Chapter 6 Dynamic Priority Servers

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Chapter 6 Dynamic Priority Servers
We looked at fixed priority servers. The
scheduling algorithms were based upon Rate
Monotonic periodic scheduling, which limits
guaranteed utilization to approximately 70. We
will now look at dynamic priority servers. The
scheduling is based upon Earliest Deadline First.
The most significant implication of this is that
scheduling has higher bounds, approaching 100
utilization.
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Scheduling Strategies for Fixed Priority
Servers(Based upon Rate Monotonic (RM)
scheduling for periodic tasks)

• Background Service
• First Come First Served (FCFS) service as
  availability permits
• Polling Server (PS)
• Pseudo periodic task(s) provides slot for
  serving aperiodic tasks
• Deferrable Server (DS)
• Unused capacity is saved for future aperiodic
  arrivals
• Priority Exchange Server (PE)
• Loans unusable capacity to ready periodic
  tasks
• Sporadic Server
• Loans unusable capacity to ready periodic tasks
  and delays recoup
• Slack Stealing
• Passive task steals unnecessary slack time in
  scheduling

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Properties of Dynamic Priority Servers(Based
upon Earliest Deadline First (EDF) scheduling for
periodic tasks)
All tasks are fully preemptable
Using Dynamic Priority Servers, the tasks are
schedulable if and only if
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Dynamic Priority Servers
Dynamic Priority Exchange Server (DPE)
Loans priority to periodic tasks, and recoups it
at a lower priority later Dynamic Sporadic Server
(DSS) Used server capacity is replenished
some time after it has been consumed Total
Bandwidth Server (TBS) Aperiodic tasks are
assigned all available bandwidth
immediately Earliest Deadline Late Server (EDL)
Like Earliest Deadline Last, periodic tasks
are scheduled as late of possible Improved
Dynamic Priority Exchange Server (IPE)
Identified EDL slack is used schedule using
Dynamic Priority Exchange
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Dynamic Priority Exchange Server
Like Priority Exchange Server, whenever there are
no aperiodic tasks to be served, Dynamic Priority
Exchange Server exchanges priorities with
periodic tasks and wastes as little time as
possible. It also facilitates using the spare
time when periodic tasks dont use their worst
case times.
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Dynamic Priority Exchange Server
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Dynamic Priority Exchange Server Example
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Dynamic Priority Exchange Reclaiming Spare Time
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Dynamic Sporatic Server
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Dynamic Sporadic Server
Dynamic Sporadic Server always schedules with a
relatively long far deadline. Therefore, the
response of aperiodic tasks may be longer than
necessary. A solution is to shorten the period of
the Dynamic Sporadic Server, at the expense of
more context switches.
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Total Bandwidth Server
Total Bandwidth Server schedules earlier
deadlines for aperiodic tasks, BUT does it such
that the overall utilization of the aperiodic
load never exceeds the maximum value of Us.
Each time an aperiodic request enters, the
total bandwidth of the server is immediately
assigned to it, WHENEVER POSSIBLE.
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Total bandwidth Server
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Earliest Deadline Late Server
Note that the Total Bandwidth Server is
conservative. There is slack time that could
have been utilized to give the aperiodic tasks
better response.
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Earliest Deadline Late Server
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Earliest Deadline Late Server Example
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Improved Dynamic Priority Exchange Server

• EDL is overhead intensive. In Improved Dynamic
  Priority Exchange, the Dynamic Exchange Server is
  modified to use the idle times of EDL, yielding
• a more efficient replenishment policy for the
  server
• the server is no longer periodic, and can be
  kept at the highest priority

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Homework 6 Lecture 5/22

• 1) Schedule the following tasks with a Dynamic
  Sporadic Server with Cs 2 and Ts 6.

2) Solve the same scheduling problem with a Total
Bandwidth Server having utilization Us
1/3. 3) Solve the same scheduling problem with
a Constant Bandwidth Server with Cs 2 and Ts
6.