1. Processes (Διεργασίες)
Chapter 2
Processes (Διεργασίες)
2.1 Processes - Διεργασίες
2.2 Interprocess communication – Διαδιεργασιακή επικοινωνία
2.3 Classical IPC problems
2.4 Scheduling - Χρονοπρογραμματισμός

2. Processes The Process Model
Pseudo-parallelism: rapid switching back and forth of the CPU between programs
A process is an executing program + values of the PC+registers+variables.
Think the processes run in parallel – each on a separate CPU.
Independent sequential processes.

3. Processes The Process Model
Multiprogramming of four programs
Conceptual model of 4 independent, sequential processes
Only one program active at any instant

4. Processes The Process Model
Processes must not be programmed with built-in assumptions about timing.
E.g. an idle loop to wait for I/O reading.
Processes are NOT programs.
E.g. someone preparing a birthday cake following a recipe (=the program). The whole activity is the process.

5. Process Creation Principal events that cause process creation
System initialization
Execution of a process creation system
User request to create a new process
Initiation of a batch job

6. Process Termination Conditions which terminate processes
Normal exit (voluntary)
Error exit (voluntary)
Fatal error (involuntary)
Killed by another process (involuntary)

7. Process Hierarchies
Parent creates a child process, child processes can create its own process
Forms a hierarchy
UNIX calls this a "process group"
Windows has no concept of process hierarchy
all processes are created equal

8. Process States - Καταστάσεις (1)
Process often need to interact with other processes.
E.g. cat chapter1 chapter2 chapter3 | grep tree
Three states: runnable, blocked, running (έτοιμη ή εκτελέσιμη, υπό αναστολή, εκτελούμενη)
Blocked != Runnable

9. Process States (2) Possible process states
running (using the CPU now)
blocked (unable to run until an event happens)
ready (runnable; temporarily stopped)
Transitions between states shown

10. Process States (3) Lowest layer of process-structured OS
handles interrupts, scheduling, i.e. scheduler does more than process scheduling
Above that layer are sequential processes

11. Implementation of Processes (1)
To implement the process model the OS maintains the process table (πίνακας διεργασιών). There is one entry for each process, keeping the process state.

12. Implementation of Processes (2)
Fields of a process table entry

13. Implementation of Processes (3)
Skeleton of what lowest level of OS does when an interrupt occurs

14. Interprocess Communication Race Conditions (συνθήκες ανταγωνισμού)
Processes frequently communicate with other processes (IPC).
Processes share common storage (read/write).
Situations where two or more processes are reading or writing some shared data and the final result depends on who runs when are called race conditions.
E.g. printing and spooler directories.

15. Interprocess Communication Race Conditions
Two processes want to access shared memory at same time

16. Critical Regions (1) – Κρίσιμα τμήματα
Prohibit more than one process to read/write
What is needed is mutual exclusion (αμοιβαίος αποκλεισμός)
Primitive operations to achieve mutual exclusion – a design choice for an OS
The part of the program that accesses shared memory is called critical region

17. Critical Regions (2) Four conditions to provide mutual exclusion
No two processes simultaneously in critical region
No assumptions made about speeds or numbers of CPUs
No process running outside its critical region may block another process
No process must wait forever to enter its critical region

18. Mutual exclusion using critical regions

19. Mutual Exclusion with Busy Waiting (1) Αμοιβαίος αποκλεισμός με ενεργό αναμονή
Disabling interrupts (απενεργοποίηση διακοπών): a process disables all the interrupts when entering the critical region and enabling them before exit
Lock variables (μεταβλητές κλειδώματος): A software solution. Unfortunately does not work.
Strict alternation (αυστηρή εναλλαγή)
Peterson’s solution
TSL instruction

20. Mutual Exclusion with Busy Waiting (2) Strict Alternation
Proposed solution to critical region problem
(a) Process (b) Process 1.

21. Mutual Exclusion with Busy Waiting (3) Peterson’s solution
Peterson's solution for achieving mutual exclusion

22. Mutual Exclusion with Busy Waiting (4)
TSL instruction= test and set lock – indivisible.
Use of a shared variable, flag (could be 0 or 1).
Entering and leaving a critical region using the
TSL instruction

23. Sleep and Wakeup (1) – Απενεργοποίηση και αφύπνιση
Previous solutions require busy waiting:
the waiting process sits in a tight loop
wastes CPU time
unexpected effects
A process may block instead of waiting
SLEEP and WAKEUP(pid) system calls
Example: producer and consumer (παραγωγός- καταναλωτής)

24. Producer-consumer problem with fatal race condition
Sleep and Wakeup (2)
Producer-consumer problem with fatal race condition

25. Semaphores(1) - Σημαφόροι
A semaphore can have the value 0 or > 0
Two operations, DOWN and UP
DOWN: if semaphore > 0 then semaphore-- otherwise it sleeps (atomic action-all in one)
UP: increments the semaphore and waking up a process – again indivisible
DOWN and UP should be atomic
The producer-consumer problem using binary semaphores

26. Semaphores(2)

27. Monitors (1) - Παρακολουθητές
Semaphores are good but there are deadlocks.
A higher level of synchronization principle
A monitor is a collection of procedures, variables and data structures grouped together in a module or package.
Processes can call procedures of the monitor but can not access internal data structures.
!Only one process can be active in a monitor!

