Interprocess Communication

Question 1

What’s the point of interprocess communication?

Answer 1

Processes often need to talk to other processes

Question 2

What are the three key things interprocess communication needs to think about?

Answer 2

1) how processes can communicate with other processes
2) making sure processes don’t get in each other’s ways
3) proper sequencing and making sure dependencies are respected

Question 3

Define race condition

Answer 3

Where two or more processes are reading or writing some shared data and the final result depends on who runs when

Question 4

What does mutual exclusion allow:

Answer 4

that if one process is using a share dvariable or file, another process will not be able to use this fle.

Question 5

Define critical section

Answer 5

the part of the program where the shared memory is accessed

Question 6

What are the four conditions to create ensure that processes cooperate correctly?

Answer 6

1) No two processes may be inside their critical section
2) No assumptions can be made about speed or number of CPUs
3) No processes running outside critical section may block other processes
4) No process should have to wait forever to enter its critical section

Question 7

What is busy waiting?

Answer 7

a technique in which a process repeatedly checks to see if a condition is true

Question 8

what happens when you disable all interrupts?

Answer 8

you have a process disable interrupts when it enters its critical section. this means no clock interrupts can occur.

Question 9

once a process has disabled all interrupts what can it do confidently?

Answer 9

access and update shared memory

Question 10

give two reasons why is disabling interrupts unattractive?

Answer 10

1) if they never turned interrupts back on again, then the CPU would never get a look in
2) if the system has multiple processors, the other processors will not know that the process on another CPU has disabled interrupts. They will keep going as normal.

Question 11

what is the spooler example?

Answer 11

when process wants to print a file, it tells the spooler directory
printer daemon checks if there are files to print
spooler has in and out variables
consider a spooler with an empty slot (7) and two files that want to print
item a reads a variable for next space to be 7
process switch immediately happens
item b reads same variable and also sets to 7. it continues and puts an item in 7 and sets next to 8
process switch
item a returns. it then puts a file in 7 and sets next space to 8
== item b has been overwritten

Question 12

what are lock variables?

Answer 12

a software solution
process setting a lock variable to 1.
process enters the critical section

Question 13

why are lock variables janky?

Answer 13

same idea as spooler directory
if one process reads lock as 0 and gets ready to make it 1, then process switches to something else and then THAT process also sets something to 1, then you’ll find yourself in the critical section for two process. NOPE.

Question 14

define busy waiting

Answer 14

continuously testing a variable until some value appears

Question 15

why should busy waiting be avoided?

Answer 15

wastes CPU time

Question 16

what is a lock that uses busy waiting called?

Answer 16

spin lock

Question 17

why is strict alternation not a good idea?

Answer 17

because you can’t assume the speed of a program. it is only sound if they are balanced.

Question 18

what kind of violation can happen with strict alternation?

Answer 18

a process wanting to enter its critical section is not allowed to enter its critical section.

Question 19

What does Peterson’s solution combine?

Answer 19

taking turns and lock variables

it uses the idea of ‘interested’ in code blocks

Question 20

define the priority inversion problem

Answer 20

where a low priority process and a low priority process are flipped. The means the low priority process is running and the high priority process is waiting

Question 21

what is the producer-consumer problem?

Answer 21

two processes
producer / consumer
producer generates data and puts on buffer
consumer takes items off buffer

Question 22

where does trouble arise for producer-consumer problem?

Answer 22

When the buffer is full and the producer then needs to sleep.
If the buffer is empty and the consumer wants to take something from it.

Question 23

what is the problem with the producer-consumer problem?

Answer 23

consumer sees the buffer is empty
then the scheduler interrupts
producer sees the buffer is empty.
producer generates an item for the buffer.
producer sees count is 0 and wakes consumer.
but consumer is not asleep.
consumer keeps reading count, sees 0, actually goes to sleep.

Question 24

define atomic operation

Answer 24

a process that is indivisible. guaranteed to be isolated from other operations that may be happening at the same time