Low Power

Two new concepts were developed to reduce the power consumed by the Linux watch.

1) work dependent timing scheme to eliminate a periodic timer tick in an OS when it is not necessary

2) controlling the power consumed by the display by varying the number of lighted pixels.

Work Dependent Timing Scheme

The Linux Operating System (OS) operates around the notion of a periodical timer interrupt, known as the tick. The frequency of this interrupt is 100Hz in most Linux systems. In the case where there's no work to be done, i.e., the system is idling, the OS typically puts the processor into a low power state in between the timer ticks. From the standpoint of power consumption, this is not an efficient timing method since it forces the processor and OS to service the timer interrupt every 10ms, even when there is no other work to carry out. In addition, there may be a finite amount of energy associated with transitioning the processor into and out of the low power state. This energy is also wasted whenever there's no work to be done. Finally, the 10ms idling period between timer ticks may limit the OS to exploit more efficient processor low power states, since the more efficient low power states may take longer than 10ms to transition into and out of.

Many of today's mobile and portable devices are what we denote as idle systems. Per our definition, an idle system is a system in which there are no active tasks running the majority of time the device is turned on. Examples of predominantly idle systems could include PDAs, sensors, etc. On many occasions, we turn PDAs on, issue a couple of requests and then we digest the information displayed on the screen and/or we let the device "just sit there". Nevertheless, the timer interrupt in the system, persistently interrupts the processor and forces the OS to needlessly service the interrupt and thus consume energy.

We address the above problem by implementing a more suitable timing scheme in the Linux kernel which better exploits the processor's most low power mode. The scheme is called work dependent timing (WDT). The basic approach is to skip timer ticks whenever there is no work to be done. So whenever code operation returns to the Linux "main idle loop", we parse the various queues and lists that may contain tasks that need to be executed. If they are all empty, then we parse the timer list which contains timers with associated timeout values. We find the timer with the nearest timeout value and then program the hardware timer accordingly to wake us up when it's time for the timer function to be executed. The following figure illustrates the low power WDT scheme (using specific power numbers from the Linux Watch implementation).
 

As may be seen from the figure, the timer ticks which do not result in any work to be carried out are skipped, thus giving the processor and OS more time to remain in the IDLE or STANDBY low power modes. The low power WDT scheme improves the battery life for the WatchPad 1.5 from around 8hrs (when running standard Linux kernel) to anywhere between 35-64hrs depending on usage pattern. The reason for the significant improvement in battery life is the fact that sufficiently long timer interrupt intervals makes it possible to utilize the STANDBY (lowest power) mode in the Linux Watch processor.

More details can be found in

A Work Dependent OS Timing Scheme for Power Management: Implementation in Linux and Modeling of Energy Savings, Claus Michael Olsen, Chandrasekhar Narayanaswami. IBM Research Report RC22784, April 2003. To appear in IEEE Transactions on Mobile Computing.

OLED Displays and Power Savings

The OLED display used in one version of the Linux Watch includes an SRAM cell per pixel and power is consumed only by pixels that are turned on. There is no backlight in the device. This feature allows us to control the power consumed by the display by changing the graphical representation of data shown on the screen. One example, shown in the figure below is the display of time. For comparable readability, digital representation of time needs more pixels to be turned on than an analog representation. Within each scheme, the figure shows how the number of lighted pixels can be changed. For example, the number of lighted pixels for an analog representation ranges from 1.07-1.33% of the total number of pixels on the screen.

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More details of this technique can be found in

Energy Trade-offs in the IBM Wristwatch Computer. N. Kamijoh, T. Inoue, C. M. Olsen, M. Raghunath, C. Narayanaswami. Proceedings of the Fifth IEEE International Symposium on Wearable Computers (ISWC'01), pp 133-140.