Deadlocks – An Introduction
• What Are DEADLOCKS ? A Blocked Process which can never be resolved
unless there is some outside Intervention.
Resource R1 is requested by Process P1 but is held by Process P2.
• For Example:-
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Deadlock
Deadlock is a situation where a process or a set of processes is blocked on an event that never occurs
Processes while holding some resources may request for additional allocation of resources which are held by other processes
Processes are in circular wait for the resources
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Deadlock vs Starvation
Starvation occurs when a process waits for a resource that becomes available continuously but is not allocated to a process
Two Main Differences - In starvation it is not certain that a process will ever get
the requested resource where as a deadlocked process is permanently blocked because required resource never become available
- In starvation the resource under contention is in continuation use where as this is not true in case of deadlock
Causes Of Deadlocks
• Mutual Exclusion – Resources being held must be in non-shareable mode.
• Hold n Wait – A Process is holding one resource and is waiting for another, which is held by another process.
• No Preemption – Resource cannot be preempted even if it is being requested.
• Circular Wait – Presence of a cycle of waiting processes.
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Models of Deadlock
Single-Unit Request Model- Process is restricted to request only one resource at a
time- Outdegree in WFG is one- Cycle in WFG means deadlock
P1 P2
P3
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Models of Deadlock ………
AND Request Model- Process can simultaneously request multiple resources- Process Remain blocked until all the resources are granted- Outdegree of WFG can be more than 1- Cycle in WFG means system is deadlocked - Process can be involved in more than one deadlock
P1 P2
P3 P4
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Models of Deadlock ………
OR Request Model- Process can simultaneously request multiple resources- Process Remain blocked until it is granted any of the requested
resources- Outdegree of WFG can be more than 1- Cycle in WFG is not a sufficient condition for the deadlock - Knot in the WFG is a sufficient condition for deadlock - Knot is a subset of graph such that starting from any node in the
subset it is impossible to leave the knot by following the edges of the graph
Cycle vs Knot
P1 P2
P3
P4
P5
Cycle but no Knot
Deadlock in AND Model
But no Deadlock in OR Model
P1 P2
P3
P4
P5
Cycle & Knot
Deadlock in both AND & OR Model
Resources
Reusable (CPU, Main-memory, I/O Devices) Consumable (Messages, Interrupt Signals
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Distributed Deadlock Detection• Assumptions:
a. System has only reusable resourcesb. Only exclusive access to resourcesc. Only one copy of each resourced. States of a process: running or blockede. Running state: process has all the resourcesf. Blocked state: waiting on one or more resource
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Resource vs Communication Deadlocks
• Resource Deadlocks• A process needs multiple resources for an activity.• Deadlock occurs if each process in a set request resources held by another process in the same set, and it must receive all the requested resources to move further.
• Communication Deadlocks• Processes wait to communicate with other processes in a set.• Each process in the set is waiting on another process’s message, and no process in the set initiates a message until it receives a message for which it is waiting.
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Graph Models
Nodes of a graph are processes. Edges of a graph the pending requests or assignment of resources.
Wait-for Graphs (WFG): P1 -> P2 implies P1 is waiting for a resource from P2.
Transaction-wait-for Graphs (TWF): WFG in databases. Deadlock: directed cycle in the graph. Cycle example:
P1 P2
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Graph Models
Wait-for Graphs (WFG): P1 -> P2 implies P1 is waiting for a resource from P2.
P1
P2
R1
R2
Request Edge
Assignment Edge
Illustrating A Deadlock
• Wait-For-Graph (WFG) Nodes – Processes in the system Directed Edges – Wait-For blocking relation
• A Cycle represents a Deadlock• Starvation - A process’ execution is permanently halted.
Process 1 Process 2
Resource 1
Resource 2Waits For
Waits For
Held By
Held By
WFG (Wait For Graph) & TWF (Transaction WF)
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AND, OR Models
AND Model A process/transaction can simultaneously request for
multiple resources. Remains blocked until it is granted all of the requested
resources.
OR Model A process/transaction can simultaneously request for
multiple resources. Remains blocked till any one of the requested resource is
granted.
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Sufficient Condition
P1 P2
P3P4
P5
P6
Deadlock ??
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AND, OR Models
AND Model Presence of a cycle.
