number the higher the priority
• Kernel processes have priorities less than 0
• User processes have priorities ≥ 0
• Kernel processes are not preemptable
• User processes are preempted after a quantum of 5 ticks

Question 19

What is the usual base priority value?

Answer 19

0

CPU(0) = 0

Question 20

What is the formula for recalculating UNIX dynamic priorities?

Answer 20

• priority(n) = base + CPU(n)
• CPU(n) = ticks_used(n) + CPU(n-1)/2.

Question 21

What is the purpose of the nice system call in UNIX?

Answer 21

It allows users to deliberately lower a process’s priority by increasing its base value.

Question 22

How are priorities grouped in WinNT?

Answer 22

• realtime priorities (16 - 31): used for time-critical threads, only available to users with administrator privilege
• dynamic priorities (0 - 15): used for application program threads

Question 23

How are dynamic priority levels in Windows NT grouped?

Answer 23

Into high (11-15), normal (6-10), and idle (2-6).

Question 24

What is the time quantum for the Windows NT scheduler on different systems?

The NT scheduler is basically a priority queue

Answer 24

120 ms on NT Server, and 20, 40, or 60 ms on NT Workstation depending on user settings.

Question 25

What happens when a thread requests a system resource in Windows NT?

Answer 25

It is blocked, and the scheduler switches to another thread. When the thread becomes unblocked, it either resumes immediately or is requeued.

Question 26

In which only group of priority levels does the Windows NT have dynamic priority tuning?

Answer 26

Only to dynamic priority levels

Question 27

What events cause priority boosts in Windows NT?

Answer 27

Keyboard or mouse wakeup (+6), other event wakeups (+1), and foreground application boosts.

Question 28

What is priority decay in Windows NT?

Answer 28

In a case in which a thread, which has been boosted, uses an entire quantum without waiting, then its priority decreases by 1 (each time) until it gets back to its original level

Question 29

How does Windows NT prevent starvation?

Answer 29

A high-priority system thread boosts low-priority threads that haven’t executed in over 3 seconds to priority 15.

Question 30

What happens after a thread receives an anti-starvation boost?

Answer 30

It doubles its quantum and immediately executes before returning to its original priority and quantum.