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• Deadlock remains until a programs is withdrawn or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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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
• Resource holding
• No preemption
• Circular wait

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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)
Figure 5.7 (c)
Figure 5.7 (b)
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Figure 5.8
Figure 5.9
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? ? ?
Figure 5.11 (a)
? ? ?
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 or No Preemption

• 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.
• 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.

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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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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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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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Detection

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

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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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Recovery

• 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.

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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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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