CS422S Home Page (Spring 2002)

Homework Assignment 3

(From Operating Systems Concepts and "Further Comments on Dijkstra's Concurrent Programming Control Problem, Eisenberg and McGuire, Communications of the ACM, Volume 15, Number 11 (1972), page 999.

A solution to the critical section problem for n processes with a lower bound on waiting of n - 1 turns was presented by Eisenberg and McGuire (reference above).

The shared data and process structure are outlined below. turn is initialized to a number between 0 and n-1 (inclusive), that is, 0 <= turn < n. All elements of the flag array, one per process, are initialized to Idle.

Prove that the algorithm satisfies all three requirements for the critical section problem.

Shared State:
  enum pstate {Idle, WantIn, InCS};
  pstate flag[n]; // All elements initialed to Idle
  int turn;       // Initialized such that 0 <= turn < n

Process i's code:

do {

  // Acquire lock
  while(1) {
    flag[i] = WantIn;
    j = turn;

    while (j != i) {
      if (flag[j] != idle)
        j = turn;
      else
        j = (j + 1) % n;
    }

    flag[i] = InCS;

    j = 0;
    while ((j < n) && (j == i || flag[j] != InCS))
      j++;

    if ((j >= n) && (turn == i || flag[turn] == Idle))
      break; // Acquired lock, Leave while loop

  }

  turn = i;


  -- Critical Section --


  // Release Lock
  j = (turn + 1) % n;
  while (flag[j] == Idle)
    j = (j + 1) % n;

  turn = j;

  flag[i] = Idle;

  -- Remainder Section

} while (1);

For deadlock prevention we can ensure that circular wait does not occur if we impose a total ordering on all resource types. Prove this assertion. Assume that we have m resources R = (R1, R2, ..., Rm) and n processes P = (P1, P2, ..., Pn). Further, assume that the resources are ordered by their type number (R1 is resource with type number 1).
To get started, assume the following relations:
Resources are ordered by their type number starting at 1 and going to m. Define the relation
```
  Ri <= Rj if and only if i <= j.
```
When a process Pq requests a resource Ri, the resource is allocated if and only if both of the conditions below are true:
a) Resource Ri is not held by another process and
b) Process Pq does not hold any other resource or if all the resources held by Pq are less than Ri (Rj <= Ri, for all resources Rj currently help by Pq).
When requesting a resource, the resource is either allocated or the requesting process goes to sleep and waits until the resource is available.
Problem 5.3 in our text.
Problem 6.9 in our text.

Date	Class #	Topic	Text Chapter
1/8	1	Introduction PPT, PS	1
1/10	2	Computer Architecture Review and OS Overview PPT, PS	1 & 2
1/15	3	The Process Model PPT, PS	2
1/17	4	Processes management in UNIX, See lecture notes from 1/15	3
1/22	5	Interprocess Communications PPT, PS	3
1/24	6	Review Project and OS Structure	Class Notes
1/29	7	Threads PPT, PS	4
1/31	8	Concurrency and Synchronization PPT,PS	5
2/5	9	Concurrency and Synchronization: Continued	5
2/7	10	Finish Concurrency and Synchronization	5
2/12	11	Deadlocks	6
2/14	12	Deadlocks PPT,PS	6
2/19	13	Scheduling PPT,	9
2/21	14	Scheduling	9
2/26	15	Review for Midterm
2/28	16	Midterm Exam. The midterm is in two parts: part one is to be taken in class and part two it to be tturned in no later then start of class Tuesday, 12 March. Part two in MS Word format, PostScript or PDF.	N/A
3/12	17	Real Time Scheduling PPT or PS	10
3/14	18	Memory Management PS, PPT	7
3/19	19	Finish Memory Management and start Virtual Memory PS, PPT	8
3/21	20	Virtual Memory	8
3/26	21	Discussion of Project 2, Quiz, File Systems PPT PS	12
3/28	22	File Systems	12
4/2	23	Midterm returned and reviewed. I/O Systems PS, PPT	11
4/4	24	Mass-Storage Structure PS, PPT	12
4/9	25	Networking Overview PPT PS	13
4/11	26	Distributed File Systems PPT, PS	13 and 14
4/16	27	Distributed Process Management, PPT, PS	14 and ClassNotes
4/18	28	Review for Final - Last Class PPT, PS.	Class Notes and Review Questions
4/26	29	Final exam, Friday 4/26, 1:00PM - 3:00PM	N/A

CS422S: Operating Systems Organization (Spring 2002)

Updates:

Tue Apr 9 13:01:44 CDT 2002

Thu Apr 4 16:44:38 CST 2002

Instructor

Graders

Locations

Course Description

Prerequisites

Prerequisite knowledge and experience:

Notes and Reference Materials

Texts

Note: Much of the course meterial has been derived from the highlighted texts below.

Grading

Homeworks, Quizes, and Class Attendance

Exams

Programming Projects

Late Submissions

Syllabus (Subject to Change)

Reference material for lectures (and source for many of the powerpoint slides):

Homework Assignments

Homework Assignment 2

Homework Assignment 3

Prove that the algorithm satisfies all three requirements for the critical section problem.

Shared State:

Process i's code:

Programming Assignments