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Today’s Schedule Assignment #4 from Chapter 3 posted Deadlock - Chapter 3
Skip multiple resources (3.4.2 & 3.5.4)
Today’s Objectives
You will be able to describe: Several causes of system deadlock The difference between preventing and
avoiding deadlocks How to detect and recover from deadlocks How to prevent deadlock
Overview A lack of process synchronization results in
deadlock or starvationDeadlock:
A system-wide tangle of resource requests that begins when two or more jobs are put on hold
Each job waiting for a vital resource to become available The jobs come to a standstill Resolved via external intervention
Starvation: Infinite postponement of a job
Starvation
Which of the following scheduling algorithms could result in starvation? If so, how?
First-come, First-served Shortest job first Round robin Priority
Deadlock
Affects more than one job, hence more serious than starvation
System (not just a few programs) is affected as resources are being tied upe.g., Traffic jam
Deadlock in Spooling Virtual device: Sharable device—e.g., a printer
transformed by installing a high-speed device, a disk, between it and the CPU
Spooling: Disk accepts output from several users and acts as a temporary storage area for all output until printer is ready to accept it
Deadlock in spooling: If printer needs all of a job's output before it will begin printing, but spooling system fills available disk space with only partially completed output
Deadlock Defined
From our more playful days … I’ve got the ball and want the batYou’ve got the bat and want the ball
“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.”
Modeling DeadlocksResource Allocation Graphs
Process P1 holds Resource R1
Process P1 requests Resource R1
Conditions Required for Deadlock
1. Mutual exclusion conditioneach resource assigned to 1 process or is available
2. Hold and wait conditionprocess holding resources can request additional
3. No preemption conditionpreviously granted resources cannot forcibly taken away
4. Circular wait conditionmust be a circular chain of 2 or more processeseach is waiting for resource held by next member of the chain
Dealing with Deadlock
Ostrich Algorithm Ignore deadlock possibility!
Detection & Recovery Dynamic Avoidance
careful resource allocation Prevention
negating one of the four necessary conditions
Ostrich Algorithm
Stick your head in the sand and pretend there is no problem at all
Reasonable if deadlocks occur very rarely cost of prevention is high
UNIX and Windows takes this approach Trade off between
convenience correctness
What is the typical use of the system? What is the probability of deadlock? Do the costs associated with dealing w/ deadlock outweigh the
benefits?
Detecting Deadlock – Find Cycle
Note the resource ownership and requests A cycle can be found within the graph, denoting
deadlock
How to Recover from Deadlock?
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
Recovery through killing processes crudest but simplest way to break a deadlock kill one of the processes in the deadlock cycle
Avoid Deadlock Resource Trajectories Show
I1 I2 I3 I5I4
I6
I8
I7
I9
PrinterPlotter
Printer
Plotter
p q
sr
t u
A
BImpossible
Unsafe
Safe/Unsafe States
Safe State Not in deadlock There is some scheduling order with which all
processes can finish
Unsafe State Not necessarily deadlock Doesn’t guarantee deadlock But can’t guarantee deadlock will be avoided
Banker’s Algorithm – Avoid Deadlock
Regulate resource allocationNo loan exceeding bank’s total capitalCustomers have a pre-set maximum creditMay not borrow over the limitTotal of all loans may not exceed bank’s
capital
Avoidance (continued)
The bank started with $10,000 and has remaining capital of $4,000 after these loans
Safe state: Bank still has enough money left after loans to satisfy the maximum requests of C1, C2, or C3
Avoidance (continued)
Table 5.5: The bank has remaining capital of only $1,000 after these loans and therefore is in an “unsafe state”
Unsafe state: Bank does not have enough money left after loans to satisfy the maximum requests of C1, C2, or C3
Avoidance (continued)
Table 5.6: A safe state: six devices are allocated and four units are still available
Same banking principles can be applied to an operating system
Avoidance (continued)
An unsafe state: only one unit is available but every job requires at least two to complete its execution
Deadlock Avoidance
To avoid deadlock, OS must make sure:Never satisfy a request that moves it from a
safe state to an unsafe oneMust identify the job with the smallest number
of remaining resourcesNumber of available resources is always
equal to, or greater than, the number needed for the selected job to run to completion
Attack Deadlock Conditions Attack Mutual Exclusion Attack Hold and Wait
Require all process to request all resources before starting execution
Resource must temporarily release all the resources it currently holds
Attack No preemption Just take away resource, does not work
Attack Circular Wait condition Process is entitled to single resource at any moment Use of global numbering
All requests made in numerical order
Summary
Resources can be preemptable & nonpreemptable
Deadlock can cause processes to halt (stop making progress)
We can detect deadlock RAGs are quite useful to detect deadlock Ostrich algorithm quite popular
Summary (cont) Trajectories can help in process scheduling to
avoid deadlock Avoid unsafe states Prevent Deadlock by attack one of four
necessary conditions Ordering resources avoids circular wait Keep processes from starving – all processes
must make some progress