Lecture Operating systems: A concept-based approach (2/e): Chapter 17 - Dhananjay M. Dhamdhere

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Chapter 17: Distributed 
Control Algorithms

Dhamdhere: Operating Systems—
A Concept­Based Approach , 2 ed

Slide No: 1
Copyright © 2008


OS control functions in a distributed environment

•

Special features of distributed OS control functions
– Mutual exclusion
* Involves synchronization of processes in different computers

– Deadlock handling
* Deadlocks may involve use of resources in different computers

– Scheduling
* Perform load balancing to ensure uniform loading of computers

– Termination detection
* Check whether all processes of a computation, which may operate
in different computers, have completed

– Election
* Elect a coordinator for a privileged function like resource allocation
Chapter 17: Distributed 
Control Algorithms

Dhamdhere: Operating Systems—
A Concept­Based Approach , 2 ed

Slide No: 2
Copyright © 2008


Nature of a distributed control algorithm

•

A distributed control function offers services to both
system and user processes
– It operates in parallel with its clients

•

Following terminology is used to differentiate between
the distributed control algorithm and its clients
– Basic computation: Operation of a client
* Interprocess messages used by it are called basic messages

– Control computation: Operation of the control algorithm
* Interprocess messages exchanged in the control computation are

called control messages

– Basic part and control part of a process
* Participate in the basic and control computations, respectively
Chapter 17: Distributed 
Control Algorithms

Dhamdhere: Operating Systems—
A Concept­Based Approach , 2 ed

Slide No: 3
Copyright © 2008


Basic and control parts of a process Pi

• The basic part of Pi interacts with basic parts of other processes through
basic messages; analogously for control part of Pi
• The control part provides services such as resource allocation to the
basic part
Chapter 17: Distributed 
Control Algorithms

Dhamdhere: Operating Systems—
A Concept­Based Approach , 2 ed

Slide No: 4
Copyright © 2008

Correctness of a distributed control algorithm

•

Processes of a distributed control algorithm exchange
control data and coordinate through control messages
– New correctness issues arise because
* Exchange of control messages incurs delays
 Control data used in processes may become stale or may
appear inconsistent

– Hence correctness has two new facets
* Liveness
 The algorithm must eventually perform the correct action
* Safety
 The algorithm must not perform wrong actions

Chapter 17: Distributed 
Control Algorithms

Dhamdhere: Operating Systems—
A Concept­Based Approach , 2 ed

Slide No: 5
Copyright © 2008


Liveness and safety of
distributed control algorithms

Chapter 17: Distributed 
Control Algorithms

Dhamdhere: Operating Systems—
A Concept­Based Approach , 2 ed

Slide No: 6
Copyright © 2008


Distributed mutual exclusion algorithms

•

At any time, at most one process may be in a CS for a
data item ds
– Permission-based algorithms
* A process seeks the permission of a set of processes and enters a
CS only when all processes in the set have granted the permission

– Token-based algorithms
* A token represents a special privilege of some kind, e.g., privilege to
enter a CS
* Only a process possessing the token may perform a privileged
operation
* Processes send the token to one another to pass on the privilege

Chapter 17: Distributed 
Control Algorithms

Dhamdhere: Operating Systems—
A Concept­Based Approach , 2 ed

Slide No: 7
Copyright © 2008


Ricart–Agrawala algorithm

•

Steps of the algorithm
1. Process wishing to enter CS sends time-stamped requests to
all other processes
2. When a process receives a request
a. If it is not interested in entering CS, it sends a ‘go ahead’
immediately
b. If it is also interested in entering CS, it sends a ‘go ahead’ only if
the received request’s time-stamp < its own time-stamp
c. If it is in a CS, it adds the request to the pending list

3. When a process receives n -1 ‘go ahead’ replies, it enters CS
4. When a process exits a CS, it sends ‘go ahead’ replies to each
request in its pending list
The algorithm requires 2 x (n – 1) messages per CS entry
Chapter 17: Distributed 
Control Algorithms

Dhamdhere: Operating Systems—
A Concept­Based Approach , 2 ed

Slide No: 8
Copyright © 2008


Basic and control actions in
Ricart–Agrawala algorithm

1, 2(b), 3

Chapter 17: Distributed 
Control Algorithms

Dhamdhere: Operating Systems—
A Concept­Based Approach , 2 ed

Slide No: 9
Copyright © 2008


Maekawa algorithm

•

Each process has a request set of processes; it seeks
the permission of only processes in the request set (Ri
represents the request set of process Pi)
– Correctness is ensured through the following rules:
* For all Pi :

