Understanding Operating Systems Fifth Edition Chapter 5 Process

Understanding Operating Systems Fifth Edition Chapter 5 Process Management

Deadlock • Resource sharing: Memory and CPU sharing • Many programs competing for limited resources • Lack of process synchronization consequences – Deadlock: “deadly embrace” • • Two or more jobs placed in HOLD state Jobs waiting for unavailable vital resource System comes to standstill OS can’t resolve the deadlock; external intervention bys users and/or sys admin needed to terminate jobs – Starvation • Infinite postponement of job Understanding Operating Systems, Fifth Edition 2

Deadlock (continued) • More serious than starvation • Affects entire system – Affects more than one job • Not just a few programs – All system resources become unavailable • Example: traffic jam (Figure 5. 1) • More prevalent in interactive systems • Real-time systems – Deadlocks quickly become critical situations • No simple and immediate solution Understanding Operating Systems, Fifth Edition 3

Deadlock (continued) Understanding Operating Systems, Fifth Edition 4

Seven Cases of Deadlock • Deadlocks usually happen when non-sharable and nonpreemptable resources (such as files, printers, and scanners) are allocated to jobs that eventually require other such resources which have been locked by other jobs. • Resource types locked by competing jobs – – – – File requests Databases Dedicated device allocation Multiple device allocation Spooling Disk sharing Network Understanding Operating Systems, Fifth Edition 5

Case 1: Deadlocks on File Requests • Jobs request and hold files for execution duration • Example (Figure 5. 2) – Two programs (P 1, P 2) and two files (F 1, F 2) – Deadlock sequence • P 1 has access to F 2 and also requires F 1 • P 2 has access to F 1 and also requires F 2 – Deadlock remains • Until one program withdrawn or • Until one program forcibly removed and file released – Other programs requiring F 1 or F 2 • Put on hold for duration of situation Understanding Operating Systems, Fifth Edition 6

Case 1: Deadlocks on File Requests (continued) Understanding Operating Systems, Fifth Edition 7

Case 2: Deadlocks in Databases • A deadlock can also occur if two processes access and lock records in a database. • Locking: Technique to guarantee the integrity of the data through which the user locks out all other users while working with the database. – Three locking levels • Entire database for duration of request • Subsection of database • Individual record until request completed Understanding Operating Systems, Fifth Edition 8

Case 2: Deadlocks in Databases (continued) • Example: two processes (P 1 and P 2) – Each needs to update two records (R 1 and R 2) – Deadlock sequence • • P 1 accesses R 1 and locks it P 2 accesses R 2 and locks it P 1 requests R 2 but locked by P 2 requests R 1 but locked by P 1 • Race between processes – Results when locking not used – Causes incorrect final version of data – Depends on process execution order Understanding Operating Systems, Fifth Edition 9

Case 2: Deadlocks in Databases (continued) Understanding Operating Systems, Fifth Edition 10

Case 3: Deadlocks in Dedicated Device Allocation • Limited number of dedicated devices (devices that can be assigned to a single process at a time) • Example – Two programs (P 1, P 2) • Need two tape drives each • Only two tape drives in system – Deadlock sequence • • P 1 requests tape drive 1 and gets it P 2 requests tape drive 2 and gets it P 1 requests tape drive 2 but blocked P 2 requests tape drive 1 but blocked Understanding Operating Systems, Fifth Edition 11

Case 4: Deadlocks in Multiple Device Allocation • Several processes request and hold dedicated devices • Example (Figure 5. 4) – Three programs (P 1, P 2, P 3) – Three dedicated devices (scanner, printer, plotter) – Deadlock sequence • • • P 1 requests and gets scanner P 2 requests and gets printer P 3 requests and gets the plotter P 1 requests printer but blocked P 2 requests plotter but blocked P 3 requests scanner but blocked Understanding Operating Systems, Fifth Edition 12

Case 4: Deadlocks in Multiple Device Allocation (continued) Understanding Operating Systems, Fifth Edition 13

Case 5: Deadlocks in Spooling • Virtual device – Dedicated device made sharable – Example: The printer has been transformed into a sharable device by installing a high-speed disk between the printer and the CPU. • Spooling – Process • The disk accepts output from several users • Acts as temporary storage for output • Output resides in disk area until printer accepts job data Understanding Operating Systems, Fifth Edition 14

Case 5: Deadlocks in Spooling (continued) • Deadlock sequence – Printer needs all job output before printing begins • • Spooling system fills disk space area No one job has entire print output in spool area Results in partially completed output for all jobs Results in deadlock Understanding Operating Systems, Fifth Edition 15

Case 6: Deadlocks in a Network • A network that’s congested or has filled a large percentage of its I/O queues can become deadlocked if it doesn’t have protocols controlling the network message flow. • Example (Figure 5. 5) – Seven computers on network • Each on different nodes – Direction of arrows • Indicates message flow – Deadlock sequence • All available buffer space fills Understanding Operating Systems, Fifth Edition 16

