RUN corresponds to the SA-1110 RUN mode when the processor is busy
executing instructions. Its power is measured when the PDA
computes discrete cosine transforms, as detailed
in [28]. IDLE206 and IDLE59 correspond to the SA-1110
IDLE mode with a core clock frequency/voltage of 206z/1.5V and
59
z/1.25V, respectively. IDLE59 is a hypothetical mode for the
Zaurus SL-5500, but is real for many other SA-1100 or SA-1110
based systems [9,22,26]. It is estimated as the display power plus the
measured non-display power at 206
scaled down using the same
ratio of the power for IDLE59 to that for IDLE206 in the Itsy
system power measurement presented in [26].
SLEEP-Display corresponds to the SA-1110 SLEEP mode when the
display is left on. Therefore, its system power consumption is
measured when the system is suspended, and the display power is
added thereafter.
The energy consumption for system usage is obtained by running its trace through the system power model with DPM/DVS. There are different ways to do DPM/DVS with such a system power model in view of user delays, as discussed next.
The Linux kernel automatically puts the system into the IDLE206 mode whenever there is no process running and returns it to the RUN mode upon interrupts. We take this as the baseline and report the performance of other techniques against it.
The most straightforward DPM/DVS technique would be to put the
system into the IDLE59 or the SLEEP-Display mode right after the
system finishes responding to the user and put it back into the
RUN mode upon a user input. These methods are called the
and
techniques, respectively. Since the IDLE59 to RUN
mode-transition delay is small, there is no concern with regard to
user productivity. However, that for SLEEP-Display to RUN will
most likely be noticed by users and decrease user productivity.
Assuming we can predict user delays absolutely accurately, we can
choose to put the system into the SLEEP-Display mode and wake it
up right before the next user input, if the delay is long enough.
This is called the technique since it gives the upper
bound on energy savings based on the system power model.
The proposed user delay models can be used to predict user delays.
There are two concerns with respect to prediction errors. First,
in the case of overestimation, the system will wake up from the
SLEEP-Display mode upon an interrupt generated by the user input
and enter the RUN mode directly. Such errors are called
lazy errors. If the delay is overestimated by more than
the human perceptual threshold, the user will experience a
noticeable mode transition delay. Such lazy errors are called
serious lazy errors. Second, in the case of
underestimation, the system wakes up and transfers to the IDLE59
mode after the predicted delay to get ready for the user input and
will not be able to fully exploit the idle time to reduce energy
by remaining in the SLEEP-Display mode. Therefore, we report the
average error for underestimations. We refer to the DPM/DVS
technique based on user delay predictions by the history-based and
psychological models as and
,
respectively.