P1 P2
P3P4
P5
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AND, OR Models
OR Model Presence of a knot. Knot: Subset of a graph such that starting from any
node in the subset, it is impossible to leave the knot by following the edges of the graph.
P1 P2
P3P4
P5
P6
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Deadlock Handling Strategies
Deadlock Prevention: difficult Deadlock Avoidance: before allocation, check for
possible deadlocks. Difficult as it needs global state info in each site (that
handles resources). Deadlock Detection: Find cycles. Focus of discussion. Deadlock detection algorithms must satisfy 2 conditions:
No undetected deadlocks. No false deadlocks.
Deadlocks in Distributed Systems
• Resource Deadlock Most Common. Occurs due to lack of requested Resource. Set of deadlocked processes, where each process waits for a
resource held by another process (e.g., data object in a database, I/O resource on a server)
• Communication Deadlock
A Process waits for certain messages before it can proceed.
Set of deadlocked processes, where each process waits to receive messages (communication) from other processes in the set.
Handling Deadlocks
• Deadlock Avoidance
Only fulfill those resource requests that won’t cause deadlock in the future.
Inefficient. Requires Prior resource requirement information for all
processes. High Cost of scalability. Every site has to maintain global state of system (extensive
overhead in storage and communication) Different sites may determine (concurrently) that state is safe, but
global state may be unsafe: verification for safe global state by different sites must be mutually exclusive
Large overhead to check for every allocation (distributed system may have large number of processes and resources
Conclusion: Deadlock avoidance impractical in distributed systems
• Drawbacks
Simulate resource allocation and determine if resultant state is safe or not.
Decision made dynamically, before allocating a resource, the resulting global system state is checked - if safe, allow allocation
Handling Deadlocks
• Deadlock Prevention
Provide all required resources from start itself.
Prioritize processes. Assign resources accordingly.
Inefficient and effects Concurrency.
Make Prior Rules: For Ex. – Process P1 cannot request resource
R1 unless it releases resource R2.
Future resource requirement unpredictable.
• Drawbacks
Starvation possible.
1.a Prevent the circular-wait condition by defining a linear ordering of resource types
A process can be assigned resources only according to the linear ordering
Disadvantages- Resources cannot be requested in the order that are needed- Resources will be longer than necessary
1.b Prevent the hold-and-wait condition by requiring the process to acquire all needed resources before starting execution
Disadvantages Inefficient use of resources Reduced concurrency Process can become deadlocked during the initial resource
acquisition Future needs of a process cannot be always predicted
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Deadlock Detection Principle of operation
Detection of a cycle in WFG proceeds concurrently with normal operation
Requirements for the deadlock detection and resolution algorithms Detection
The algorithm must detect all existing deadlock in finite time The algorithm should not report non-existent (phantom) deadlock
Resolution (recovery) All existing wait-for dependencies in WFG must be removed, i.e. roll-
back one or more processes that are deadlocked and give their resources to other blocked processes
Observation Deadlock detection is the most popular strategy for handling
deadlocks in distributed systems
Handling Deadlocks
• Deadlock Detection
Resource allocation with an optimistic outlook. Periodically examine process status. Detect then break the Deadlock.
• Resolution – Roll back 1 or More processes and break dependency.
CSE - 8344
Issues in distributed systems Special issues in distributed systems
Resources are distributed across many sites The control processes that control access to
resources do not have complete, up-to-date information on the global state of the system
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Distributed Deadlocks Centralized Control
One control node (Coordinator) maintains Global WFG and searches for cycles A control site constructs wait-for graphs (WFGs) and checks for directed cycles. WFG can be maintained continuously (or) built on-demand by requesting WFGs
from individual sites. Distributed Control
Each node equally responsible in maintaining Global WFG and detecting Deadlocks.
WFG is spread over different sites.Any site can initiate the deadlock detection process.
Hierarchical Control Nodes organized in a tree, where each site detects deadlocks involving only its
descendants. Sites are arranged in a hierarchy. A site checks for cycles only in descendents.
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Deadlock in resource allocation:Algorithms for distributed deadlock detection 3) Deadlock Detection (cont.)
