Chapter 4 : Deadlock 1By : Jigar M. Pandya. Deadlock 2 A Process Must request a resource before...
-
Upload
max-christman -
Category
Documents
-
view
229 -
download
1
Transcript of Chapter 4 : Deadlock 1By : Jigar M. Pandya. Deadlock 2 A Process Must request a resource before...
1
Chapter 4 : Deadlock
By : Jigar M. Pandya
By : Jigar M. Pandya 2
DeadlockA Process Must request a resource
before using it.And Process release the resource after
using it.
Sequence of events required to use a resource:
1.Request the resource.2.Use the resource.3.Release the resource.
By : Jigar M. Pandya 3
DeadlockExamaple :
By : Jigar M. Pandya 4
Deadlock
A deadlock consists of a set of blocked processes, each holding a resource and waiting to acquire a resource held by another process in the set.
By : Jigar M. Pandya 5
Deadlock Characterization Deadlock can arise if four conditions hold simultaneously.
1. Mutual exclusion: only one process at a time can use a resource
By : Jigar M. Pandya 6
Deadlock Characterization Deadlock can arise if four conditions hold simultaneously.
2. Hold and wait: a process holding at least one resource is waiting to acquire additional resources held by other processes
By : Jigar M. Pandya 7
Deadlock Characterization Deadlock can arise if four conditions hold simultaneously.
3. No preemption: a resource can be released only voluntarily by the process holding it after that process has completed its task
By : Jigar M. Pandya 8
Deadlock Characterization Deadlock can arise if four conditions hold simultaneously.
4. Circular wait: there exists a set {P0, P1, …, P0} of waiting processes such that P0 is waiting for a resource that is held by P1, P1 is waiting for a resource that is held by P2, …, Pn–1 is waiting for a resource that is held by Pn, and Pn is waiting for a resource that is held by P0
9By : Jigar M. Pandya
Resource-Allocation Graph
V is partitioned into two types:
◦ P = {P1, P2, …, Pn}, the set consisting of all
the processes in the system
◦ R = {R1, R2, …, Rm}, the set consisting of all
resource types in the system
request edge – directed edge P1 -> Rj
assignment edge – directed edge Rj -> Pi
A set of vertices V and a set of edges E.
By : Jigar M. Pandya 10
Resource-Allocation Graph
Process
Resource Type with 4 instances
Pi requests instance of Rj
Pi is holding an instance of Rj
Pi
Pi
By : Jigar M. Pandya 11
Resource Allocation Graph With A Deadlock
Before P3 requested an instance of R2
After P3 requested an instance of R2
A Cycle In the Graph May cause Deadlock
By : Jigar M. Pandya 12
Graph With A Cycle But No Deadlock
Process P4 may release its instance of resource type R2. That resource can then be allocated to P3, thereby breaking the cycle.
By : Jigar M. Pandya 13
Deadlock
By : Jigar M. Pandya 14
Methods for Handling Deadlocks
Prevention ◦ Ensure that the system will never enter a
deadlock stateAvoidance
◦ Ensure that the system will never enter an unsafe state
Detection ◦ Allow the system to enter a deadlock state and
then recover
Do Nothing ◦ Ignore the problem and let the user or system
administrator respond to the problem; used by most operating systems, including Windows and UNIX
By : Jigar M. Pandya 15
Deadlock Prevention
By : Jigar M. Pandya 16
Deadlock PreventionTo prevent deadlock, we can restrain the ways that a request can be made
Do not allow one of the four conditions to occur.
Mutual Exclusion :
• if no resource were ever assigned to a single process exclusively ,we would never have deadlock.
• Shared entities (read only files) don't need mutual exclusion (and aren’t susceptible to deadlock.)
• Prevention not possible, since some devices are naturally non-sharable
By : Jigar M. Pandya 17
Deadlock PreventionTo prevent deadlock, we can restrain the ways that a request can be made
Do not allow one of the four conditions to occur.
Hold And Wait : we must guarantee that whenever a process requests a resource, it does not hold any other resources
• Require a process to request and be allocated all its resources before it begins execution
• allow a process to request resources only when the process has none
Result: Low resource utilization; starvation possible
By : Jigar M. Pandya 18
Deadlock PreventionTo prevent deadlock, we can restrain the ways that a request can be made
Do not allow one of the four conditions to occur.
No Preemption : If a process that is holding some resources
requests another resource that cannot be immediately allocated to it, then all resources currently being held are released
Pi Rj Pj Rk A process will be restarted only when it can
regain its old resources, as well as the new ones that it is requesting.
Allow preemption - if a needed resource is held by another process, which is also waiting on some resource, steal it. Otherwise wait.
By : Jigar M. Pandya 19
Deadlock PreventionTo prevent deadlock, we can restrain the ways that a request can be made
Do not allow one of the four conditions to occur.
Circular Wait : • impose a total ordering of all resource types, and
require that each process requests resources in an increasing order of enumeration.
