Deadlock and Starvation
Deadlock
 Permanent blocking of a set of processes
 Processes are waiting on events, but the events can
only be triggered by other blocked processes
 No simple, efficient solution
2
Bridge Crossing Example
 Traffic only in one direction.
 Each section of a bridge can be viewed as a resource.
 If a deadlock occurs, it can be resolved if one car backs
up (preempt resources and rollback).
 Several cars may have to be backed up if a deadlock
occurs.
 Starvation is possible.
3
Can deadlock occur?
Process P
…
Get A
…
Get B
…
Release A
…
Release B
….
Process Q
…
Get B
…
Get A
…
Release B
…
Release A
….
Joint Progress
Diagram (Fig.6.2)
The occurrence of
deadlock depends
on both of the
execution rates and
the details of the
application.
Can deadlock occur?
Process P
…
Get A
…
Release A
…
Get B
…
Release B
….
Process Q
…
Get B
…
Get A
…
Release B
…
Release A
….
Types of Resources
 Reusable vs. Consumable
 Preemptible vs Non-preemptible
7
Reusable resources
 Description:
 Used by one process at a time and not depleted by that
use
 Processes obtain resources that they later release for
reuse by other processes
 Example:
 Processor time, I/O channels, main and secondary
memory, files, databases, and semaphores, printers
 Deadlock occurs if each process holds one resource
and requests the other
8
Deadlock with Reusable Resources
 P1
 P1
Request disk
Lock disk
…
Request tape
Lock tape
…
Unlock disk
…
Unlock tape
Request tape
Lock tape
…
Request disk
Lock disk
…
Unlock tape
…
Unlock disk
Consumable Resources
 Description:
 Created (produced) and destroyed (consumed) by a
process
 Examples:
 Interrupts, signals, messages, and information in I/O
buffers
 Deadlock:
 May occur if a Receive message is blocking
 May take a rare combination of events to cause deadlock
10
Example of Deadlock with a
Consumable Resource
• Deadlock occurs if at some point each is waiting to
receive a message from the other, assuming Receive
is blocking.
• Caused by a design error.
Process P
…
Receive (Q)
…
Send (Q, message1)
Process Q
…
Receive (P)
…
Send (P, message2)
Preemptible Resources
 Can be taken away (preempted) from the owning
process with no ill effects
 Can be restored to the process at a later time
 Examples:
 memory
 processor
 Cannot cause deadlock
12
Nonpreemptible Resources
 Cannot be taken away from the owning process
without adversely affecting its computation
 Examples:
 printer
 tape
 disk
 semaphore
 Can cause deadlock
13
Four Conditions for Deadlock

Necessary policies
1.
2.
3.

Sufficient circumstance
4.


Mutual exclusion must be enforced.
No preemption – no resource can be forcibly removed from the
process.
Hold and Wait – a process may hold a resource while waiting for
another.
Circular wait – closed chain of processes exists, such that each
process holds at least one resource needed by the next process in
the chain.
Conditions 1-3 create a situation that could result in
deadlock
Condition 4 indicates that deadlock has occurred.
Resource Allocation Graphs
 A directed graph used to represent the current snapshot of
a system, i.e. current allocations and pending requests.
 A circle represents a process
 A square represents a resource (can have more than one instance of
a resource)
 A graph edge from a resource instance to a process implies that the
resource instance is allocated to the process
 A graph edge from a process to a resource implies that the process is
requesting an instance of the resource
 Can be used to determine if a system is deadlocked.
RAG Example with Deadlock
16
RAG Example with No Deadlock
17
Basic Facts
 If graph contains no cycles
 no deadlock.
 If graph contains a cycle
 if only one instance per resource type, then deadlock.
 if several instances per resource type, possibility of
deadlock.
18
Approaches to Deadlock Handling
 Ostrich Approach
 Prevention
 Avoidance
 Detection and Recovery
19
Ostrich Approach
 Treat deadlock as a rare annoyance by ignoring it
 Used by most operating systems, including Windows
and UNIX
20
Ostrich Approach Not Always Viable
 A real-time control system monitoring a gasoline refinery
ensures the safe and proper operation of the refinery.
 An avoidance strategy or prevention strategy must be employed.
 A computerized pacemaker – can’t afford to miss a single beat
 deadlock avoidance or prevention must be employed
 Embedded computers – cell phones, PDAs.
 These are SoCs or system on chips i.e. everything is on one chip and
SoCs are characterized by a small set of resources and real-time
processing constraints.
 Can’t have a system administrator come in and decide which process
to abort after deadlock has been detected.
 Either a prevention or avoidance strategy must be employed.
Deadlock Prevention
 Restrain ways a request can be made
 Indirect method: prevent any one of the three
necessary conditions from occurring
 Mutual Exclusion
 No preemption
 Hold and Wait
 Direct method: prevent circular wait from occurring
22
Prevention – Indirect Method
 Option 1 – Don’t enfoce mutual exclusion
 Rarely an option
 Option 2 – Allow preemption
 Resource(s) must be preemptible
 Priority must be used to decide which process should be
preempted.
Prevention – Indirect Method
(cont.)
 Option 3 – Don’t allow “Hold and wait”
 One approach – a process must request all its resources at the
same time. If it does not get all of them, then it must release
all and block until all resources simultaneously available.
 Another approach – a process, if denied a resource, must
release all of its previously allocated resources. If necessary, it
must request them all again together with the next resource.
 Drawbacks:



