Synchronization Protocols in EndtoEnd Scheduling

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Synchronization Protocols in EndtoEnd Scheduling
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Scope

• Job-shop model
• Each task needs to execute on a set of processors
  in a certain order
• Each task may require a different order
• Problems in End-to-End scheduling
• Priority assignment
• Assign fixed priorities to tasks so that the
  system is schedulable
• Synchronization of tasks
• Control the releases of subtask instances
  (non-first subtasks)
• Schedulability analysis
• For a given priority assignment and a given
  synchronization protocol, whether every instance
  of each task meets its deadline

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The Synchronization Problem

• Given that
• Priorities are assigned to subtasks in a task
  chain using some fixed priority assignment
  algorithm
• How do we coordinate the release of subtasks in a
  task chain so that
• Precedence constraints among subtasks are
  satisfied
• subtask deadlines are met
• end-to-end deadlines are met

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Synchronization Protocols

• Direct Synchronization (DS) Protocol
• Simple and straightforward
• Phase Modification (PM) Protocol
• Proposed by Bettati
• Used by flow-shop tasks
• Extension called Modified Phase Modification
  (MPM) Protocol
• Release Guard Protocol
• Proposed by Sun

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Synchronization Protocol - Example
P1
P2
(4,2)
T1
(6,2)
T2,2
(6,2)
T2,1
(6,3)
T3
Ti,j jth subtask of task Ti
Task T3 has a phase of 4 time units
(period,execution time)
Period relative deadline of parent task
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Direct Synchronization Protocol

• Greedy strategy
• On completion of subtask
• A synchronization signal sent to the next
  processor
• Successor subtask competes with other
  tasks/subtasks on the next processor

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Direct Synchronization Illustrated
T1
T2,1
On P1
On P2
T2,2
T3 misses deadline
Phase of T3
T3
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Phase Modification Protocol

• Proposed by Bettati
• Release subtasks periodically
• According to the periods of their parent tasks
• Each subtask given its own phase
• Phase determined by subtask precedence constraints

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Phase Modification Protocol Illustrated (1/2)
T1,1
T1,2
T1,3
T1,1
p1
T1,2
p1
T1,3
p1
Phase of T1,2
Phase of T1,3
Actual response time
Estimated worst case response time
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Phase Modification Protocol Illustrated (2/2)
T1
T2,1
On P1
On P2
Phase of T2,2
T2,2
Phase of T3
T3
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Phase Modification Protocol - Analysis

• Periodic Timer interrupt to release subtasks
• Centralized clock or strict clock synchronization
• Task overruns could cause Precedence constraint
  violations

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Modified PM Protocol Illustrated (1/2)
T1,1
T1,2
T1,3
T1,1
p1
Overrun ?
T1,2
p1 ?
Actual response time
Estimated worst case response time
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Modified PM Protocol Illustrated (2/2)
T1
Synch signal delayed
T2,1
On P1
On P2
T2,2
Phase of T3
T3
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Modified PM Protocol - Analysis

• MPM protocol behavior the same as PM under ideal
  conditions
• Ideal conditions Clocks synchronized, no
  overrun
• MPM protocol does not need clock synchronization
• Precedence constraints preserved even in the case
  of overruns
• Upper bound on End-to-End Response time of task Ti

Ri,k is the response time of the kth subtask of
Ti ni is the number of subtasks for the task Ti

• Lower bound on End-to-End Response time of task
  Ti

Actual Response time of nith subtask

• Lower bound high, hence high average EER time

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Release Guard Protocol

• Proposed by Sun
• A guard variable release guard - associated
  with each subtask
• Release guard used to control release of each
  subtask
• Contains next release time of subtask
• Synchronization signals just like MPM
• Release guard updated
• On getting synchronization signal
• During idle time

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Release Guard Protocol Illustrated
T1
T2,1
On P1
On P2
g1,2 4610
g1,2 9
T2,2
Idle time detected
Phase of T3
T3
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Release Guard Protocol - Analysis

• Shares the same advantages as MPM
• Upper bound on EER still the same as MPM
• Since upper bound on release time enforced by
  release guard

Ri,k is the response time of the kth subtask of
Ti ni is the number of subtasks for the task Ti

• Lower bound on EER less than that of MPM
• If there are idle times
• Results in lower average EER

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Comparison of Protocols
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Classification of Protocols
Synchronization Protocols
Without execution control
With execution control
DS
PM
MPM
RG
Task release controlled by predecessor
processor by delaying the synch signal
Task release controlled by same processor by
using guard variables
Phase modification
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References

• Synchronization Protocols in Distributed
  Real-Time Systems,
• Jun Sun and Jane Liu, ICDCS 96
• Fixed Priority End-to-End Scheduling in
  Distributed Real-Time Systems,
• Jun Sun, PhD Thesis