OS - Processes

Question 1

is “reading from disk” an example of an abstraction the os provides?

Answer 1

Yes! it is machine independent, easier and hiding unnecessary details

Question 2

name what relates to user-mode,kernel mode and hardware: editors, system calls, compilers, shell, process management, I/O, file system, disks, CPU. In addition, why the operating system is named also an “extended machine”?

Answer 2

User interface(shell, editors,compilers,etc) is where users can program. they ask the kernel for info using the library interface(open,close, fork, etc). the library interface is using the system call interface in order to move into “kernel mode” where we have UNIX operating system(process management, memory management, the file system, I/O, etc). the kernel mode then reads/writes the information from/to the Hardware(CPU,memory, disks, terminals, etc..) The OS is referred to as an “extended machine” because right from the hardware it gets extended to “kernel mode” and then to “user mode”

Question 3

“Recipe”, “Baker”,”Ingredients”, “Baking the pie”. who is the process, program,processor and data?

Answer 3

Recipe = Program Baker = Processor Ingredients = data Baking the pie = Process

Question 4

What are the goals of the OS regarding processes?

Answer 4

1. interleave processes executions to maximize processor utilization 2. provide reasonable response time 3. allocate resources to processes. 4. support inter-process communication and synchronization.

Question 5

when is a new process created?

Answer 5

1. System initialization(userinit) 2. Exec 3. fork 4. batch job(?)

Question 6

when does a process terminate?

Answer 6

1. normal exit (voluntary) 2. error exit (voluntary) 3. fatal exit (involuntary) 4. killed by another process (involuntary)

Question 7

what are the process states?

Answer 7

Running - using the CPU Runnable - temporarily stopped to let another process use the CPU Blocked - unable to run until some external(!) event happens.

Question 8

can a process block itself? or run itself?

Answer 8

it can block itself, setting its state field to BLOCKED but it can’t run itself because the scheduler skips it as long it is in blocked state.

Question 9

For what reason does a process become blocked?

Answer 9

1. waits for some event(other process to end its job, etc) 2. waits for input from the kernel.

Question 10

Scheduling: what is a single Blocked Queue?

Answer 10

the scheduler holds a “Ready Queue” for process that are ready to be executed. in addition it holds a “blocked queue” for processes that are waiting for some event.

Question 11

Scheduling: what is a multiple Blocked Queue?

Answer 11

same idea as “Single Blocked Queue” just that here each blocked process is placed in a specific event queue.

Question 12

what is a suspended process?

Answer 12

A process which has been moved from memory to disk

Question 13

why is a context switch called a context switch?

Answer 13

because the processor operates in terms of context - registers, program counter, etc. when we switch processes we switch contexts

Question 14

what happens in a context switch?

Answer 14

1. we save the processor context and save it one the old process struct. 2. update old process state 3. move old process to appropriate queue. 4. select another process for execution(remove from queue) 5. update new process state 6. restore new process context

Question 15

what does “fork” do?

Answer 15

it duplicate a process for a given program to create a new process, to perform other tasks: 1. parent check if process table is full, if not - it allocates a spot for the child 2. parent tries to allocate memory to child’s data and stack. 3. parent copies its code, data and stack to the child’s memory. 4. parent sets eax register of child to 0, so the “fork()” call returns 0 for the child 5. return pid of child.

Question 16

what is exec for?

Answer 16

executing a new program; the program code is loaded to the process’ image: 1. fork() creates a new process 2. execvp replaces the process core image with that of another executable program

Question 17

from shell(sh), which system calls occur in order to exec “ls”?

Answer 17

1. fork() - create more process of sh 2. exec(“ls”,args) - use the new process for the new program - ls.

Question 18

What is a signal? how is it generated?

Answer 18

It is a software interrupt. they are generated via: 1. keyboard - ctrl-C, ctrl-Z 2. kill syscall 3. command line - kill

Question 19

who can a process send signals to?

Answer 19

To all process within its “process group”

Question 20

What can a process do once receiving a signal?

