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Architecture of Computing Systems (Lecture Notes in Computer ...

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Hierarchical Distributed Control <strong>of</strong> Power and Performances 47<br />

resources. This could be exploited to configure the hardware devices on the best<br />

feasible operat<strong>in</strong>g mode that supports the resources demand. A positive effect <strong>of</strong><br />

this method is the energy sav<strong>in</strong>g that could be achieved by select<strong>in</strong>g, for each QoS<br />

demand, the optimal work<strong>in</strong>g mode not only with respect to a multi-objective<br />

performances optimization policy but also consider<strong>in</strong>g the system-wide power<br />

consumption, which can be associated to every FSC.<br />

Dependency track<strong>in</strong>g. CPM allows to track hardware dependencies among different<br />

subsystems <strong>of</strong> a SoC that may prevent a correct operation <strong>of</strong> a system.<br />

Instead <strong>of</strong> patch<strong>in</strong>g each device driver to adapt to the platform, developers declare<br />

platform DWRs to solve dependencies issues. In that way code portability<br />

is improved.<br />

Identification <strong>of</strong> Feasible System-wide Configurations. The automatic computation<br />

<strong>of</strong> the FSCs allows to identify all the feasible work<strong>in</strong>g po<strong>in</strong>ts <strong>of</strong> an entire<br />

platform. This is done by exploit<strong>in</strong>g the <strong>in</strong>formation def<strong>in</strong>ed, <strong>in</strong>dependently, <strong>in</strong><br />

each device driver code. Other approaches to PM require to code all the work<strong>in</strong>g<br />

po<strong>in</strong>ts by hand. Consider<strong>in</strong>g that <strong>in</strong> the presented use case the total number<br />

<strong>of</strong> FSCs was 415, we understand how <strong>in</strong>terest<strong>in</strong>g is the ability to automatically<br />

compute these po<strong>in</strong>t. Thus, this is a relevant result by itself. Moreover, it improves<br />

the portability <strong>of</strong> the solution across different platforms because allows<br />

to reuse drivers def<strong>in</strong>ed <strong>in</strong>formations.<br />

Additive aggregation. This is a new concept, <strong>in</strong>troduced <strong>in</strong> CPM, to solve a limitation<br />

present <strong>in</strong> the implementation <strong>of</strong> QoSPM where even for resources that<br />

are <strong>in</strong>tr<strong>in</strong>sically additive (e.g., bandwidth) the aggregation function is <strong>of</strong> type<br />

m<strong>in</strong>/max: not allow<strong>in</strong>g to keep a correct view <strong>of</strong> system resources and br<strong>in</strong>g<strong>in</strong>g<br />

to select devices’ WMs that actually can’t support the QoS level required,<br />

e.g., if two applications require 300 kb/s each, QoSPM aggregates with the max<br />

thus with a f<strong>in</strong>al value <strong>of</strong> 300 Kb/s, i.e. an <strong>in</strong>correct view <strong>of</strong> the resource’s<br />

requirement.<br />

5 Conclusions<br />

We have presented CPM, L<strong>in</strong>ux kernel framework for system-wide power and resources’<br />

optimization. The proposed method efficiently implements a well-known<br />

formal technique to solve optimization problems. The cross-layer design <strong>of</strong> the<br />

framework allows to collect and aggregate QoS requirements from the application<br />

layer and to coord<strong>in</strong>ate the reconfiguration <strong>of</strong> device drivers work<strong>in</strong>g mode.<br />

A system-wide optimization, <strong>of</strong> both perceived performances and energy consumption,<br />

is supported by the def<strong>in</strong>ition <strong>of</strong> a global dynamic and multi-objective<br />

policy.<br />

As revealed both theoretically and experimentally, the CPM approach allows<br />

to capture energy sav<strong>in</strong>gs while fulfill<strong>in</strong>g QoS constra<strong>in</strong>ts, thanks to a systemwide<br />

cross-layer dynamic optimization. Work is <strong>in</strong> progress, with<strong>in</strong> a FP7 EUfunded<br />

project, to extend the approach towards multi-core architectures.

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