28. Monitors (2)
Example of a monitor

29. Monitors (3)
Monitors are programming constructs => compilers job to implement (binary semaphores)
Monitors achieve mutual exclusion but processes must block when they can not proceed.
Condition variables (μεταβλητών συνθήκης) + WAIT + SIGNAL
Ensure: no two active processes in the monitor.
WAIT and SIGNAL are similar to SLEEP and WAKEUP, but without the known bug.
Problem: it is a programming language concept

30. Monitors (4) Outline of producer-consumer problem with monitors
only one monitor procedure active at one time
buffer has N slots

31. Solution to producer-consumer problem in Java (part 1)
Monitors (5)
Solution to producer-consumer problem in Java (part 1)

32. Solution to producer-consumer problem in Java (part 2)
Monitors (6)
Solution to producer-consumer problem in Java (part 2)

33. Message Passing (1) – Μεταβίβαση μηνύματος
Uses 2 primitives: SEND, RECEIVE
SEND (destination, &message)
RECEIVE(source, &message)
These are system calls, like semaphores
Many interesting problems and design issues
lost messages
naming processes
authentication

34. The producer-consumer problem with N messages
Message Passing (2)
The producer-consumer problem with N messages

35. Message Passing (3) Implementing Message Passing: buffers
Mailboxes - γραμματοκιβώτιο
Rendezvous – συνάντηση
pipes - σωληνώσεις

36. Equivalence of Primitives (1)
Using semaphores to implement monitors
Associated with each monitor is a binary semaphore, mutex, initially 1, to control entry to the monitor and an additional semaphore, initially 0, per condition variable
Associated with each process is a semaphore, initially 0, on which it will block when a SEND or RECEIVE must wait for completion.

37. Equivalence of Primitives (2)
Using monitors to implement semaphores
Using monitors to implement message passing
Using message passing to implement semaphores
Using message passing to implement monitors

38. Dining Philosophers (1)
Philosophers eat/think
Eating needs 2 forks
Pick one fork at a time
How to prevent deadlock

39. Dining Philosophers (2)
A nonsolution to the dining philosophers problem

40. Dining Philosophers (3)
Modifications:
after taking the left fork, the program checks to see if the right fork is available. If not, put down the left fork and wait for some time. Starvation.
waiting random time – critical applications.
protect the five statements following the call to think by a binary semaphore. Performance problem.

41. Dining Philosophers (4)
Solution to dining philosophers problem (part 1)

42. Dining Philosophers (5)
Solution to dining philosophers problem (part 2)

43. The Readers and Writers Problem (1)
Big database system (e.g. airline reservation)
Processes compete to write and read
Many readers concurrently
Only one writer at any time – no readers and writers

44. The Readers and Writers Problem (2)
A solution to the readers and writers problem

45. The Sleeping Barber Problem (1)

46. The Sleeping Barber Problem (2)
Solution to sleeping barber problem.

47. Scheduling- χρονοπρογραμματισμός Introduction to Scheduling (1)
Bursts of CPU usage alternate with periods of I/O wait
a CPU-bound process
an I/O bound process

48. Scheduling Introduction to Scheduling (1)
Schedulers, scheduling algorithms
Goals: fairness, efficiency, response time, turnaround, throughput (δικαιοσύνη, αποδοτικότητα, χρόνος απόκρισης, κύκλος διεκπεραίωσης, ρυθμός απόδοσης)
Processes are unique and unpredicatable
Preemptive scheduling vs. run to completion
Interactive systems vs. batch systems

49. Introduction to Scheduling (2)
Scheduling Algorithm Goals

50. Round Robin Scheduling –Εκ περιτροπής
One of the oldest, fairest, most widely used algorithms
Each process is given a time interval, called quantum
Round Robin Scheduling
list of runnable processes
list of runnable processes after B uses up its quantum
Length of quantum could be too short or too long
Process switch or context switch

51. Priority Scheduling (1) - Προτεραιότητες
Round robin assumes all processes equal
May have different groups with priorities
Idea: Each process is assigned a priority and the runnable process with the highest priority is allowed to run
The scheduler may decrease the priority of each process at each clock tick (Unix: nice)
Priorities can be assigned dynamically (I/O)

52. Priority Scheduling (2)
A scheduling algorithm with four priority classes

53. Multiple Queues – Πολλαπλές ουρές
Set up priority classes
Processes in the highest class run for one quantum. In the next highest class run for two quanta, four quanta, etc.
Minimize swaps

54. Shortest Job First Mainly batch systems, running times are known
An example of shortest job first scheduling:
14m
11m
4 jobs with running time: a,b,c,d. Completion time is: 4a+3b+2c+d.
Minimum average response time. Can it be used for interactive processes?

55. Policy-Driven Scheduling – Εγγυημένος χρονοπρογραμματισμός
Make promises to user and live up to them
Example: if there are n users in the system you will get 1/n CPU power
Keep track of how much CPU time a user has had for all his processes since login
A similar idea can be applied to real-time systems where there are deadlines

56. Scheduling in Real-Time Systems
Schedulable real-time system
Given
m periodic events
event i occurs within period Pi and requires Ci seconds
Then the load can only be handled if

57. Two Level Scheduling Some processes may have to be kept in disk.
Major implications – Algorithms must change
Two level-scheduler:
Some subset of runnable processes into memory. Lower-level scheduler picks from these.
Periodically a higher-level scheduler removes processes from main memory to disk and vice versa
How long has it been since the process was swapped in/out
How much CPU the has the process had recently?
How big is the process? What is its priority?

58. Three Level Scheduling

59. Policy versus Mechanism
Separate what is allowed to be done with how it is done
a process knows which of its children threads are important and need priority
Scheduling algorithm parameterized
mechanism in the kernel
Parameters filled in by user processes
policy set by user process