Processes can be held up for a long time waiting to acquire all
resources
Processes can hold on to resources they do not immediately need
Processes may not know in advance all the resources they need
Prevention – Direct Method
 Prevent circular wait
 Organize resources into groups that are linearly ordered:
G1 , G2 , G3 … Gn.
 A process can request a resource from G1 and then from
G2 and then from G3 and so on, but not vice versa.
 Drawbacks
 Processes may have to request and hold on to resources
that may be used much later, thus wasting resources.
 Processes must know in advance all the resources they
need.
Deadlock Avoidance
 Two strategies:
 Process initiation denial
 Resource allocation denial
 Each process declares the maximum number of resources
of each type that it may need
 Requires that system has a priori information
 The deadlock-avoidance algorithm dynamically examines
the resource-allocation state to ensure that there can never
be a circular-wait condition
 Resource-allocation state is defined by the number of
available and allocated resources, and the maximum
demands/claims of the processes
26
Process Initiation Denial
 The following conditions will apply
 All resources are either available or allocated.
 No process can claim more than the total amount of
resources.
 No process can claim or hold more resources than it
originally claimed.
 Start a process only if all the needs of the current
processes and the new process can be met.
 Pessimistic approach as the assumption is that all
processes will require their maximum claims at the
same time.
Resource Allocation Denial:
Bank Analogy
Ann
Mark
Judy
Greg




Credit limit
$10000
$6000
$3000
$6000
Allocated
$1000
$2000
$1000
$1000
Bank had a total of $12,000.
Current total = $7000.
Should Ann’s request for $5000 be granted?
If yes, amount remaining is $2000.
Resource Allocation Denial:
Bank Analogy (cont.)
Ann
Mark
Judy
Greg




Credit limit
$10000
$6000
$3000
$6000
Allocated
$6000
$2000
$1000
$1000
Bank had a total of $12,000.
Current total = $2000.
Can any other customer get her/his max credit limit?
Judy can take the $2000 and finish, leaving $3000.
Resource Allocation Denial:
Bank Analogy (cont.)
Ann
Mark
Greg




Credit limit
$10000
$6000
$6000
Allocated
$6000
$2000
$1000
Bank had a total of $12,000.
Current total = $3000.
No other customer can get her/his max credit limit
According to Banker’s algorithm, Ann’s original
request for $5000 should not have been granted.
Avoidance Using the Bankers algorithm
 A request is granted only if the request keeps the
system in a safe state
 Otherwise, the process blocks
 A safe state is one which there is at least one
sequence of resource allocations such that every
process can claim its maximum resources and
complete.
 An unsafe state does not imply deadlock, only the
possibility of deadlock
 All processes do not request maximum claims. Even if
they do, deadlock will occur depending on the sequence
in which these demands are made.
Determination of a Safe State
32
Determination of a Safe State
(cont.)
33
Determination of a Safe State
(cont.)
34
Determination of a Safe State
(cont.)
35
Deadlock Detection and Recovery
 Construct a resource allocation graph
 If only one instance of each resource exists
 Circular wait implies deadlock
 If there are two or more instances of any resource
 Circular wait could mean deadlock
 Must reduce the RAG to see if deadlock exists
Recovery from Deadlock
1. Abort all deadlocked processes.
2. Back up each deadlocked process to some previously
defined checkpoint and restart all processes.


Widely used in current database systems.
Possibility of the same situation occurring again.
3. Successively abort all deadlocked processes, until
deadlock no longer exists.

Must decide which process to abort first.
4. Successively pre-empt resources until deadlock no
longer exists.

Must decide which process to pre-empt from
Recovery from Deadlock (cont.)
 To choose which process to abort or to pre-empt a
device from:
 Least amount of processor time consumed so far
 Least amount of output produced so far
 Most estimated remaining time
 Least total resources allocated so far
 Lowest priority
An Integrated Approach
 Group resources into a number of classes.
 Use linear ordering to prevent circular wait between
classes.
 Within a class use an algorithm that is most
appropriate for the class.
Dining Philosophers
 Five philosophers who
alternately think and
eat spaghetti/sushi
 Share a fork/chopstick
with each neighbor
 Assume each
philosopher enters,
picks up left utensil,
then right utensil, then
eats
 Deadlock if all enter at
once
 Figure 6.11, page 276
Dining Philosophers’ problem
Solution 1: Show with an example why this solution is not a correct solution.
semaphore fork[5] = {1,1,1,1,1};
Philosopher(int i){
while(true){
Think();
semwait(fork[i]);
semwait(fork[(i+1)mod5]);
eat();
semsignal(fork[(i+1)mod5]);
semsignal(fork[i]);
}
}
int main(){
parbegin(Philosopher(0), Philosopher(2),
Philosopher(3), Philosopher(4),
Philosopher(1));
}
Solution 2: Show with an example why this solution is not a correct solution.
semaphore x = 1;
Philosopher(int i){
while(true){
Think();
semwait(x);
pick up fork[i];
pick up fork[(i+1)mod5];
semsignal(x);
eat();
semwait(x);
place fork[i] back on the table;
place fork[(i+1)mod5] back on the table;
semsignal(x);
}
}
int main(){
parbegin(Philosopher(1), Philosopher(2), Philosopher(3),
Philosopher(4), Philosopher(5));
}
Solution 3: Provide a correct solution for the dining philosophers’
problem.
Solutions to Dining Philosophers’
Problem
 Buy more utensils
 Equivalent to increasing resources
 Put utensil down if 2nd utensil busy
 Can produce “livelock” if philosophers stay synchronized
 Room attendant
 Only let 4 of the philosophers into the room at once
 May have 4 philosophers in room, but only 1 can eat
 Invite some left-handed philosophers
 Grab forks in the other order (right fork, then left fork)
 Any mix will avoid deadlock (effective linear ordering on
forks)