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DEADLOCK

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Resources• Examples of computer resources

– printers

– tape drives

– tables

• Processes need access to resources in reasonable order• Suppose a process holds resource A and requests

resource B– at same time another process holds B and requests A

– both are blocked and remain so

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Resources (1)

• Deadlocks occur when …

– processes are granted exclusive access to devices– we refer to these devices generally as resources

• Preemptable resources– can be taken away from a process with no ill effects

• Nonpreemptable resources– will cause the process to fail if taken away

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Resources (2)

• Sequence of events required to use a resource1. request the resource

2. use the resource

3. release the resource

• Must wait if request is denied– requesting process may be blocked– may fail with error code

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Introduction to Deadlocks

• Formal definition :A set of processes is deadlocked if each process in the set is waiting for an event that only another process in the set can cause

• Usually the event is release of a currently held resource• None of the processes can …

– run

– release resources

– be awakened

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Deadlock Modeling

• Modeled with directed graphs

– resource R assigned to process A

– process B is requesting/waiting for resource S

– process C and D are in deadlock over resources T and U

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THE DEADLOCK PHENOMENON

Resource allocation graphs. (a) Holding a resource. (b) Requesting a resource. (c) Deadlock.

CONDITIONS FOR DEADLOCK:1. Mutual exclusion No private copy of resource; each resource assigned to 1 process or is available2. Hold and wait May hold a resource and ask for more3. Non preemption Serial usage of every resource; previously granted resources cannot be taken away4. Circular wait Must be a circular chain of 2 or more processes; each is waiting for resource held

by next member of the chain

8How deadlock occurs

A B C

Deadlock Modeling

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Deadlock Modeling

How deadlock can be avoided

(o) (p) (q)

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An example of how deadlock occurs and how it can be avoided.(A,B) (B,C) (C,A) have only one resource in common. By allowing any pair to proceed and blocking the third, no deadlock is possible.

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HOW TO DEAL WITH DEADLOCK

1. User/System administrator intervention

2. Detection and recovery by O.S.

3. Prevention Break any of the four conditions for deadlock by

administrative rules; must work for any set of user resource requirements

4. Avoidance Exploit user resource usage information to formulate

deadlock-free allocation policy

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PREVENTION

Summary of approaches to deadlock prevention.

•Spooling not always possible because of spooler memory limitations and transaction serializability requirements.

•Resource needs may not be known a priori.

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Attacking the Mutual Exclusion Condition

• Some devices (such as printer) can be spooled– only the printer daemon uses printer resource– thus deadlock for printer eliminated

• Not all devices can be spooled• Principle:

– avoid assigning resource when not absolutely necessary

– as few processes as possible actually claim the resource

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Attacking the Hold and Wait Condition

• Require processes to request resources before starting– a process never has to wait for what it needs

• Problems– may not know required resources at start of run– also ties up resources other processes could be using

• Variation: – process must give up all resources– then request all immediately needed

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Attacking the No Preemption Condition

• This is not a viable option

• Consider a process given the printer– halfway through its job– now forcibly take away printer– !!??

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Detection with One Resource of Each Type (1)

• Note the resource ownership and requests• A cycle can be found within the graph, denoting deadlock

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Detection with One Resource of Each Type (2)

Data structures needed by deadlock detection algorithm

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Detection with One Resource of Each Type (3)

An example for the deadlock detection algorithm

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Attacking the Circular Wait Condition

• Numerically ordered resources

• A resource graph

(a) (b)

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Requests must be consistent with numerical order

(a) Numerically ordered resources. (b) A resource graph.

i'

i j'

j

A B

i'>i j'>j

A loop of increasing resource indices is impossible!

May request only resources ranking lower than (have bigger numbers) those already acquired

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Attacking the Circular Wait Condition (1)

Summary of approaches to deadlock prevention

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DEADLOCK AVOIDANCE

Two process resource trajectories.Both using printer

Both using plotter

Unsafe region bound to end up in deadlock

S is safe because the trajectory can be extended to u without going through deadlock region

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Safe and Unsafe States (1)

Demonstration that the state in (a) is safe

(a) (b) (c) (d) (e)

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Safe and Unsafe States (2)

Demonstration that the sate in b is not safe

(a) (b) (c) (d)

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The Banker's Algorithm for a Single Resource

• Three resource allocation states– safe– safe– unsafe

(a) (b) (c)

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Banker's Algorithm for Multiple Resources

Example of banker's algorithm with multiple resources

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BANKER’S ALGORITHM

Three resource allocation states: (a) Safe. (b) Safe. (c) Unsafe.

