- Origin
H.ย Simonis
- Constraint
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- Arguments
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- Restrictions
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- Purpose
Consider a set of tasks described by the
collection where each task has a given priority chosen in the
range .
Let denote the subset of tasks of
that all have a priority less than or equal to .
For each set , the
constraint forces that at each point in time, the cumulated
height of the set of tasks that overlap that point, does not exceed
a given limit. A task overlaps a point if and only if
(1)ย its origin is less than or equal to , and
(2)ย its end is strictly greater than .
Finally, it also imposes for each task of the constraint
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- Example
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Figureย 5.100.1 shows the cumulated
profile associated with both levels of priority.
To each task of the
constraint corresponds a set of rectangles containing the same number
(i.e.,ย the position of the task within the collection):
the sum of the lengths of the rectangles corresponds to the duration of the task,
while the height of the rectangles (i.e.,ย all the rectangles associated with a task
have the same height) corresponds to the resource consumption of the task.
Tasks that have a priority of 1 are coloured in pink,
while tasks that have a priority of 2 are coloured in blue.
The constraint holds since:
At each point in time the cumulated resource consumption profile of the tasks of priority 1
does not exceed the upper capacity 2 enforced by the first item of the collection.
At each point in time the cumulated resource consumption profile of the tasks of priority 1
and 2 does not exceed the upper capacity 3 enforced by the second item of the
collection.
- Typical
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- Symmetries
Items of are permutable.
can be increased to any value .
can be decreased to any value .
One and the same constant can be added to the and attributes of all items of .
can be increased.
- Arg. properties
Contractible wrt. .
- Usage
The constraint
was suggested by problems from the telecommunication
area where one has to ensure different levels of quality of service.
For this purpose the capacity of a transmission link is
split so that a given percentage is reserved to each level.
In addition we have that, if the capacities allocated to levels
is not completely used, then level can use the corresponding spare capacity.
- Remark
The constraint
can be modelled by a conjunction of constraints.
As shown by the next example, the consistency for all variables
of the constraints does not implies
consistency for the corresponding
constraint.
The following constraint
where the domains of , and are respectively equal to
, and corresponds to the following
conjunction of constraints
Even if the constraint could
achieve arc-consistency, the previous
conjunction of constraints would not detect the fact that
there is no solution.
- See also
common keyword:
ย (resource constraint).
used in graph description:
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- Keywords
characteristic of a constraint:
derived collection.
constraint type:
scheduling constraint,
resource constraint,
temporal constraint.
modelling:
zero-duration task.
- Derived Collection
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