Chapter 5 - Process Management

1. Chapter 5 - Process Management

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 processsynchronizationcan 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

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. 5.7 - 10 p.108-110

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