Termination Detection

Goal

• Study the development of a protocol for termination detection with the help of invariants.

Termination Detection

• Rules:– A process is either active or passive– An active process can become passive at any

time– A passive process can become active only if it

receives an computation message– Only active processes can send computation

messages. All processes can receive them

• A system is said to be terminated if– All processes are passive

– No application messages are in transit

• We distinguish between computation messages and messages sent for detecting termination. Any process can send and receive them. These messages do not change the status of a process

Application

• A solution for termination detection allows one to ensure that all tasks in a system are indeed complete, even though the tasks may create additional tasks that are run at other processors in the system

Observation

• Termination detection is a stable property– Once true, it remains true forever

• Detecting such properties is important for many problems including– Garbage collection– Deadlock detection

We will consider two algorithms

• Based on the idea of diffusion

• Based on the idea of global snapshot

– We will study these aspects later.

Approach 1: Dijkstra Scholten

• Assumptions– Initially one process is active– No failures, lost messages etc.

• Each process j maintains a variable P.j that is its parent in the tree– At root, P.root = root– Initially for all other processes,

• P.j = NULL

Predicate in Invariant (1)

• The set of active processes form a tree– True in the initial state– Ensure that this remains true during

computation

When a Process becomes active

• Consider the case when j changes from Passive to Active– It must be the case that j received a

computation message from some process, say k• P.j = k

• Become active

Action (1)

P.j = NULL j receives a message from k

P.j = k, j becomes active

When a Process Becomes Passive

• Consider the case when j changes from Active to Passive – It must be the case that j has no children

Action (2)

P.j = NULL j receives a message from k

P.j = k, j becomes active

j is active j wants to become passive j has no children

j becomes passive, P.j = NULL

Problem?

• Does not deal with messages.

Predicate in Invariant (2)

• The set of active processes form a tree

• If j is passive then all messages it sent have been received– True initially– Preserve this predicate during computation

Action (3)

• Maintain a variable oc.j that denotes the number of messages that j has sent and are not yet acknowledged

Action (2) corrected

P.j = NULL j receives a message from k

P.j = k, j becomes active

j is active j wants to become passive j has no children oc.j = 0

j becomes passive, P.j = NULL

• The actions on previous slide can be used to implement termination detection.

• Consider second actionj is active j wants to become passive j has no children

oc.j = 0j becomes passive, P.j = NULL

• Is it possible to drop ` j has no children’ from the guard?

Answer

• We could if we guarantee that – oc.j = 0 j has no children– Same as

• j has children oc.j > 0

• Could be achieved if the child does not respond to at least one of parent’s message (first one?)

• Thus, checking oc.j is 0 sufficient

Action (3)

P.j = NULL j receives a message from kP.j = k, j becomes active (Don’t send ack to this message)

j is active j wants to become passive j has no children oc.j = 0j becomes passive, P.j = NULL; send ack to parent

j is active j receives a message from kSend ack to k

Other simple actions for maintaining oc.j

Summarizing Approach 1

• Goal– Active processes form a rooted tree

• If process k activates j then j sets its parent to k

– If a process is passive, all messages it sent have been received

• Acknowledge each message (at some time)

– A process becomes passive only when all its children are passive; in other words, force a process to wait for its children to become passive.

• This is achieved if the children do not send an acknowledgment for the first message received from the parent until they become passive.

Actions

• Passive Active– If j is passive and receives a computation

message from k then• P.j = k

• Become active

Actions

• Active Active– If j is active and receives a computation

message from l• Send an acknowledgment

Actions

• Message send– If j wants to send message (it must be active)

• oc.j ++

• (Number of outstanding acknowledgments is increased)

• Acknowledgement receive– oc.j = oc.j – 1

Actions

• Active passive– If j wants to be passive and oc.j = 0

• Send an acknowledgment to P.j (Observe that the first message from parent was not immediately acknowledged)

• Become passive

• If j is the root then declare termination

Diffusing Computation

• Crucial for various applications• General outline

– root(?) sends the diffusion message– Every node that receives the diffusion message for the first time

forwards it to its neighbors• First node from which diffusion is received is called parent

– All subsequent diffusion messages are acknowledged– Upon receiving acknowledgements form all neighbors, a node

completes the diffusion and sends acknowledgment to parent– When root completes the diffusion, the diffusion computation is

complete

• We will see application of diffusion computation later in the class.