Managing Static (Leakage) Power - Computer Architecture Techniques for Power-Efficiency - page 151

FIGURE 5.10: Feedback control for AMC. The performance factor PF defines a target error (decay-
induced miss rate) as a percentage of the “ideal” miss rate (miss rate without decay). The actual miss
rate is allowed to move inside a band around the target miss rate (ideal misses target error) without

changing the decay interval. But when the actual miss rate ventures outside this band the decay interval
is changed (increased if the actual rate overshoots the upper band limit or decreased in the opposite case)
to bring it back into the limits. Reproduced from [ 250 ]. Copyright 2001 IEEE.

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supplied. The decay interval does not change as long as the miss rate of the cache remains
within a specific range (band) around the target error. If the miss rate ventures outside the band
limits, the decay interval is changed to bring the miss rate back inside.
Zhou et al. report that AMC yields similar benefits to adaptive cache decay but with a
much more robust control over performance. Whereas adaptive per-cache-line decay settles in
a power-performance point that is controlled only indirectly, AMC sets a specific ceiling in
performance loss and adjusts decay under that constraint.
Control theoretic techniques : Velusamy, Sankaranarayanan, Parikh, Abdelzaher, and
Skadron formalized the Adaptive Mode Control technique using control theory [ 219 ]. The
contribution of their work is to develop a formal method for feedback-control for cache decay.
The method is also based on identifying and controlling decay-induced misses by monitoring
the tags.
Like ACM, the tags are kept powered-on at all times. Velusamy et al. show that both
their formal method and AMC adapt very well to changing application behavior as opposed
to statically selecting a fixed decay interval. The latter leads to unwelcome behavior during
different application phases. The formal controller, called Integral Miss Control (IMC), is
almost as good in practice as AMC but easier to design and tune. This gives a strong incentive
to use control-theoretic approaches when designing closed-loop responses.
Comparison : To summarize, Table 5.2 shows the main differences between the adaptive
decay methods. Although the per-cache-line adaptive decay is finer-grain and thus more