Pi is included in Ri

* For all Pi, Pj: Ri ∩ Rj is non-null

– The algorithm requires 2 x √n messages per CS entry

Chapter 17: Distributed 
Control Algorithms

Dhamdhere: Operating Systems—
A Concept­Based Approach , 2 ed

Slide No: 10
Copyright © 2008


Token-based algorithm for a ring

•

Only the process holding the token can enter a CS
– An abstract ring is superimposed on a system (see next slide)
– A process wishing to enter a CS sends its request along the ring
and enters the CS only when it receives the token
* A Process not holding the token simply forwards the request to the
next process
* If the process holds the token and is not in a CS
 It sends the token to the next process
 The token travels over the ring until it reaches the requester
* If the process holds the token and is in a CS

 The request is entered in a request queue in the token
 When the process finishes the CS, it sends token to the next
process for delivery to the first process in its request queue

Chapter 17: Distributed 
Control Algorithms

Dhamdhere: Operating Systems—
A Concept­Based Approach , 2 ed

Slide No: 11
Copyright © 2008


Token-based algorithm for a ring

(a) The system
(b) Abstract ring for the system: P4 has the token, requests by P2 and
P6 exist in the token’s request queue
Chapter 17: Distributed 
Control Algorithms

Dhamdhere: Operating Systems—
A Concept­Based Approach , 2 ed

Slide No: 12
Copyright © 2008


Raymond’s token-based algorithm

•

Features of the algorithm
– The algorithm uses an abstract inverted tree to reduce the
number of messages. It has three invariants
*

Pholder, the process holding the token, is at root of the tree

*

Each process in the system belongs to the tree

*

Each process other than the Pholder has only one edge, which points
to its parent in the tree

Consequently, each process has a path that ends on Pholder
– Each process has a local request queue
*

When it receives a request, it puts the requestor’s id in the queue

*

When it makes a request, it puts its own id in the queue

Chapter 17: Distributed 

Control Algorithms

Dhamdhere: Operating Systems—
A Concept­Based Approach , 2 ed

Slide No: 13
Copyright © 2008


Raymond’s token-based algorithm

(a) A system
(b) Abstract inverted tree for the system: P5  holds the token

Chapter 17: Distributed 
Control Algorithms

Dhamdhere: Operating Systems—
A Concept­Based Approach , 2 ed

Slide No: 14
Copyright © 2008


Raymond’s token-based algorithm
1. Process wishes to make a request:
a. It enters the request in its local queue
b. Sends it on the out-edge if it has not sent a request earlier

2. When a process receives a request:

a. It enters the request in its local queue
b. Sends it on the out-edge if it has not sent a request earlier

3. When a process completes execution of a CS:
a. It removes first requester id from its queue,
b. sends the token to that process and inverts the edge to it

4. When a process receives the token:
a. It removes first requester id from its queue.
b. If it is its own id, it enters a CS; otherwise, it sends the token to the
process with that id and inverts the edge to it
Chapter 17: Distributed 
Control Algorithms

Dhamdhere: Operating Systems—
A Concept­Based Approach , 2 ed

Slide No: 15
Copyright © 2008


An example of Raymond's algorithm

(a) Process P5 is in CS. Requests made by P3 and P1 have reached it 
(b) Process P5 finishes execution of CS and passes token to P4

Chapter 17: Distributed 
Control Algorithms

Dhamdhere: Operating Systems—

A Concept­Based Approach , 2 ed

Slide No: 16
Copyright © 2008


Distributed deadlock handling

•

A distributed computation may wish to use resources
located in many nodes of the system
– Information about allocated resources and pending requests in
many nodes has to be collected
– Correctness problems may arise due to
* Delays in obtaining information
* Consistency of information

– Consider building a global wait-for graph (WFG) by collecting
information about wait-for relationships in all nodes
* Inconsistent information due to delays may cause phantom
deadlocks, i.e., declaration of deadlock when none exists
* Next slide contains an example
Chapter 17: Distributed 
Control Algorithms

Dhamdhere: Operating Systems—
A Concept­Based Approach , 2 ed

Slide No: 17

Copyright © 2008


Phantom deadlock

1. P5  requests and obtains r5  in node N3
2. P6  requests and obtains r4  in N3
3. P5  requests and obtains r6  in N4
4. P4  requests r5  in N3
5. P5   requests r4  in N3
Chapter 17: Distributed 
Control Algorithms

