The document discusses the complexities and approaches of multiprocessor real-time scheduling, noting that research is less developed compared to uniprocessor systems. It outlines two main scheduling categories: global scheduling, which allows any task to run on any processor, and partitioned scheduling, which assigns specific tasks to designated processors. The document concludes that multiprocessor scheduling remains complex and necessitates ongoing research as these systems become more prevalent.

1. Multiprocessor Real-
Multiprocessor Real-
Time Scheduling
Time Scheduling
Aaron Harris
Aaron Harris
CSE 666
CSE 666
Prof. Ganesan
Prof. Ganesan

2. Multiprocessor Scheduling
Multiprocessor Scheduling
 As more systems incorporate multiple
As more systems incorporate multiple
processors, interest in scheduling algorithms
processors, interest in scheduling algorithms
increases.
increases.
 Still relatively little research done (compared to
Still relatively little research done (compared to
uniprocessor scheduling).
uniprocessor scheduling).
 Due to increased complexity, much research
Due to increased complexity, much research
makes simplifications of their systems.
makes simplifications of their systems.

3. When and Where
When and Where
 Multiprocessor scheduling requires determining
Multiprocessor scheduling requires determining
both which task to run at a given time and on
both which task to run at a given time and on
which processor to run it on.
which processor to run it on.
 Added complexity as compared to scheduling in
Added complexity as compared to scheduling in
a uniprocessor environment.
a uniprocessor environment.
 Scheduling must account for not only the status
Scheduling must account for not only the status
of tasks, but the status of processors.
of tasks, but the status of processors.

4. Schedule Management
Schedule Management
 In a standard system, the schedule is controlled by the
In a standard system, the schedule is controlled by the
processor.
processor.
 Multiprocessor systems follow two general approaches:
Multiprocessor systems follow two general approaches:
master-slave and dedicated scheduler.
master-slave and dedicated scheduler.
 In master-slave, one processor handles all schedule
In master-slave, one processor handles all schedule
processing and also processes normal tasks.
processing and also processes normal tasks.
 Dedicated scheduler systems allocate an entire
Dedicated scheduler systems allocate an entire
processor to nothing but schedule processing. This can
processor to nothing but schedule processing. This can
be more efficient in complex systems with many
be more efficient in complex systems with many
processors.
processors.

5. Algorithm Selection
Algorithm Selection
 In a single processor system, rate-monotonic
In a single processor system, rate-monotonic
(for static priorities) and earliest-deadline-first
(for static priorities) and earliest-deadline-first
(for dynamic priorities) algorithms are the
(for dynamic priorities) algorithms are the
accepted best overall algorithms.
accepted best overall algorithms.
 There is no agreed-upon best algorithm for
There is no agreed-upon best algorithm for
multiprocessor systems (due to increased
multiprocessor systems (due to increased
complexity of algorithms and lack of research).
complexity of algorithms and lack of research).

6. Algorithm Categories
Algorithm Categories
 Most multiprocessor scheduling algorithms fall
Most multiprocessor scheduling algorithms fall
into one of two categories: global scheduling and
into one of two categories: global scheduling and
partitioned scheduling.
partitioned scheduling.

7. Partitioned Scheduling
Partitioned Scheduling
 All tasks are divided among the processors at compile
All tasks are divided among the processors at compile
time.
time.
 Each task can only execute on its assigned processor.
Each task can only execute on its assigned processor.
 Advantages: This turns each processor into a single
Advantages: This turns each processor into a single
processor system as far as scheduling is concerned. All
processor system as far as scheduling is concerned. All
research and theory of single processor scheduling then
research and theory of single processor scheduling then
applies to each processor.
applies to each processor.
 Disadvantages: The process of assigning tasks to
Disadvantages: The process of assigning tasks to
processors can be complicated and extremely inefficient
processors can be complicated and extremely inefficient
if done poorly. This can lead to poor system
if done poorly. This can lead to poor system
performance.
performance.

