1. Chapter 5 - Process Management
CIS106
Microcomputer Operating Systems
Gina Rue
CIS Faculty
Ivy Tech State College Northwest Region 01

2. Introduction - Process Management
Problems are caused when many processes compete for relatively few resources & the system is unable to service all the processes of the system
A lack of process synchronization can result in two extreme conditions
deadlock
starvation
See Illustration p.99

3. Process Management - Deadlock
In early OSs, deadlock was known as “deadly embrace”
begins when two or more jobs requests a resource
jobs are put on hold, each waiting for vital resources to become available, jobs come to a standstill

4. Process Management - Deadlock
Deadlock is complete if the rest of the system comes to a standstill as well
Usually can’t be resolved by the OS and requires intervention
operators/users terminate job
Deadlock described as a narrow staircase in a building
Starvation is an extreme case of indefinite postponement

5. Deadlock More serious than indefinite postponement or starvation
affects more than one job
affects resources of the entire system
were more infrequent in early batch systems
became more prevalent with growing use of interactive systems
See Fig. 5.1 p. 101

6. Seven Cases of Deadlock
Deadlock usually occurs when non-sharable, non-preemptable resources, such as files, printers are allocated jobs that eventually require--the resources that have been locked by other jobs
also with resources such as tape drives, disks, & databases
What is an operating system?

7. Seven Cases of Deadlock
A Deadlock can occur with
File Requests - if jobs are allowed to request and hold files for the duration of their execution
Databases - if two processes access and lock records in a database. Note: Locking & Race
Device Allocation - use of a group of dedicated devices, tape drive
Multiple Device Allocation - can happen when several processes request, & hold two devices while other processes act in a similar manor

8. Seven Cases of Deadlock (con’t.)
A Deadlock can occur with
Spooling - where a disk, between the device (printer) and CPU, accepts output from several users (network) and acts as a temporary storage area for all output until the printer is ready to accept it
Disk Sharing - two processes to be accessing different areas of the same disk causing competing processes that send conflicting commands
Network - congested or has large % of its I/O buffer space full if no protocol controls flow of traffic

9. Conditions For Deadlock
Deadlock is proceeded by the simultaneous occurrence of 4 conditions the OS can recognize
mutual exclusion
resource holding
no preemption
circular wait

10. Conditions For Deadlock
Stairway example:
mutual exclusion, the act of allowing only one person (or process) to have access to a step (or resource)
resource holding, two people met on the stairs & each waited for the other to retreat
no preemption, each step is dedicated to the climber (or descender) as long as needed
circular wait, each person (or process) involved is waiting for another to release the step (or resource)

11. Modeling Deadlocks Directed Graphs Holt (1972) two kinds of symbols
processes represented by circles
resources represented by squares
solid lines
from a resource to a process means that the process is holding that resource
from a process to a resource means that the process is waiting for that resource
arrows
direction indicates the flow
See Fig p

12. Strategies for Handling Deadlocks
Requests & releases are received in an unpredictable order, which makes it difficult to design a foolproof preventive policy.
An OS uses 1 of 3 strategies to deal with deadlocks:
Prevent one user of the 4 conditions from occurring
Avoid deadlock if it becomes probable
Detect the deadlock when it occurs & recover from it gracefully

13. Strategies for Handling Deadlocks
Deadlock recovery strategies require that at least one victim, an expendable job, when removed from the system, will free the system. Victim selection should have the least effect on the system:
Priority of the job
CPU time used by the job
The number of jobs

14. Starvation
The result of conservative allocation of resources where a single job is prevented from execution because it is waiting for resources that never become available
an algorithm designed to detect starving jobs can be implemented, which tracks how long each job has been waiting for resources (this is the same as aging described in Ch.4)

15. Summary
Every OS must dynamically allocate a limited number of resources while avoiding the 2 extremes of deadlock & starvation
Methods of dealing with deadlock
prevention
avoidance
detection
recovery

16. Summary
Deadlocks can be prevented by not allowing the 4 conditions of deadlock to occur in the system at the same time
mutual exclusion
resource holding
no preemption
circular wait
By eliminating at least one of the 4 conditions, a system can be kept deadlock-free

17. Summary
Prevention algorithms are complex & to routinely execute them involves high overhead
Avoid by identifying safe & unsafe states
Prepare to detect and recover from deadlock
“victim” a job must be terminated before it finishes execution & must be started again