Case 6: Deadlocks in a Network (continued) Understanding Operating Systems, Fifth Edition 17

Case 7: Deadlocks in Disk Sharing • Competing processes send conflicting commands – Scenario: disk access • Example (Figure 5. 6) – Two processes – Each process waiting for I/O request • One at cylinder 20 and one at cylinder 310 – Deadlock sequence • Neither I/O request satisfied • Device puts request on hold while attempting to fulfill other request for each request – Livelock results Understanding Operating Systems, Fifth Edition 18

Case 7: Deadlocks in Disk Sharing (continued) Understanding Operating Systems, Fifth Edition 19

Conditions for Deadlock • Four conditions simultaneously occurring prior to deadlock or livelock – Mutual exclusion – Resource holding – No preemption – Circular wait • All needed by operating system – Must recognize simultaneous occurrence of four conditions • Resolving deadlock – Removal of one condition resolves the deadlock. Although this concept is obvious, it isn’t easy to implement. Understanding Operating Systems, Fifth Edition 20

Conditions for Deadlock (continued) • Mutual exclusion – Allowing only one process access to dedicated resource • Resource holding – A process holds on to a resource, does not release it and waits for the other job to retreat • No preemption – A resource is allocated to a process for as long as needed. Lack of temporary reallocation of resources. Understanding Operating Systems, Fifth Edition 21

Conditions for Deadlock (continued) • Circular wait – Each process involved in the impasse is waiting for another process to voluntarily release a resource so at least one process can continue • All four conditions are required for deadlock occurrence but if one condition is removed then the deadlock is broken. Understanding Operating Systems, Fifth Edition 22

Modeling Deadlocks • Directed graphs – – Used to model deadlocks Circles represent processes Squares represent resources Solid arrow from a resource to a process • Process is holding the resource – Solid arrow from a process to a resource • Process is waiting for the resource – Arrow direction indicates flow – If there’s a cycle in the graph then there’s a deadlock involving the processes and the resources in the cycle Understanding Operating Systems, Fifth Edition 23

Modeling Deadlocks (continued) Understanding Operating Systems, Fifth Edition 24

Modeling Deadlocks (continued) • Three graph scenarios to help detect deadlocks – System has three processes (P 1, P 2, P 3) – System has three resources (R 1, R 2, R 3) • Scenario one: no deadlock – Resources released before next process request • Scenario two: deadlock – Processes waiting for resource held by another • Scenario three: no deadlock – Resources released before deadlock Understanding Operating Systems, Fifth Edition 25

Modeling Deadlocks (continued) • No deadlock – Resources released before next process request Understanding Operating Systems, Fifth Edition 26

Modeling Deadlocks (continued) Understanding Operating Systems, Fifth Edition 27

Modeling Deadlocks (continued) • Deadlock – Processes waiting for resource held by another Understanding Operating Systems, Fifth Edition 28

Modeling Deadlocks (continued) Understanding Operating Systems, Fifth Edition 29

Modeling Deadlocks (continued) • No deadlock – Resources released before deadlock Understanding Operating Systems, Fifth Edition 30

Modeling Deadlocks (continued) Understanding Operating Systems, Fifth Edition 31

Modeling Deadlocks (continued) • Examples studied so far involved the allocation of one or more resources of different types (e. g. a printers and a disk drive) to a process. • The directed graphs can be expanded to include several resources of the same type (e. g. a tape drive) that can be allocated individually or in groups to the same process. • These graph clusters devices of the same type into one entity (shown in Fig. 5. 11 as a rectangle). Understanding Operating Systems, Fifth Edition 32

Modeling Deadlocks (continued) Understanding Operating Systems, Fifth Edition 33

Strategies for Handling Deadlocks • The requests and releases of resources are received in an unpredictable order making it very difficult to design a foolproof preventive policy. Operating systems use one of these strategies to deal with deadlocks: • Prevention – Prevent occurrence of one condition • Mutual exclusion, resource holding, no preemption, circular wait • Avoidance – Avoid deadlock if it becomes probable • Detection and Recovery – Detect deadlock when it occurs – Recover gracefully Understanding Operating Systems, Fifth Edition 34

Strategies for Handling Deadlocks (continued) • Prevention eliminates one of four conditions – Complication: same condition can’t be eliminated from every resource – Mutual exclusion • Some resources must allocated exclusively to one user (e. g. memory, CPU) • Bypassed if I/O device uses spooling (e. g. using a print spooler on a printer). However, we may be trading one type of deadlock for another (Case 3: Deadlocks in Dedicated Device Allocation for Case 5: Deadlocks in Spooling) – Resource holding • Job holds on to one resource while waiting for another that’s not yet available. • Sidestepped if jobs request every necessary resource at creation time • Multiprogramming degree significantly decreased • Idle peripheral devices • Can be successfully used in batch systems but doesn’t work well in interactive systems Understanding Operating Systems, Fifth Edition 35

