Chapter 5 : Process Management

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Chapter 5 Process Management

• Deadlock
• 7 Cases of Deadlock
• Conditions for Deadlock
• Modeling Deadlocks
• Strategies for Handling Deadlocks
• Avoidance
• Detection
• Recovery
• Starvation

• Process Synchronization
• Deadlock Starvation
• Management Management

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A Lack of Process Synchronization Causes Deadlock
or Starvation

• Deadlock (deadly embrace) -- a system-wide
  tangle of resource requests that begins when 2
  jobs are put on hold.
• Each job is waiting for a vital resource to
  become available.
• Needed resources are held by other jobs also
  waiting to run but cant because theyre waiting
  for other unavailable resources.
• The jobs come to a standstill.
• The deadlock is complete if remainder of system
  comes to a standstill as well.
• Resolved via external intervention.

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Deadlock

• Deadlock is more serious than indefinite
  postponement or starvation because it affects
  more than one job.
• Because resources are being tied up, the entire
  system (not just a few programs) is affected.
• Requires outside intervention (e.g., operators or
  users terminate a job).

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Seven Cases of Deadlocks

• 1. Deadlocks on file requests
• 2. Deadlocks in databases
• 3. Deadlocks in dedicated device allocation
• 4. Deadlocks in multiple device allocation
• 5. Deadlocks in spooling
• 6. Deadlocks in disk sharing
• 7. Deadlocks in a network

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Case 1 Deadlocks on File Requests

• If jobs can request and hold files for duration
  of their execution, deadlock can occur.
• Any other programs that require F1 or F2 are put
  on hold as long as this situation continues.
• Deadlock remains until a programs is withdrawn or
  forcibly removed and its file is released.

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Case 2 Deadlocks in Databases

• 1. P1 accesses R1 and locks it.
• 2. P2 accesses R2 and locks it.
• 3. P1 requests R2, which is locked by P2.
• 4. P2 requests R1, which is locked by P1.

• Deadlock can occur if 2 processes access lock
  records in database.
• 3 different levels of locking
• entire database for duration of request
• a subsection of the database
• individual record until process is completed.
• If dont use locks, can lead to a race condition.

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Race Condition
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Case 3 Deadlocks in Dedicated Device Allocation

• Deadlock can occur when there is a limited number
  of dedicated devices.
• E.g., printers, plotters or tape drives.

• 1. P1 requests tape drive 1 and gets it.
• 2. P2 requests tape drive 2 and gets it.
• 3. P1 requests tape drive 2 but is blocked.
• 4. P2 requests tape drive 1 but is blocked.

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Case 4 Deadlocks in Multiple Device Allocation

• Deadlocks can happen when several processes
  request, and hold on to, dedicated devices while
  other processes act in a similar manner.

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Case 5 Deadlocks in Spooling

• Most systems have transformed dedicated devices
  such as a printer into a sharable device by
  installing a high-speed device, a disk, between
  it and the CPU.
• Disk accepts output from several users and acts
  as a temporary storage area for all output until
  printer is ready to accept it (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, then a deadlock can occur.

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Case 6 Deadlocks in Disk Sharing

• Disks are designed to be shared, so its not
  uncommon for 2 processes access different areas
  of same disk.
• Without controls to regulate use of disk drive,
  competing processes could send conflicting
  commands and deadlock the system.

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Case 7 Deadlocks in a Network

• A network thats congested (or filled large of
  its I/O buffer space) can become deadlocked if it
  doesnt have protocols to control flow of
  messages through network.

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Four Conditions for Deadlock

• Deadlock preceded by simultaneous occurrence of
  four conditions that operating system could have
  recognized
• Mutual exclusion something cannot be shared
• Resource holding hold and wait
• No preemption resources cannot be relocated
• Circular wait a cycle in the graph

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• Mutual exclusion -- the act of allowing only one
  process to have access to a dedicated resource.
• Resource holding -- the act of holding a resource
  and not releasing it waiting for the other job
  to retreat.
• No preemption -- the lack of temporary
  reallocation of resources once a job gets a
  resource it can hold on to it for as long as it
  needs.
• Circular wait -- each process involved in
  impasse is waiting for another to voluntarily
  release the resource so that at least one will be
  able to continue.