Control for distributed deadlock detection can be:a. Centralized
b. Distributed
c. Hierarchical
a.1 Centralized deadlock detection algorithms A central control site constructs the global WFG and searches for cycles Control site an maintain WFG continuously (with every assignment) or when
running deadlock detection (and asking all sites for WFG updates) Disadvantages: single point of failure and congestion
a.2
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Deadlock in resource allocation:Algorithms for distributed deadlock detection 3) Deadlock Detection (cont.)
b. Hierarchical deadlock detection algorithms Sites organized in a tree structure with one site at the root of the
tree Each node (except for leaf nodes) has information about the
dependent nodes Deadlock is detected by the node that is the common ancestor of
all sites which have resource allocations in conflict Deadlock is detected at the lowest level
Deadlock Detection Algorithms
• Centralized Deadlock Detection
• Distributed Deadlock Detection
• Hierarchical Deadlock Detection
Ho-Ramamoorthy’s one and two phase algorithms. Completely Centralized Algorithm
Obermarck’s Path Pushing Algorithm. Chandy-Misra-Haas Edge Chasing algorithm.
Menasce-Muntz Algorithm. Ho-Ramamoorthy’s Algorithm.
•
The completely centralized
algorithm All sites request resources and release resources by sending
corresponding messages to control site Control site updates WFG for each request/release For every new request edge added to WFG, control site checks WFG for
deadlock Alternative: each site maintain its WFG and update control site periodically
or on request
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Centralized Algorithms
Ho-Ramamurthy 2-phase Algorithm Each site maintains a status table of all processes initiated at
that site: includes all resources locked & all resources being waited on.
Controller requests (periodically) the status table from each site. Controller then constructs WFG from these tables, searches for
cycle(s). If no cycles, no deadlocks. Otherwise, (cycle exists): Request for state tables again. Construct WFG based only on common transactions in the 2
tables. If the same cycle is detected again, system is in deadlock. Later proved: cycles in 2 consecutive reports need not result in
a deadlock. Hence, this algorithm detects false deadlocks.
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Centralized Algorithms...
Ho-Ramamoorthy 1-phase Algorithm Each site maintains 2 status tables: resource status table and
process status table. Resource table: transactions that have locked or are waiting
for resources. Process table: resources locked by or waited on by
transactions. Controller periodically collects these tables from each site. Constructs a WFG from transactions common to both the
tables. No cycle, no deadlocks. A cycle means a deadlock.
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Distributed Algorithms
Path-pushing: resource dependency information disseminated through designated paths (in the graph) [Examples : Menasce-Muntz & Obermarck]
Edge-chasing: special messages or probes circulated along edges of WFG. Deadlock exists if the probe is received back by the initiator. [Examples :CMH for AND Model , Sinha-Natarajan]
Diffusion computation: queries on status sent to process in WFG. [Examples :CMH for OR Model, Chandy-Herman]
Global state detection: get a snapshot of the distributed system. [Examples :Bracha-Toueg,Kshemkalyani-Singhal]
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Path-pushing Obermarck’s Algorithm (AND model)
Path Propagation Based Algorithm Based on a database model using transaction
processing Sites which detect a cycle in their partial WFG views
convey the paths discovered to members of the (totally ordered) transaction
Algorithm can detect phantoms due to its asynchronous snapshot method
S1 S2
S4 S3
Obermark’s Algorithm Example
Intial State
Obermark’s Algorithm Example
Iteration 1
Obermark’s Algorithm Example
Iteration 2
Obermark’s Algorithm Example
Iteration 3
Obermark’s Algorithm Example
Iteration 4
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Edge-Chasing Algorithm
Chandy-Misra-Haas’s Algorithm (AND MODEL): A probe(i, j, k) is used by a deadlock detection process Pi.
This probe is sent by the home site of Pj to Pk. This probe message is circulated via the edges of the graph.
Probe returning to Pi implies deadlock detection. Terms used:
Pj is dependent on Pk, if a sequence of Pj, Pi1,.., Pim, Pk exists.
Pj is locally dependent on Pk, if above condition + Pj,Pk on same site.
Each process maintains an array dependenti: dependenti(j) is true if Pi knows that Pj is dependent on it. (initially set to false for all i & j).
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Chandy-Misra-Haas’s AlgorithmSending the probe:
if Pi is locally dependent on itself then deadlock.else for all Pj and Pk such that (a) Pi is locally dependent upon Pj, and (b) Pj is waiting on Pk, and (c ) Pj and Pk are on different sites, send probe(i,j,k) to the home site of Pk.