Strategies for Handling Deadlocks (continued) • Prevention (continued) – No preemption • Bypassed if operating system allowed to deallocate resources from jobs • Okay if job state easily saved and restored • Not accepted to preempt dedicated I/O devices or files during the modification process – Circular wait • Bypassed if operating system prevents circle formation • Requires jobs to anticipate resource request order • Difficult to satisfy all users Understanding Operating Systems, Fifth Edition 36

Strategies for Handling Deadlocks (continued) • Avoidance: even if one of the 4 conditions to prevent deadlock cannot be removed, the OS can avoid deadlock if the OS knows ahead of time the sequence of requests associated with each active process. • Dijkstra’s Bankers Algorithm – Algorithm to regulate resource allocation to avoid deadlock. It is based on a bank with a fixed amount of capital that operates on the following principles: • No customer granted loan exceeding bank’s total capital • All customers given maximum credit limit • No customer allowed to borrow over limit • Sum of all loans will not exceed bank’s total capital Understanding Operating Systems, Fifth Edition 37

Strategies for Handling Deadlocks (continued) Understanding Operating Systems, Fifth Edition 38

Strategies for Handling Deadlocks (continued) Understanding Operating Systems, Fifth Edition 39

Strategies for Handling Deadlocks (continued) Understanding Operating Systems, Fifth Edition 40

Strategies for Handling Deadlocks (continued) Understanding Operating Systems, Fifth Edition 41

Strategies for Handling Deadlocks (continued) • Operating systems deadlock avoidance assurances – Never satisfy request if job state moves from safe to unsafe • Identify job with smallest number of remaining resources • Number of available resources >= number needed for selected job to complete • Block request jeopardizing safe state Understanding Operating Systems, Fifth Edition 42

Strategies for Handling Deadlocks (continued) • Problems with the Banker’s Algorithm – Jobs must predict maximum number needed resources. This is impractical for interactive systems. – Requires constant number of total resources for each class. – Number of jobs must remain fixed. This is not possible in an interactive system. – Possible high overhead cost incurred by running the avoidance algorithm because it must be invoked for every request. – Resources not well utilized • Algorithm assumes worst case – Scheduling suffers • Result of poor resource utilization • Jobs kept waiting for resource allocation Understanding Operating Systems, Fifth Edition 43

Strategies for Handling Deadlocks (continued) • Detection: build directed resource graphs – Look for cycles • Algorithm detecting circularity – Executed whenever appropriate • Detection algorithm used to reduce directed graphs. – Remove process using current resource and not waiting for one – Repeat step until all connecting lines removed Understanding Operating Systems, Fifth Edition 44

Strategies for Handling Deadlocks (continued) Understanding Operating Systems, Fifth Edition 45

Strategies for Handling Deadlocks (continued) Understanding Operating Systems, Fifth Edition 46

Strategies for Handling Deadlocks (continued) • Recovery – Deadlock untangled once detected – System returns to normal quickly • All recovery algorithms require at least one victim (an expendable job) • Recovery methods – Terminate every job active in system • Restart all jobs from beginning – Terminate only jobs involved in deadlock • Ask users to resubmit jobs – Identify jobs involved in deadlock • Terminate jobs one at a time, checking to see if the deadlock is broken after each removal, until the deadlock is resolved. Understanding Operating Systems, Fifth Edition 47

Strategies for Handling Deadlocks (continued) • Recovery methods (continued) – Select a non-deadlocked job • Preempt its resources • Allocate resources to a deadlocked process – Stop new jobs from entering system • • Allow non-deadlocked jobs to complete Releases resources when complete No victim Will work only if the number of available resources exceeds that needed by at least one of the deadlocked jobs to run. Understanding Operating Systems, Fifth Edition 48

Strategies for Handling Deadlocks (continued) • Factors to consider – Select victim with least-negative effect on the system – Most common • Job priority under consideration: high-priority jobs usually untouched • CPU time used by job: jobs close to completion usually left alone • Number of other jobs affected if job selected as victim • Jobs modifying data: usually not selected for termination (a database issue) because a database that is only partially updated is only partially correct (consistency and validity of the database must not be compromised). Understanding Operating Systems, Fifth Edition 49

Starvation • Job execution prevented – Waiting for resources that never become available – Results from conservative resource allocation • Example – “The dining philosophers” by Dijkstra • Starvation avoidance – Implement algorithm tracking how long each job waiting for resources (aging) – Block new jobs until starving jobs satisfied Understanding Operating Systems, Fifth Edition 50

Starvation (continued) Understanding Operating Systems, Fifth Edition 51

Starvation (continued) Understanding Operating Systems, Fifth Edition 52