Transcript Chapter 6 Notes

Deadlocks
Deadlock handling
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Deadlock prevention
Deadlock avoidance
Deadlock detection
Deadlock recovery
How deadlocks arise
• Conflicting need for resource by two or
more processes
• All process are unable to proceed
Deadlock Example
Precess P
Process Q
• Get A
• Get B
• Get B
• Get A
• Release B
• Release B
• Release A
• Release A
Four conditions for deadlocks
• Mutual exclusion of resources
• Hold and wait
– A process can hold a resource and wait for
another
• No preemption
– A process will release resources voluntarily
• Circular wait
– There is a circular wait among the processes
Deadlock prevention
• Make one of the four conditions false aways
• No deadlock!
• Which condition? Only circular wait
condition is doable
Make circular wait condition
false always
• Order all the resources
• Processes can request resources that are
ordered strictly higher than what they are
holding
• Can prove that there are cycles in wait for
graph
Deadlock avoidance
• Allows more concurrency than prevention
• A decision is made dynamically whether the
current resource allocation request will
potentially lead to a deadlock (if granted)
• Needs knowledge of future resource needs
Two rules of deadlock avoidance
• Do not start a process if its demands might
lead to a deadlock
• Do not grant an incremental resource
request to a process if doing so might lead
to a deadlock
Initial denial
• R = (R1, R2, …, Rm) set of system
resources; m types of resources
• A = (V1, V2, …, Vm) available reources
(not allocated to any process) at any time
• Claim matrix; maximum requirement of
each process for each resource
• Current allocation matrix; what each
process holds currently
Relations
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Ri = Vi + sum(A_ki) k going from 1 to n
C_kl <= R_i for all k, I
A_ki <= C_ki for all k, i
(Strong condition): Start a new process
P_n+1 only if R_I >= C_(n+1)i+sum(C_ki)
k going from 1 to n; for all i
Resource allocation denial
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Banker’s algorithm
Safe and unsafe states
Detect if a state is unsafe
Go from one safe state to another safe state
Deadlock recovery
• Forcibly terminate some processes to
recover from deadlocks.
• Deadlock breaking set: Set of processes,
which when terminated, will lead to the
other processes’ normal termination.
• Cost of forcibly terminating a process
Deadlock recovery Cont’d
• Cost of deadlock breaking set: sum of the
cost of aborting processes in the deadlock
breaking set.
• Optimal deadlock breaking set: Deadlock
breaking set of least cost.
• Finding the optimal deadlock breaking set is
an NP-hard problem.