F : R NF(ri) < F(rj)
• P1 r1 P2 r2 P3……. Pn-1 rn Pn
For example: F(tape drive) = 1 F(disk drive) = 5 F(printer) = 12
By : Jigar M. Pandya 20
Deadlock Prevention
• EACH of these prevention techniques may cause a decrease in utilization and/or resources. For this reason, prevention isn't necessarily the best technique.
• Prevention is generally the easiest to implement.
21
Deadlock Avoidance
Process must declare all the required resource before starting the execution.
P1 Requires A,B.P2 Requires B,A.
By : Jigar M. Pandya
P1
B
P2
A
1) P1 A
2) P2 BWait Wait Wait
3) P1 B
4) P2 B
5) P2 A
Process P1 Complete Release Resource
Process P2 Complete Release Resourece
22
Deadlock Avoidance
By : Jigar M. Pandya
Simplest and most useful model requires that each process declare the maximum number of resources of each type that it may need
The deadlock-avoidance algorithm dynamically examines the resource-allocation state to ensure that there can never be a circular-wait condition
A resource-allocation state is defined by the number of available and allocated resources, and the maximum demands of the processes
23
Deadlock Avoidance
By : Jigar M. Pandya
Possible states are:
Deadlock No forward progress can be made.
Unsafe state A state that may allow
deadlock. Safe state A state is safe if a sequence of
processes exist such that there are enough resources for the first to finish, and as each finishes and releases its resources there are enough for the next to finish.
The rule is simple: If a request allocation would cause an unsafe state, do not honor that request.
24
Deadlock Avoidance
By : Jigar M. Pandya
NOTE: All deadlocks are unsafe, but all unsafes are NOT deadlocks.
SAFEDEADLOCK
UNSAFE
Only with luck will processes avoid
deadlock.
O.S. can avoid deadlock.
25
Deadlock Avoidance
By : Jigar M. Pandya
EXAMPLE:
There exists a total of 12 resources. Each resource is used exclusively by a process. The current state looks like this:
• In this example, < p1, p0, p2 > is a workable sequence. • Free = 2
• Suppose p2 requests and is given one more resource. What happens then?
• Free= 1
Process Max Needs Allocated Current Needs
P0 10 5 5
P1 4 2 2
P2 9 3 7
Process Max Needs Allocated Current Needs
P0 10 5 5
P1 4 2 2
P2 9 4 7
By : Jigar M. Pandya 26
Avoidance algorithms
For a single instance of a resource type, use a resource-allocation graph
For multiple instances of a resource type, use the banker’s algorithm
By : Jigar M. Pandya 27
Resource-Allocation Graph withClaim Edges
Requestedge
Assignmentedge
Claimedge
Claimedge
By : Jigar M. Pandya 28
Unsafe State In Resource-Allocation Graph
Assignmentedge
Requestedge
AssignmentedgeClaim
edge
By : Jigar M. Pandya 29
Resource-Allocation Graph Algorithm
Suppose that process Pi requests a resource Rj
The request can be granted only if converting the request edge to an assignment edge does not result in the formation of a cycle in the resource allocation graph
30By : Jigar M. Pandya
Data Structures for the Banker’s Algorithm
Available: Vector of length m. If available [j] = k, there
are k instances of resource type Rj available.
Max: n x m matrix. If Max [i,j] = k, then process Pi may
request at most k instances of resource type Rj.
Allocation: n x m matrix. If Allocation[i,j] = k then Pi is
currently allocated k instances of Rj.
Need: n x m matrix. If Need[i,j] = k, then Pi may need k
more instances of Rj to complete its task.
Let n = number of processes, and m = number of resources types.
31By : Jigar M. Pandya
Banker’s Algorithm
A method used to determine if a particular state is safe. It's safe if there exists a sequence of processes such that for all the processes, there’s a way to avoid deadlock:
The algorithm uses these variables:
Need[I] – the remaining resource needs of each process.Work - Temporary variable – how many of the resource
are currently available.Finish[I] – flag for each process showing we’ve analyzed
that process or not.
need <= available + allocated[0] + .. + allocated[I-1] Sign of success
Let work and finish be vectors of length m and n respectively.
Safety Algorithm
32By : Jigar M. Pandya
Banker’s Algorithm
1. Initialize work = availableInitialize finish[i] = false, for i = 1,2,3,..n
2. Find an i such that:
finish[i] == false and need[i] <= work
If no such i exists, go to step 4. 3. work = work + allocation[i]
finish[i] = truegoto step 2
4. if finish[i] == true for all i, then the system is in a safe state.
Safety Algorithm
33
Banker’s Algorithm
By : Jigar M. Pandya
Context Of Matrix need Is Calculated As Need = Max - Allocation
By : Jigar M. Pandya 34
Banker’s Algorithm
By : Jigar M. Pandya 35
Banker’s Algorithm
By : Jigar M. Pandya 36
Banker’s Algorithm