Download Preventing Too Much Milk: A Case Study on Process Cooperation and Synchronization in OS - and more Study notes Operating Systems in PDF only on Docsity!
CS 414 : Operating Systems
UNIVERSITY OF VIRGINIA
Department of Computer Science
Fall 2005
Topic 3: Cooperating Processes and Synchronization
Synchronization: using atomic operations to ensure correct cooperation between processes.
The ‘‘Too Much Milk’’ problem:
Person A Person B 3:00 Look in fridge. Out of milk. 3:05 Leave for store. 3:10 Arrive at store. Look in fridge. Out of milk. 3:15 Leave store. Leave for store. 3:20 Arrive home, put milk away. Arrive at store. 3:25 Leave store. 3:30 Arrive home. OH, NO!
One of the most important things in synchronization is to figure out what you want to achieve.
In the given problem: somebody gets milk, but we don’t get too much milk.
Mutual exclusion: Mechanisms that ensure that only one person or process is doing certain
things at one time (others are excluded). E.g. only one person goes shopping at a time.
Critical section: A section of code, or collection of operations, in which only one process may
be executing at a given time. E.g. shopping.
There are many ways to achieve mutual exclusion. Most involve some sort of locking
mechanism: prevent someone from doing something. For example, before shopping, leave a note on the refrigerator.
Three elements of locking:
- Must lock before using. leave note
- Must unlock when done. remove note
- Must wait if locked. don’t shop if note
1st attempt at computerized milk buying:
Processes A & B 1 if (NoMilk) { 2 if (NoNote) { 3 Leave Note; 4 Buy Milk; 5 Remove Note; 6 } 7 }
Will it work?
If not, why not?
Making the problem less likely to occur - that is typical of first attempts at solutions to syn-
chronization problems.
What happens if we leave the note at the very beginning: does this make everything work?
3rd attempt: use 2 notes.
Process A 1 Leave NoteA; 2 if (NoNoteB) { 3 if (NoMilk) { 4 Buy Milk; 5 } 6 } 7 Remove NoteA;
Process B is the same except interchange NoteA and NoteB. What can you say about this solution?
4th attempt: in case of tie, B will buy milk. Process A stays the same as before.
Process B 1 Leave NoteB; 2 while (NoteA) DoNothing; 3 if (NoMilk) { 4 Buy Milk; 5 } 6 Remove NoteB;
Does this solution work? Any disadvantages?
