Processes and Threads

Question 1

Processes

Answer 1

• A process is an abstraction of a running program
• It enables doing several things at the same time
• support the ability to have (pseudo) concurrent operation
• Ex. consider a user PC ??
• In a multiprogramming system, the CPU switches from process to
  process quickly, running each for tens or hundreds of milliseconds

Question 2

Process and Program

Answer 2

• A process is an activity of some
  kind
• A program is something that
  may be stored on disk, not doing
  anything
• Ex: baking vs. recipe

Question 3

The Process Model

Answer 3

• All the runnable software on the computer, sometimes including the
  operating system, is organized into a number of sequential processes
• To understand the system, it is much easier to think about a collection
  of processes running in (pseudo) parallel
• This rapid switching back and forth is called multiprogramming

Question 4

The Process Model

Answer 4

a) Multiprogramming four programs.
b) Conceptual model of four independent, sequential processes.
c) Only one program is active at once

Question 5

Process Creation

Answer 5

• Events which cause processes creation:
  1. System initialization.
  2. Execution of a process creation system call by a running process.
  3. A user request to create a new process.
  4. Initiation of a batch job.
• In all these cases, a new process is created by having an existing
  process execute a process creation system call.

Question 6

Process Termination

Answer 6

• Events which cause process termination:
  1. Normal exit (voluntary).
  2. Error exit (voluntary).
  3. Fatal error (involuntary).
  4. Killed by another process (involuntary).

Question 7

Process States

Answer 7

• Three states a process may be in:
  1. Running (actually using the CPU at that instant).
  2. Ready (runnable; temporarily stopped to let another process run).
  3. Blocked (unable to run until some external event happens)

Question 8

Implementation of Processes

Answer 8

• The lowest layer of a process-structured operating system handles
  interrupts and scheduling.
• Above that layer are sequential processes.

Question 9

Process Table

Answer 9

One entry per process
* Contains important information
about the process’ state
* Information that must be saved
when the process is switched
from running to ready or blocked
state so that it can be restarted
later

Question 10

Interrupt Handling and Scheduling

Answer 10

1. Hardware stacks program counter, etc.
2. Hardware loads new program counter from interrupt vector.
3. Assembly language procedure saves registers.
4. Assembly language procedure sets up new stack.
5. C interrupt service runs (typically reads and buffers input).
6. Scheduler decides which process is to run next.
7. C procedure returns to the assembly code.
8. Assembly language procedure starts up new current process.

Question 11

Threads

Answer 11

• In many applications, multiple activities are going on at once
• Threads give the ability for the parallel entities to share an address
  space and all of its data
• not possible with multiple processes (with their separate address spaces)
• Threads are lighter weight than processes, so they are easier (i.e.,
  faster) to create and destroy
• Threads allow computing and IO activities to overlap

Question 12

Thread Usage - A Word Processor

Answer 12

• First thread interacts with the user.
• Second thread handles reformatting in the background.
• Third thread handles the disk backups.

Question 13

Thread Usage - A Multithreaded Web Server

Answer 13

The dispatcher reads incoming requests for work from the
network and chooses an idle (i.e., blocked) worker thread to
hand the request

Question 14

The Classical Thread Model

Answer 14

• Multithreading is the situation of allowing multiple threads in the
  same process

Question 15

Advantages of Thread over Process

Answer 15

• Responsiveness:
• If the process is divided into multiple threads, one thread can complete its
  execution and its output can be immediately returned.
• Faster context switch:
• Context switch time between threads is lower compared to process context
  switch. Process context switching needs more CPU overhead.
• Effective utilization of multiprocessor system:
• If we have multiple threads in a single process, then we can schedule multiple
  threads on multiple processor.
• Resource sharing:
• Resources like code, data, and files can be shared among all threads within a
  process. (Stack and registers can’t be shared)
• Communication:
• Communication between multiple threads is easier, as the threads shares
  common address space. while in process we have to follow some specific
  communication technique for communication between two process.
• Enhanced throughput of the system:
• If a process is divided into multiple threads, and each thread function is
  considered as one job, then the number of jobs completed per unit of time is
  increased, thus increasing the throughput of the system.

Question 16

Inter-process Communication (IPC)

Answer 16

Processes frequently need to
communicate with other
processes:
* To pass information from process
to another
* To enable proper sequencing
when dependencies are present
* To ensure that no two processes
are ever in their critical regions at
the same time.

Question 17

Mechanisms for IPC:

Answer 17

• Atomic read/write
• Locks
• Semaphores
• Monitors
• Message Passing

Question 18

Race Conditions

Answer 18

• Two or more processes are
  reading or writing some shared
  data and the final result depends
  on who runs precisely when
• Ex. two processes want to access
  the printer spooler directory at
  the same time

Question 19

Conditions Required to Avoid Race Condition

Answer 19

1. No two processes may be simultaneously inside their critical regions.
   – Mutual Exclusion (mutex)
2. No assumptions may be made about speeds or the number of CPUs.
   – No Assumption
3. No process running outside its critical region may block other processes.
   – Progress
4. No process should have to wait forever to enter its critical region.
   – No Starvation

Question 20

Critical region

Answer 20

the part of the program where shared variables are
accessed

Question 21

Mutual Exclusion with Busy Waiting

Answer 21

• Disabling interrupts
• Each process disables all interrupts just after entering its critical region and
  re-enable them just before leaving it
• In a multicore (i.e., multiprocessor system) disabling the interrupts of one
  CPU does not prevent other CPUs from interfering with operations the first
  CPU is performing
• Lock variables
• A single, shared (lock) variable, process sets it to 1 and enters the critical
  region. If the lock is already 1, the process just waits until it becomes 0.
• Has the same fatal flaw that we saw in the spooler directory.
• Strict alternation
• two processes strictly alternate in entering their critical regions
• it is not really a serious candidate as a solution because it violates condition 3
• Peterson’s solution
• The TSL instruction