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Modeling Deadlocks Using Directed Graphs (Holt,
1972)

• Processes represented by circles.
• Resources represented by squares.
• Solid line from a resource to a process means
  that process is holding that resource.
• Solid line from a process to a resource means
  that process is waiting for that resource.
• Direction of arrow indicates flow.
• If theres a cycle in the graph then theres a
  deadlock involving the processes and the
  resources in the cycle.

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Directed Graph Examples
Figure 5.7 (a)P1 is holding R1
Figure 5.7 (c)
Figure 5.7 (b)P1 is waiting for R1
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1. P1 requests and is allocated R1
2. P1 releases R1
3. P2 requests and is allocated R2
4. P2 releases R2
5. P3 requests and is allocated R3
6. P3 releases R3
7. P1 requests R2
8. P2 requests R3
9. P3 requests R1

Figure 5.8 free of deadlock if all resources are
released before theyre requested by the next
process
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1. P1 requests and is allocated R1
2. P2 requests and is allocated R2
3. P3 requests and is allocated R3
4. P1 requests R2
5. P2 requests R3
6. P3 requests R1

Figure 5.9 deadlocked when P3 request R1
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? ? ?
Figure 5.11 (a)
Cluster of resources
? ? ?
Figure 5.11 (b)
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Strategies for Handling Deadlocks

• Prevent one of the four conditions from
  occurring.
• Avoid the deadlock if it becomes probable.
• Detect the deadlock when it occurs and recover
  from it gracefully.

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Prevention of Deadlock

• To prevent a deadlock OS must eliminate 1 out of
  4 necessary conditions.
• Same condition cant be eliminated from every
  resource.
• Mutual exclusion is necessary in any computer
  system because some resources (memory, CPU,
  dedicated devices) must be exclusively allocated
  to 1 user at a time.
• Might be able to use spooling for some devices.
• May trade 1 type of deadlock (Case 3) for another
  (Case 5).

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Prevention of Resource Holding

• Resource holding can be avoided by forcing each
  job to request, at creation time, every resource
  it will need to run to completion.
• Significantly decreases degree of
  multiprogramming.
• Peripheral devices would be idle because
  allocated to a job even though they wouldn't be
  used all the time.

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Prevention of No Preemption

• No preemption could be bypassed by allowing OS to
  deallocate resources from jobs.
• OK if state of job can be easily saved and
  restored.
• Bad if preempt dedicated I/O device or files
  during modification.
• Starvation if not well scheduled

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Prevention of Circular Wait

• Circular wait can be bypassed if OS prevents
  formation of a circle.
• Havenders solution (1968) is based on a
  numbering system for resources such as printer
  1, disk 2, tape 3.
• Forces each job to request its resources in
  ascending order.
• Any number one devices required by job
  requested first any number two devices
  requested next
• Require that jobs anticipate order in which they
  will request resources.
• A best order is difficult to determine.

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Deadlock Avoidance

• Even if OS cant remove 1 conditions for
  deadlock, it can avoid one if system knows ahead
  of time sequence of requests associated with each
  of the active processes.
• Dijkstras Bankers Algorithm (1965) used to
  regulate resources allocation to avoid deadlock.
• Safe state -- if there exists a safe sequence of
  all processes where they can all get the
  resources needed.
• Unsafe state -- doesnt necessarily lead to
  deadlock, but it does indicate that system is an
  excellent candidate for one.

