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

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58 J. Zeppenfeld and A. Herkersdorf<br />

Objective Value<br />

Avg. Frequency (MHz)<br />

20%<br />

15%<br />

10%<br />

5%<br />

0%<br />

140<br />

120<br />

100<br />

80<br />

60<br />

40<br />

20<br />

0<br />

0 s<br />

0 s<br />

4 ms<br />

4 ms<br />

8 ms<br />

8 ms<br />

12 ms<br />

12 ms<br />

16 ms<br />

16 ms<br />

20 ms<br />

20 ms<br />

24 ms<br />

24 ms<br />

28 ms<br />

28 ms<br />

DVFS Pipe<br />

DVFS R2C<br />

Auto Global<br />

Lower is better<br />

DVFS Pipe<br />

DVFS R2C<br />

Auto Global<br />

Fig. 6. Objective value and average CPU frequency <strong>of</strong> dynamic and autonomic systems across<br />

various traffic scenarios. Lower values are better.<br />

3.2 Comparison <strong>of</strong> Autonomic and DVFS <strong>Systems</strong><br />

Figure 6 compares the globally optimized autonomic system presented <strong>in</strong> the previous<br />

section with two systems that employ dynamic voltage and frequency scal<strong>in</strong>g<br />

(DVFS). DVFS allows the non-autonomic system to adjust its operat<strong>in</strong>g frequency <strong>in</strong><br />

a fashion similar to the autonomic frequency actuator. The pipel<strong>in</strong>ed dynamic system<br />

corresponds to a DVFS-enhanced version <strong>of</strong> the hand-optimized static system from<br />

section 3.1. The dynamic run-to-completion system comb<strong>in</strong>es all tasks necessary for<br />

the process<strong>in</strong>g <strong>of</strong> a packet onto a s<strong>in</strong>gle CPU, allow<strong>in</strong>g each <strong>of</strong> the system’s three<br />

CPUs to completely process any packet type.<br />

Look<strong>in</strong>g at the objective function at the top <strong>of</strong> Figure 6, it can be seen that the autonomic<br />

system achieves results quite similar to those <strong>of</strong> the dynamic run-to-completion<br />

system. The difference <strong>in</strong> objective and frequency values for the first burst is due to<br />

the fact that only two CPUs are utilized by the run-to-completion system, s<strong>in</strong>ce the<br />

process<strong>in</strong>g time <strong>of</strong> the <strong>in</strong>com<strong>in</strong>g packets is less than twice the <strong>in</strong>terarrival rate. The<br />

first CPU has therefore completed process<strong>in</strong>g before the third packet arrives, which<br />

means that at least one CPU is idle at any po<strong>in</strong>t <strong>in</strong> time. This results <strong>in</strong> two CPUs<br />

perform<strong>in</strong>g all the work, and keeps their workload high enough that the DVFS controller<br />

does not further reduce their frequency. S<strong>in</strong>ce the m<strong>in</strong>imum frequency <strong>of</strong> a<br />

CPU <strong>in</strong> these simulations is 50 MHz, even for one that is idle, this results <strong>in</strong> a

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