Managing Static (Leakage) Power - Computer Architecture Techniques for Power-Efficiency - page 150

FIGURE 5.9: Adaptive Mode Control (adapted from [ 250 ]). The decay interval changes by changing
the global control register (GCR) which holds the threshold for the local counters (LICs). The global
counter (LIC Update Interval) does not change. Adaptation is controlled performance feedback loop
(adaptive mechanism). Reproduced from [ 250 ]. Copyright 2001 IEEE.
Control Logic (MCL)—shown next to the cache lines in Figure 5.9. The threshold, which is
the same for all cache lines, is kept in a global control register, the GCR. 12
Adaptation of the decay interval is simply a matter of setting the GCR to the appropriate
threshold for the LICs. For this, a feedback loop monitoring decay induced misses is employed.
The feedback loop works by adjusting the decay interval to keep the decay-induced miss rate
around a desired target which is given as a percentage of the application's true miss rate.
Decay-induced misses are distinguished from normal misses by keeping the tags powered
on at all times and detecting accesses to decayed lines (i.e., accesses that would be hits if the line
was still powered on). The decay-induced miss rate is measured within a time window, and at
the end of the window the decay interval is adjusted. If the decay-induced miss rate is less than
the target, the decay interval is decreased; otherwise, it is increased. Hitting the exact target
is difficult to do. Most often, the feedback loop simply oscillates around the target, constantly
adjusting the decay interval. To avoid this needless exercise, the decay interval is not allowed
to change if the miss rate falls within a band enveloping the target miss rate.
Figure 5.10 shows the behavior of the feedback loop. Two bold lines in the graph
represent, respectively, the “ ideal ”or true miss rate, which is the miss rate without decay, and
the “ target error ” which is the additional miss rate of the decay-induced misses. The target error
is defined as the fraction of the ideal miss rate via the performance factor (PF) which is externally
12 Note that although the hardware can accommodate individual decay intervals per cache line, adaptation is based
on a single global decay interval.