Transcript Chapter 5 : Process Management

Chapter 5 : Process Management
• Deadlock
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7 Cases of Deadlock
Conditions for Deadlock
Modeling Deadlocks
Strategies for Handling
Deadlocks
– Avoidance
– Detection
– Recovery
Process Synchronization
Deadlock
Management
Starvation
Management
• Starvation
Understanding
Operating Systems
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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
can’t because they’re 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.
Understanding
Operating Systems
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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).
Understanding
Operating Systems
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Seven Cases of Deadlocks
1.
2.
3.
4.
5.
6.
7.
Deadlocks on file requests
Deadlocks in databases
Deadlocks in dedicated device allocation
Deadlocks in multiple device allocation
Deadlocks in spooling
Deadlocks in disk sharing
Deadlocks in a network
Understanding
Operating Systems
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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programs is withdrawn or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.
Understanding
Operating Systems
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Case 2 : Deadlocks in Databases
1. P1 accesses R1 and locks it.
• Deadlock can occur if 2 processes
access & lock records in database.
2. P2 accesses R2 and locks it.
• 3 different levels of locking :
3. P1 requests R2, which is locked
by P2.
4. P2 requests R1, which is locked
by P1.
– entire database for duration of
request
– a subsection of the database
– individual record until process is
completed.
• If don’t use locks, can lead to a
race condition.
Understanding
Operating Systems
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Case 3: Deadlocks in Dedicated
Device Allocation
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.
• Deadlock can occur when
there is a limited number
of dedicated devices.
• E.g., printers, plotters or
tape drives.
4. P2 requests tape drive 1 but is
blocked.
Understanding
Operating Systems
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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.
Understanding
Operating Systems
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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.
Understanding
Operating Systems
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Case 6 : Deadlocks in Disk Sharing
P1
Read records at
cylinder 20
P2 Write to file at
cylinder 310
I/O
Channel
Disk
control
unit
• Disks are designed to be shared, so it’s 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.
Understanding
Operating Systems
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Case 7: Deadlocks in a Network
• A network that’s
congested (or filled large
% of its I/O buffer space)
can become deadlocked if
it doesn’t have protocols
to control flow of
messages through
network.
Understanding
Operating Systems
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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
Understanding
Operating Systems
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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.
Understanding
Operating Systems
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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 there’s a cycle in the graph then there’s a deadlock
involving the processes and the resources in the cycle.
Understanding
Operating Systems
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Directed Graph Examples
R1
P1
Figure 5.7 (a)
R1
P2
P1
R2
R1
Figure 5.7 (b)
Understanding
Operating Systems
P1
Figure 5.7 (c)
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Figure 5.8
R1
R1
R2
R2
R3
R3
P1
P1
P2
P2
P3
P3
Figure 5.9
R1
R2
R3
R1
R2
R3
P1
P2
P3
P1
P2
P3
Understanding
Operating Systems
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P1



P3
Figure 5.11 (a)
P2
P1



P3
Figure 5.11 (b)
P2
Understanding
Operating Systems
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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.
Understanding
Operating Systems
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Prevention of Deadlock
• To prevent a deadlock OS must eliminate 1 out of 4
necessary conditions.
– Same condition can’t 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).
Understanding
Operating Systems
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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.
Understanding
Operating Systems
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Prevention of Circular Wait
• Circular wait can be bypassed if OS prevents formation of
a circle.
• Havender’s 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.
Understanding
Operating Systems
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Avoidance
• Even if OS can’t 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.
• Dijkstra’s 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 -- doesn’t necessarily lead to deadlock, but it
does indicate that system is an excellent candidate for one.
Understanding
Operating Systems
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Banker’s 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 bank’s 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 won’t exceed the bank’s 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 < = number of
available resources
Understanding
Operating Systems
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A Bank’s Safe and Unsafe States
Safe
Customer
Loan amount
C1
0
C2
2,000
C3
4,000
Total loaned: $6,000
Total capital fund: $10,000
Maximum credit
4,000
5,000
8,000
Remaining credit
4,000
3,000
4,000
Customer
Loan amount
C1
2,000
C2
3,000
C3
4,000
Total loaned: $9,000
Total capital fund: $10,000
Maximum credit
4,000
5,000
8,000
Remaining credit
2,000
2,000
4,000
Unsafe
Understanding
Operating Systems
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Problems with Banker’s 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 aren’t 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.
Understanding
Operating Systems
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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.
Understanding
Operating Systems
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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 that’s waiting only for resource classes that
aren’t fully allocated.
– Process isn’t contributing to deadlock since eventually gets
resource it’s 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.
Understanding
Operating Systems
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R1
P2
P3
R1
P2
P3
P1
R2
R3
P1
R2
R3
Figure 5.12 (a)
Figure 5.12 (b)
R1
P2
P3
R1
P2
P3
P1
R2
R3
P1
R2
R3
Figure 5.12 (c)
Understanding
Operating Systems
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 that’s active in system and restart
them from beginning.
2. Terminate only the jobs involved in deadlock and ask their
users to resubmit them.
Understanding
Operating Systems
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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 it’s
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 nondeadlocked jobs to run to completion so they’ll release
their resources (no victim).
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Select Victim with Least-Negative Effect
• Priority of job under consideration—high-priority jobs are
usually untouched.
• CPU time used by job—jobs 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.
Understanding
Operating Systems
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Starvation
• Starvation -- result of conservative allocation of
resources where a single job is prevented from execution
because it’s 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).
Understanding
Operating Systems
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Terminology
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avoidance
circular wait
deadlock
deadly embrace
detection
directed graphs
locking
mutual exclusion
no preemption
Understanding
Operating Systems
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prevention
process synchronization
race
resource holding
safe state
spooling
starvation
unsafe state
victim
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