Receiving the probe:if (d) Pk is blocked, and (e) dependentk(i) is false, and (f) Pk has not replied to all requests of Pj,then begin dependentk(i) := true;
if k = i then Pi is deadlockedelse ...
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Chandy-Misra-Haas’s Algorithm
Receiving the probe:…….
else for all Pm and Pn such that (a’) Pk is locally dependent upon Pm, and (b’) Pm is waiting on Pn, and (c’) Pm and Pn are on different sites, send probe(i,m,n) to the home site of Pn.
end.
Performance:For a deadlock that spans m processes over n sites, m(n-1)/2 messagesare needed. Size of the message 3 words.Delay in deadlock detection O(n).
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C-M-H Algorithm: Example
P1 P2 P6 P7
P3
P4 P5
Site 1
Site 2
Site 3
( 1,1,2 )
( 1,2,3 )
( 1,2,4 )
( 1,4,5 )
( 1,5,6 )
( 1,6,7 )
( 1,7,1 )
P1 initiates Deadlock Detection by sending Probe Message (1,1,2) to P2
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Diffusion-based AlgorithmCMH Algorithm for OR
ModelInitiation by a blocked process Pi: send query(i,i,j) to all processes Pj in the dependent set DSi of Pi; num(i) := |DSi|; waiti(i) := true;
Blocked process Pk receiving query(i,j,k): if this is engaging query for process Pk /* first query from Pi */
then send query(i,k,m) to all Pm in DSk;numk(i) := |DSk|; waitk(i) := true;
else if waitk(i) then send a reply(i,k,j) to Pj.
Process Pk receiving reply(i,j,k) if waitk(i) then
numk(i) := numk(i) - 1;if numk(i) = 0 then if i = k then declare a deadlock. else send reply(i, k, m) to Pm, which sent the engaging query.
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Diffusion Algorithm: Example
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Engaging Query
How to distinguish an engaging query? query(i,j,k) from the initiator contains a unique
sequence number for the query apart from the tuple (i,j,k).
This sequence number is used to identify subsequent queries.
(e.g.,) when query(1,7,1) is received by P1 from P7, P1 checks the sequence number along with the tuple.
P1 understands that the query was initiated by itself and it is not an engaging query.
Hence, P1 sends a reply back to P7 instead of forwarding the query on all its outgoing links.
Mitchell-Merritt Algorithm(Edge-Chasing Category)
Each Node has two labels : Public & Private Private Label is unique to node but may change Initially both private and public label values are same Guarantees that only one process will detect the deadlock Process/Node/Site responsible for deadlock detection propagates
public label in reverse direction When a blocked transaction reads the public label of waiting upon
process it changes its public label if its own public label value is less than read value.
When a initiator process reads the message with public label equals to its own then deadlock is detected.
Mitchell-Merritt AlgorithmThe algorithm exhibits 4 nondeterministic
state transitions
u v
State BeforeState After
Outdegree =0
1. Block State
x vx
Value x should be computed as per function inc(u,v) i.e. any value which is larger than both u,v
1. This block step occurs when a process begins to wait on some resource held by other [ Creates an edge in WFG]
2. Label change occurs in this step for waiting process
3. Both public and private labels of the waiting process are increased to a value greater than their previous values & greater than the public label of the process being waited on.
State Before
2. Activate
• Earlier there is an edge in the before state, but there will be no edge in the after state
• Edge disappeared [Either process may be allocated resource, or timed out or owner of the resource may have changed]
State After
u v
State Before
3. Transmit State
v v
1. When a waiting process reads public variable of waiting upon process
2. If the public label of waiting process is smaller than the public label of the process upon whom it is waiting, then waiting process will change its public label equal to the public label of the process upon whom it is waiting.
