Copyright by William Lloyd Bircher 2010 - The Laboratory for ...

critical observation since the increase in power for going from 800 MHz to 1.1 GHz is
much smaller than the increase in performance. The major cause is that static power
represents a large portion of total power consumption. Since voltage dependence exists
between all cores in a package, power is only saved through the frequency reduction.
There is no possibility to reduce static power since voltage is not decreased on the idle
cores.
Performance
120%
100%
80%
60%
40%
20%
Frequency
Sensitive
Region
LocalMem
RemoteMem
Frequency Insensitive Range
0%
800 1000 1200 1400 1600 1800 2000
Idle Core Frequency
Figure 6.2 Remote and Local Probe Sensitivity
Using the same synthetic kernel the effect of p-states is isolated from c-states. Since the
p-state experiments show that indirect performance loss is significant below the
breakover point, now c-state settings that do not impose the performance loss are
considered. To eliminate the effect of this performance loss the processor can be
configured for ramped probe response. In this mode, idle cores increase their frequency
before responding to probe requests. To obtain an optimal tradeoff between performance
and power settings, this setting mode can be modulated using hysteresis, implemented by
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