Answer 20

1. ignore it - not always 2. let the system take default action 3. catch it by process’ signal header

Question 21

How defining a signal behavior is done?

Answer 21

by calling: signal(signum, [function|SIG_IGN,SIG_DFL])

Question 22

how does a process receive a signal?

Answer 22

the kernel sets signal bits in the process struct upon receiving signals.

Question 23

when is a “sigabrt” signal(core dump) received?

Answer 23

When there’s fault occurred in the program - usually due to an invalid pointer value(it is likely you are using an uninitialized pointer).

Question 24

when is a “sigalrm” signal(alarm clock) received?

Answer 24

when sleep was called and time is up.

Question 25

when is a “sigsegv” signal(segmentation fault) received?

Answer 25

Accessing memory that “does not belong to you”. that is, you are doing something wrong with memory - accessing memory that has already been freed, writing to a read-only portion of the memory, dereference a null pointer, dereference a pointer that ceased to exist(maybe was removed by the garbage collector) etc.

Question 26

when is a “sigill” signal(illegal instruction) received?

Answer 26

when an attempt is made to execute an invalid, privileged, or ill-formed instruction

Question 27

when does SIGCHILD is sent to parent?

Answer 27

upon child process termination

Question 28

what happens if the parent terminates before the child?

Answer 28

the child is passed to the init process

Question 29

what’s the name for a terminated child process for which its parent didn’t call wait()

Answer 29

A zombie! because it doesn’t get clean by the kernel via “wait()” system call which is called by the parent. that is, it still holds a process table entry.

Question 30

what does a process control block hold? and what from that is moved to the Thread control block?

Answer 30

PCB: code, data, files, registers, user stack and kernel stack. TCB: registers, user stack and kernel stack.

Question 31

what are the fields of a thread struct?

Answer 31

context trapframe kstack state

Question 32

Why is a thread the basic unit for the CPU?

Answer 32

Because it holds all that CPU needs for running commands: program counter; register set; stack space

Question 33

What are the benefits of threads?

Answer 33

1. less time to create(vs process) 2. less time to terminate 3. less time to switch between threads within the same process 4. threads within the same process share memory and files -> they can communicate without invoking the kernel!

Question 34

Does suspending a process means suspending all of its threads?

Answer 34

of course! all threads share the same PTE .

Question 35

In a thread based system, what happens if fork() is called?

Answer 35

Only thread calling fork is duplicated.

Question 36

what are the difference between User-Level Threads to Kernel Level Threads?

Answer 36

User-Level Threads: all management is done by the user. that is, the kernel is not aware of the existence of threads.

Question 37

Advantages of User-Level-Threads

Answer 37

Thread switching does not require kernel mode privileges (faster) and the scheduler can be user specific(can be more efficient)

Question 38

Disadvantages of User-Level-Threads

Answer 38

1. A system call by a thread blocks the process. that means that if one threads reads from disk - all other threads are blocked. 2. Page fault - all other threads are blocked! 3. Time limit - cannot handle clock interrupts per thread. 4. stack growth fault - kernel is not aware of which thread’s stack caused the fault.

Question 39

Kernel-Level Threads. How does it work?

Answer 39

1. kernel maintains context information for the process and the threads. 2. kernel can schedule different threads of the same process to different processors. 3. switching between threads requires the kernel

Question 40

What is the gain of using a multi-core?

Answer 40

Pipelining permits instruction-level parallelism. Multi-core permits both ILP and Thread-Level-Parallelism on same CPU.

Question 41

What is a hardware threads?

Answer 41

It is an hardware in the core which appears to the OS as a logical processor and if the scheduler is aware of HW threads it would be much more efficient. The core stores more registers and logic for much faster thread context switch. For instance, some chips contain 8 cores, each comprising 8 HW cores for a total of 64 concurrent threads!

Question 42

A system call can be roughly grouped into five major categories. What are they?

Answer 42

• Process control
• File management(read/write)
• Device management(attach a device)
• Information maintainance(set time or date)
• Communications(send messages)

Question 43

What is a trapframe

Answer 43

A backup of the process’ current execution state.