3. Waiting process’s private label remains unchanged
State After
If u < v
u u
State Before
4. Detect State
u u
1. When a process sees its own public label comes back to itself
2. When a process reads a public label of the waiting upon process and finds that the public label value of waiting upon process is equals to its own public label value then it determines that a cycle exists and declares deadlock
State After
uu
Mitchell-Merritt Algorithm Example
public
privateNode
(public-value,node-id)
(private-value,node-id)
1,1
1,1
3,3
3,3
5,5
5,5
4,4
4,4
Initially both public and private label values at each node are equal
P1
P5
P3
P4
Mitchell-Merritt Algorithm Example cont…
4,1
4,1
3,3
3,3
5,5
5,5
4,4
4,4
P1
P5
P3
P4
Now suppose P1 is waiting for P3 (P1 P3)
Block state will occur for P1
Block
Mitchell-Merritt Algorithm Example cont…
4,1
4,1
3,3
3,3
6,5
6,5
4,4
4,4
P1
P5
P3
P4
Now suppose P5 is waiting for P1 (P5 P1)
Block state will occur for P5
Block
Block
Mitchell-Merritt Algorithm Example cont…
4,1
4,1
7,3
7,3
6,5
6,5
4,4
4,4
P1
P5
P3
P4
Block
BlockBlock
Now suppose P3 is waiting for P5 (P3 P5)
Block state will occur for P3
Mitchell-Merritt Algorithm Example cont…
7,34,1
7,3
7,3
6,5
6,5
4,4
4,4
P1
P5
P3
P4
Now P3 initiates Transmit Phase
P3 will transmit its public label to P1 (Reverse Direction)
Transmit
Here P1 reads public label of P3
P1’s public label =(4,1)
P3’s public label =(7,3)
So P1 will change is public label to (7,3)
But No change for private label of P1
Mitchell-Merritt Algorithm Example cont…
7,34,1
7,3
7,3
7,36,5
4,4
4,4
P1
P5
P3
P4
P1 will transmit its public label to P5 (Reverse Direction)
Transmit
P1’s public label =(7,3)
P5’s public label =(6,5)
So P5 will change is public label to (7,3)
But No change for private label of P5
Transmit
Mitchell-Merritt Algorithm Example cont…
7,34,1
7,3
7,3
7,36,5
4,4
4,4
P1
P5
P3
P4
P5 will transmit its public label to P3 (Reverse Direction)
Transmit
P5’s public label =7,3
P3’s public label =7,3
So P3 Detects DeadlockTransmit
Transmit
Hierarchical Deadlock Detection
• Menasce-Muntz Algorithm
Sites (controllers) organized in a tree structure.
Leaf controllers manage local WFG.
Upper controllers handle Deadlock Detection. Each Parent node maintains a Global WFG,
union of WFG’s of its children. Deadlock detected for its children.
Changes propagated upwards in the tree.
CSE - 8344
• Ho-Ramamoorthy’s Algorithm
Hierarchical Deadlock Detection
Sites grouped into clusters.
Periodically 1 site chosen as central control site: Central control site chooses controls site
for other clusters. Control site for each cluster collects the status
graph there: Ho-Ramamoorthy’s 1-phase algorithm
centralized DD algorithm used. All control sites forward status report to Central
Control site which combines the WFG and performs cycle search.
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Hierarchical Deadlock Detection
• Follows Ho-Ramamoorthy’s 1-phase algorithm. More than 1 control site organized in hierarchical manner. • Each control site applies 1-phase algorithm to detect (intracluster) deadlocks.• Central site collects info from control sites, applies 1-phase algorithm to detect intracluster deadlocks.
Central Site
Controlsite
Controlsite
Controlsite
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Persistence & Resolution
Deadlock persistence: Average time a deadlock exists before it is resolved.
Implication of persistence: Resources unavailable for this period: affects utilization Processes wait for this period unproductively: affects response
time. Deadlock resolution:
Aborting at least one process/request involved in the deadlock. Efficient resolution of deadlock requires knowledge of all
processes and resources. If every process detects a deadlock and tries to resolve it
independently -> highly inefficient ! Several processes might be aborted.
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Deadlock Resolution
Priorities for processes/transactions can be useful for resolution. Consider priorities introduced in Obermarck’s algorithm. Highest priority process initiates and detects deadlock
(initiations by lower priority ones are suppressed). When deadlock is detected, lowest priority process(es) can
be aborted to resolve the deadlock. After identifying the processes/requests to be aborted,
All resources held by the victims must be released. State of released resources restored to previous states. Released resources granted to deadlocked processes.
All deadlock detection information concerning the victims must be removed at all the sites.
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