A state is safe if there exists a sequence of states extending from it which ends in all processes completing.(b) is safe because Marvin can proceed and then Barbara (or Suzanne).(c) is unsafe because none can proceed if maximum is required for each.Notice that look-ahead until everybody finishes is required to avoid unsafe states. Ex. suppose in state (b), a new process Pete joins the system, and then Barbara asks for 1 more unit. Pete can still proceed (one-step lookahead) if Barbara’s request is granted, but we know that granting Barbara’s request is unsafe.

Andy

Pete 0 1

…

Available: 1

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Existing EquipmentPossessedE-P

The banker’s algorithm with multiple resources.

1. Find a row in MN that is no greater than the availability vector A.

2. Allow the corresponding process to complete and release its resources.

3. Update the tables and go to step 1.

If every process completes, there is no deadlock.

MP MN

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The banker’s algorithm with multiple resources.

E=(6342)P=(5332)A=(1010)

E=(6342)P=(4221)A=(2121)

E=(6342)P=(6331)A=(0011)

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The banker’s algorithm with multiple resources.

E=(6342)P=(4221)A=(2121)

E=(6342)P=(5321)A=(1021)

E=(6342)P=(1210)A=(5132)

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The banker’s algorithm with multiple resources.

E=(6342)P=(4310)A=(2032)

E=(6342)P=(0100)A=(6242)

E=(6342)P=(0212)A=(6130)

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Deadlock Handling in Distributed Systems

R1

BA

R2

Detector Site

Reports R2 StatusReports R1 Status

Site 1 Site 1

Process A

1. Lock R1

2. Lock R2

3. Release R1

4. Release R2

Process B

1. Lock R2

2. Lock R1

3. Release R1

4. Release R2

Lock and release requests are performed by message passing. Site performs lock and release operations on local resources.

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Detection Techniques

• Path pushing

• Edge tracing

• Global state determination

• Diffusion computation

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Centralized Path Pushing

• Each site tracks lock and release operations of resources on the site

• Each site reports the Resource Allocation Graph (RAG) fragment it tracks to central detector

• Central detector detects cycles from the RAG fragments reported to it

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Commanded Edge Tracing

• On command of central controller, detector on each site propagates “probes” along the out-edges of the RAGs it tracks. Each probe carries originating site's Id.

• Unblocked processes on the site discard the probes targeted at them. Blocked process waiting for a resource propagates probe backward to site that currently holds the resource

• If probe returns to its originating site, detector has found a cycle

• Central controller initiates edge tracing when deadlock is suspected.

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In distributed systems, centralized detection may have false alarms

R1

BA

R2

Detector Site

Reports R2 StatusReports R1 Status

Site 1 Site 1

Process A

1. Lock R1

2. Release R1

3. Lock R2

4. Release R2

Process B

1. Lock R1

2. Release R1

3. Lock R2

4. Release R2

Notice that A and B both follow prevention protocol

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Site 1 Site 2 Detector

A

B

R1

A1

B1

A

R1

B

R1

BA2

R1

B2

A

R2

B

B3

A3

A

R1

B

R2

Cure: Edge TracingFalse Alarm!

R1 released

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Efficiency Considerations

• Deadlock detection incurs message and time overheads in distributed systems. Overhead may be high especially if application processes are mobile

• Hierarchical allocation of resources may help if hierarchy is consistent with the spatial locality of processes to resources they use

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Recovery from Deadlock (1)

• Recovery through preemption– take a resource from some other process– depends on nature of the resource

• Recovery through rollback– checkpoint a process periodically– use this saved state – restart the process if it is found deadlocked

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Recovery from Deadlock (2)

• Recovery through killing processes– crudest but simplest way to break a deadlock– kill one of the processes in the deadlock cycle– the other processes get its resources – choose process that can be rerun from the beginning

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Two-Phase Locking in Database Systems Uses Rollback

• Phase One– process tries to lock all records it needs, one at a time– if needed record found locked, start over– (no real work done in phase one)

• If phase one succeeds, it starts second phase, – performing updates– releasing locks

• Note similarity to requesting all resources at once• Algorithm works only if system can allow processes to be

stopped and rolled back

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Nonresource Deadlocks• Possible for two processes to deadlock

– each is waiting for the other to do some task

• Can happen with semaphores– each process required to do a down() on two

semaphores (mutex and another)– if done in wrong order, deadlock results

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Starvation• Algorithm to allocate a resource

– may be to give to shortest job first

• Works great for multiple short jobs in a system

• May cause long job to be postponed indefinitely– even though not blocked

• Solution:– fair schedulers