6. N3  sends local WFG
7. P6  releases r4  in N3
8. P6  requests r6  in N4
9. N4  sends local WFG

Dhamdhere: Operating Systems—
A Concept­Based Approach , 2 ed

Slide No: 18
Copyright © 2008


Diffusion computation-based deadlock detection

•

Diffusion computation: used to collect info about nodes

– Diffusion phase
* Computation that has originated in one node, spreads to other
nodes
 A control message called a query is sent along each edge
 The first query received by a node is called an engaging query.
On receiving it, the node sends queries along all its out-edges

– Information collection phase
* Each node sends information in response to each query
 It sends a dummy reply that contains null information for a nonengaging query
 It collects information from all replies it received, adds its own
information, and sends the result as the reply to the engaging
query. We call it an engaging reply
Chapter 17: Distributed 
Control Algorithms

Dhamdhere: Operating Systems—
A Concept­Based Approach , 2 ed

Slide No: 19
Copyright © 2008


Diffusion computation-based deadlock detection
1. When a process becomes blocked on a resource request: It
initiates a diffusion computation as follows:
a. Send queries along all out-edges in WFG
b. Remember the number of queries sent; await replies
c. After receiving a matching number of replies, declare a deadlock if
it has been continuously in blocked state after sending queries

2. When a process receives an engaging query: If it is blocked, it
performs the following actions:
a. Send queries along all out-edges in WFG
b. Remember the number of queries sent; await replies
c. After receiving a matching number of replies, compute and send
an engaging reply, if continuously blocked after sending queries

3. When a process receives a non-engaging query:
a. Send dummy reply if continuously blocked after sending queries
Chapter 17: Distributed 
Control Algorithms

Dhamdhere: Operating Systems—
A Concept­Based Approach , 2 ed

Slide No: 20
Copyright © 2008


Illustration of diffusion computation-based
distributed deadlock detection

•  P2, P3  are blocked. P1 becomes blocked and sends a query but does
   not receive a reply because P4  is not blocked
•  P4  requests a resource held by P1, becomes blocked and sends a query.
   It would receive a reply and declare a deadlock
Chapter 17: Distributed 
Control Algorithms

Dhamdhere: Operating Systems—
A Concept­Based Approach , 2 ed

Slide No: 21
Copyright © 2008


Mitchell–Merritt algorithm
for distributed deadlock detection

•

It is an edge chasing algorithm—control messages are
sent over WFG edges to detect cycles
– A provision is made to ensure that the cycle has not been
broken before it was detected
* Each process is assigned a public label and a private label
 The labels are identical when a process is created
 The public label of a process changes when it gets blocked on
a resource request
 It also changes when it waits for a process having a larger
public label
* A wait-for edge with a specific relation between public and private
labels of its source and destination processes indicates presence of
a deadlock

Chapter 17: Distributed 
Control Algorithms

Dhamdhere: Operating Systems—

A Concept­Based Approach , 2 ed

Slide No: 22
Copyright © 2008


Rules of Mitchell–Merritt algorithm

•  Block rule changes the labels of a process when it blocks; z = inc(u, x),
   where inc generates a unique label larger than both u and x
•  The transmit rule changes public label of a process waiting for a process
   with a larger public label
•  Out­edge of the process with largest label in a cycle satisfies detect condition
Chapter 17: Distributed 
Control Algorithms

Dhamdhere: Operating Systems—
A Concept­Based Approach , 2 ed

Slide No: 23
Copyright © 2008


Distributed deadlock prevention

•

Cycles are prevented as follows:
– A pair (local time, node id) is used to time-stamp creation of a
process

– When process Pi requests a resource allocated to Pj, timestamps of Pi, Pj are used to decide whether Pi can wait for Pj

•

Two approaches
– Wait-or-die
* Pi is allowed to wait if it is older than Pj; otherwise, it is killed

– Would-or-wait
* Pi is allowed to wait if it is younger than Pj; otherwise Pj is killed

Chapter 17: Distributed 
Control Algorithms

Dhamdhere: Operating Systems—
A Concept­Based Approach , 2 ed

Slide No: 24
Copyright © 2008


Distributed scheduling algorithms

•

Computational load in nodes is balanced through the
technique of process migration

•  Process Pi  is migrated from N1 to N2 during the interval t1–t2
•  It is preemptive migration

Chapter 17: Distributed 
Control Algorithms

Dhamdhere: Operating Systems—
A Concept­Based Approach , 2 ed

Slide No: 25
Copyright © 2008