1. G.Anuradha Reference: William Stallings
Process concurrency
G.Anuradha
Reference: William Stallings

2. Contents
• Principles of concurrency • Mutual Exclusion-hardware approaches • Mutual Exclusion-software support • Semaphores • Monitors • Message Passing • Readers/Writers problem • Deadlock and starvation • Principles of deadlock • Deadlock prevention • Deadlock avoidance • Deadlock detection • An integrated Deadlock Strategy • Dining Philosophers Problem

3. Introduction
Multiprogramming:- management of multiple processes within a set of processor system
Multiprocessing : Management of multiple processes within a multiprocessor
Distributed Processing: Management of multiple processes executing on multiple distributed computer system

4. Key terms related to concurrency

5. Concurrency What is it? Communication among processes
Sharing of and competing for resources
Synchronization of activities of multiple processes
Allocation of processor time to processes

6. When concurrency arises
Multiple applications:
Structured applications
Operating system structure

7. Race Conditions
A Race Condition occurs, if two or more processes/threads access and
manipulate the same data concurrently, and the outcome of the execution depends on the particular order in which the access takes place.
Synchronization is needed to prevent race conditions from happening

8. Principles of concurrency
Problems encountered during concurrency
Sharing of global resources is fraught with peril
Difficulty in managing the allocation of resources optimally
Difficulty in locating a programming error because results are typically not deterministic and reproducible

9. Problem can be solved by controlled access to shared resource
Simple example
Echo program is loaded in global memory and shared by applications
Problem can be solved by controlled access to shared resource

10. OS Concerns
What design and management issues are raised by concurrency?
OS should keep track of active processes
OS should allocate and deallocate resources to active processes
Processor time/memory/files/I-O devices
OS should protect against the interference by other processes
Result of a process should be independent of the speed of execution relative to other concurrent process (Process interaction)

11. Process interaction

12. Competition among Processes for Resources
3 control problems
Mutual exclusion:- eg. Printer
Mutual exclusion leads to two more additional problems
Deadlock
Starvation
Mutual exclusion can be achieved by locking a resource prior to its use.

13. Mutual exclusion

17. Cooperation among processes by sharing
Eg:- shared variables/files/database
Data items may be accessed in reading and writing mode and only the writing mode must be mutually exclusive
Requirement: data coherence

18. Cooperation among processes by Communication
Communication provides a way to synchronize or coordinate the various activities
This is done by messaging.
So Mutual exclusion is not a control requirement for this sort of cooperation
Has deadlock and starvation problems

19. Requirements of mutual exclusion(ME)
ME should be enforced
A process that halts in its noncritical section should not interfere with other processes
No deadlock and starvation
When no process is there in the CS, a process requiring CS should be granted permission
Process remains in CS only for finite time

20. Ways to arrive at mutual exclusion
Software approaches
Leave the responsibility to the processes that wish to execute concurrently.
Disadv is high processing overhead and bugs
Hardware approaches
Special purpose machine instructions
Adv of reducing overhead
Some level of support within the OS or programming language
Semaphores
monitors

22. First Attempt
Buzy Waiting : waiting process repeatedly reads the value of turn(global memory location) until its allowed to enter its critical section
Disadvantage:- Pace of execution is dictated by the slower of the two processes
If one process fails the other process is permanently blocked.

23. Second attempt
If one process fails outside the critical section, other process is not blocked. But if it fails within the critical section or after setting the flag then it blocks the other process

24. Third attempt
Eliminates the problems in second attempt. This guarantees mutual exclusion but creates deadlock, because each process can insist on its right to enter its critical section.

25. Livelock
The process keeps setting and resetting the flags alternatively and gets neither process could enter its critical section.
Alteration in the relative speed of the two processes will break this cycle and allow one to enter the critical section
This is called as livelock

26. Peterson’s Solution Two process solution
The two processes share two variables:
int turn;
Boolean flag[2]
The variable turn indicates whose turn it is to enter the critical section.
The flag array is used to indicate if a process is ready to enter the critical section. flag[i] = true implies that process Pi is ready!

27. Algorithm for Process Pi

31. Mutual exclusion-Hardware approach
Interrupt Disabling
Special machine instructions
Compare and swap
exchange

32. Interrupt Disabling
In an uniprocessor system concurrent processes can have only interleaved execution
Process runs until its interrupted
To guarantee ME its enough to prevent a process from being interrupted
Disadvantages
Degree of interleaving is limited
Does not work in a multiprocessor architecture

33. Special machine instructions
Processor designers have proposed several machine instructions that carry out two actions automatically(Read-Write/Read-Test) through a single instruction fetch cycle
Two such instruction
Compare and swap
Exchange instruction

34. Compare and swap
Checks a memory location (*word) against a test value(testval).
If memory location currval=testval, its replaced by newval
Otherwise left unchanged

35. Compare and swap Shared variable bolt is initialized to 0
The only process that can enter critical section is one that finds bolt equal to 0
All other processes would go to buzy waiting mode

36. Exchange

37. Exchange contd… Shared variable bolt is initialized to zero
Each process has a local variable key that is initialized to 1
Process that find bolt =0 alone enters the critical section
Excludes all other processes by setting bolt=1
Once on exiting it resend bold to 0

38. Properties of machine-instruction approroach

39. Disadvantage Buzy waiting is employed Starvation is possible
Deadlock is possible