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Bankers Algorithm

• Based on a bank with a fixed amount of capital
  that operates on the following principles
• No customer will be granted a loan exceeding
  banks total capital.
• All customers will be given a maximum credit
  limit when opening an account.
• No customer will be allowed to borrow over the
  limit.
• The sum of all loans wont exceed the banks
  total capital.
• OS (bank) must be sure never to satisfy a request
  that moves it from a safe state to an unsafe one.
• Job with smallest number of remaining resources lt
  number of available resources

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A Banks Safe and Unsafe States
Safe
Unsafe
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Data Structures for Bankers Algorithm
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Safety Algorithm
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Resource Allocation Algorithm for Pi
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Example of Bankers Algorithm
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Example of Bankers Algorithm (contd)
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Example of Bankers Algorithm (contd)P1
requests (1,0,2)
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Problems with Bankers Algorithm

• 1. As they enter system, jobs must state in
  advance the maximum number of resources needed.
• 2. Number of total resources for each class must
  remain constant.
• 3. Number of jobs must remain fixed.
• 4. Overhead cost incurred by running the
  avoidance algorithm can be quite high.
• 5. Resources arent well utilized because the
  algorithm assumes the worst case.
• 6. Scheduling suffers as a result of the poor
  utilization and jobs are kept waiting for
  resource allocation.

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Deadlock Detection

• Allow system to enter deadlock state
• Use directed graphs to show circular wait which
  indicates a deadlock.
• Algorithm used to detect circularity can be
  executed whenever it is appropriate.
• Recovery scheme

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Reducing Directed Resource Graphs

• 1. Find a process that is currently using a
  resource and not waiting for one. Remove this
  process from graph and return resource to
  available list.
• 2. Find a process thats waiting only for
  resource classes that arent fully allocated.
• Process isnt contributing to deadlock since
  eventually gets resource its waiting for, finish
  its work, and return resource to available
  list.
• 3. Go back to Step 1 and continue the loop until
  all lines connecting resources to processes have
  been removed.

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R1
R2
R3
Figure 5.12 (a)
Figure 5.12 (b)
R1
R1
R2
R3
R2
R3
Figure 5.12 (c)
Figure 5.12 (d)
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Example

• Is the system deadlocked?
• Are there any blocked process?
• What is the resulting graph after reduction by
  P1?
• What is the resulting graph after reduction by
  P2?
• Both P1 and P2 have requested R2
• What is the status of the system if P2s request
  is granted before P1s?
• What is the status of the system if P1s request
  is granted before P2s?

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Recovery from Deadlock

• Once a deadlock has been detected it must be
  untangled and system returned to normal as
  quickly as possible.
• There are several recovery algorithms, all
  requiring at least one victim, an expendable job,
  which, when removed from deadlock, frees system.
• 1. Terminate every job thats active in system
  and restart them from beginning.
• 2. Terminate only the jobs involved in deadlock
  and ask their users to resubmit them.

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Recovery Algorithms - 2

• 3. Terminate jobs involved in deadlock one at a
  time, checking to see if deadlock is eliminated
  after each removal, until it has been resolved.
• 4. Have job keep record (snapshot) of its
  progress so it can be interrupted and then
  continued without starting again from the
  beginning of its execution.
• 5. Select a non-deadlocked job, preempt resources
  its holding, and allocate them to a deadlocked
  process so it can resume execution, thus breaking
  the deadlock
• 6. Stop new jobs from entering system, which
  allows non-deadlocked jobs to run to completion
  so theyll release their resources (no victim).

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Select Victim with Least-Negative Effect

• Priority of job under considerationhigh-priority
  jobs are usually untouched.
• CPU time used by jobjobs close to completion are
  usually left alone.
• Number of other jobs that would be affected if
  this job were selected as the victim.
• Programs working with databases deserve special
  treatment.
• Jobs that are modifying data should not be
  selected for termination

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Starvation

• Starvation -- result of conservative allocation
  of resources where a single job is prevented from
  execution because its kept waiting for resources
  that never become available.
• The dining philosophers Dijkstra (1968).
• Avoid starvation via algorithm designed to detect
  starving jobs which tracks how long each job has
  been waiting for resources (aging).

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The Dining Philosophers Problem
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Terminology

• avoidance
• circular wait
• deadlock
• deadly embrace
• detection
• directed graphs
• locking
• mutual exclusion
• no preemption

• prevention
• process synchronization
• race
• resource holding
• safe state
• spooling
• starvation
• unsafe state
• victim