8. Global Scheduling
Global Scheduling
 All tasks are allowed to execute on all processors.
All tasks are allowed to execute on all processors.
 Tasks are scheduled from a global queue to individual
Tasks are scheduled from a global queue to individual
processors.
processors.
 Advantages: This technique allows for better processor
Advantages: This technique allows for better processor
utilization and overall system performance.
utilization and overall system performance.
 Disadvantages: The main disadvantage is “migration”.
Disadvantages: The main disadvantage is “migration”.
This is when a task is preempted on one processor and
This is when a task is preempted on one processor and
resumes execution on a different processor. This
resumes execution on a different processor. This
requires the context of the task to be moved from one
requires the context of the task to be moved from one
processor to another, which can be costly.
processor to another, which can be costly.

9. Problems: Dhall’s Effect
Problems: Dhall’s Effect
 Something interesting is the relation of ultization
Something interesting is the relation of ultization
to scheduliblity.
to scheduliblity.
 One may think that to be schedulable, the
One may think that to be schedulable, the
utilization of a set of tasks on
utilization of a set of tasks on n
n processors
processors
should simply be ≤
should simply be ≤ n
n.
.
 Dhall proved that the number must actually be
Dhall proved that the number must actually be
between
between n
n*ln(2) and
*ln(2) and n
n (Dhall’s Effect).
(Dhall’s Effect).

10. Problems: Schedule Complexity
Problems: Schedule Complexity
 Many types of scheduling (e.g. dynamic
Many types of scheduling (e.g. dynamic
priorities) that require extensive run-time
priorities) that require extensive run-time
calculations can become too complicated in a
calculations can become too complicated in a
multiprocessor environment.
multiprocessor environment.
 Due to this, most of the scheduling algorithms
Due to this, most of the scheduling algorithms
that have been researched use compile-time
that have been researched use compile-time
determined priorities.
determined priorities.

11. Problems: Resource Synchronization
Problems: Resource Synchronization
 Resource synchronization becomes a much
Resource synchronization becomes a much
larger issue in multiprocessor systems.
larger issue in multiprocessor systems.
 Even in a non-preemptive multiprocessor
Even in a non-preemptive multiprocessor
system, resource sharing can lead to task
system, resource sharing can lead to task
blocking.
blocking.
 More thought must be put into how and what
More thought must be put into how and what
resources are shared between tasks in a
resources are shared between tasks in a
multiprocessor system.
multiprocessor system.

12. Conclusions
Conclusions
 Multiprocessor scheduling algorithms are more
Multiprocessor scheduling algorithms are more
complex than uniprocessor systems.
complex than uniprocessor systems.
 There is still no decisively best multiprocessor
There is still no decisively best multiprocessor
scheduling algorithm.
scheduling algorithm.
 Multiprocessor systems will continue to become
Multiprocessor systems will continue to become
more and more prevalent, and as a result,
more and more prevalent, and as a result,
multiprocessor scheduling research and theory
multiprocessor scheduling research and theory
will have to continue to improve.
will have to continue to improve.

13. References
References
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Khemka A., Shyamasundar R. K., "An Optimal Multiprocessor Real-Time
Scheduling Algorithm", Journal of Parallel and Distributed Computing 43, 37-
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 G. Manimaran, C. Siva Ram Murthy, "An Efficient Dynamic Scheduling
G. Manimaran, C. Siva Ram Murthy, "An Efficient Dynamic Scheduling
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 J. Carpenter et al., “A Categorization of Real-Time Multiprocessor Scheduling
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Problems and Algorithms,”
Problems and Algorithms,”
http://www.cs.uga.edu/~shelby/pubs/carpenterFHSAB2003.pdf.
http://www.cs.uga.edu/~shelby/pubs/carpenterFHSAB2003.pdf.
 S. K. Dhall, C. L. Liu, “On a Real-Time Scheduling Problem,” Operations
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14. Questions?
Questions?