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savings domains. Mobile multimedia devices (e.g. Smartphones) fall in this cate-
gory. The effective trade-off between very high performance, for instance to ensure
audio and video capabilities, and ultra low-power operation is a challenging task.
Last, applications for which energy savings are of primarily relevance but in which
performance can be lower than in any other case exist: wireless nodes in a Wireless
Sensor Network belong to this set.
The remainder of the chapter is organized as follows: Section
6.2
gives an insight
of the problem of managing resources at run-time, while discussion on the support
from the OS is given in Sect.
6.3
. A more detailed review of the existing Power
Management (PM) frameworks based on the Linux kernel is presented in Sect.
6.4
.
Conclusions are given in Sect.
6.6
.
6.2
Run-Time Resource Management (RTRM)
So far we have seen how modern MPSoC architectures provide an exceptional flex-
ibility, with a huge set of physical and logical resources. The presence of multiple
applications that can be potentially integrated within the same chip makes the prob-
lem of managing such resources a challenging problem. By “Run-Time Resource
Management” (briefly, RTRM) we hereby mean the set of processes, techniques,
methodologies and instruments that allow to
use
the available resources, provided
an objective function. Generally speaking, resource utilization is subject to specific
objectives and constraints, depending on both the effective hardware implementa-
tion and the considered application. One of the objectives of the Resource Manager
is thus to ensure that all such constraints and requirements are met, while satisfy-
ing the Quality-of-Service (QoS). A RTRM will perform this action dynamically,
because there will be no chance to know which will be the applications (and, as a
consequence, the requested resources) a priori.
Purpose of this section is to introduce in a more formal way the concepts related
to the problem of managing the resources, the run-time component and the role of
the Run-Time Manager.
6.2.1
Problem Overview
The concept of
resource
is at the same time enough general to hide some details
on its real nature (e.g., physical or logical implementation) and enough detailed
to allow us to employ it in a constrained optimization problem. Thus, a generic
resource can either refer to a processor, a memory subsystem in a memory hierarchy
or it can also refer to the computation time fraction assigned to a task. Either ways,
each resource has a precise meaning within its own context. The set of available
resources in a system can be defined as the set
R
of all those
N
resources we have access to. Each resource
r
j
in
R
can be of any kind, and we
= {
r
1
,
r
2
,
...
,
r
N
}
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