What is batching?

“Batching” refers to storing sensor events in a hardware FIFO before reporting them through the HAL instead of reporting them immediately.

Batching can enable significant power savings by preventing the SoC from waking up to receive each event. Instead, the events can be grouped and processed together.

The bigger the FIFOs, the more power can be saved. Implementing batching is an exercise of trading off hardware memory for reduced power consumption.

Batching happens when a sensor possesses a hardware FIFO (sensor_t.fifoMaxEventCount > 0) and we are in one of two situations:

  • max_report_latency > 0, meaning the sensor events for this specific sensor can be delayed up to max_report_latency before being reported through the HAL.
  • or the SoC is in suspend mode and the sensor is a non-wake-up sensor, meaning events must be stored while waiting for the SoC to wake up.

See the paragraph on the HAL batch function for more details.

The opposite of batching is the continuous operation, where events are not buffered, meaning they are reported immediately. Continuous operation corresponds to:

  • when max_report_latency = 0 and the events can be delivered to the application, meaning
    • the SoC is awake
    • or the sensor is a wake-up sensor
  • or when the sensor doesn’t have a hardware FIFO (sensor_t.fifoMaxEventCount = 0), in which case
    • the events are reported if the SoC is awake or the sensor is a wake-up sensor
    • the events are lost when the SoC is asleep and the sensor is not a wake-up sensor

Wake-up FIFOs and non-wake-up FIFOs

Sensor events from wake-up sensors must be stored into a wake-up FIFO. There can be one wake-up FIFO per sensor, or, more commonly, one big shared wake-up FIFO where events from all wake-up sensors are interleaved. Other options are also possible, with for example some wake-up sensors having a dedicated FIFO, and the rest of the wake-up sensors all sharing the same one.

Similarly, sensor events from non-wake-up sensors must be stored into a non-wake-up FIFOs, and there can be one or several non-wake-up FIFOs.

In all cases, wake-up sensor events and non-wake-up sensor events cannot be interleaved into the same FIFO. Wake-up events go in wake-up FIFOs, and non-wake-up events go in non-wake-up FIFOs.

For the wake-up FIFO, the “one big shared FIFO” design provides the best power benefits. For the non-wake-up FIFO, there is no preference between the “one big shared FIFO” and “several small reserved FIFOs”. See FIFO allocation priority for suggestions on how to dimension each FIFO.

Behavior outside of suspend mode

When the SoC is awake (not in suspend mode), the events can be stored temporarily in their FIFO, as long as they are not delayed by more than max_report_latency.

As long as the SoC doesn’t enter the suspend mode, no event shall be dropped or lost. If internal hardware FIFOs is getting full before max_report_latency elapsed, then events are reported at that point to ensure that no event is lost.

If several sensors share the same FIFO and the max_report_latency of one of them elapses, all events from the FIFO are reported, even if the max_report_latency of the other sensors didn’t elapse yet. The general goal is to reduce the number of times batches of events must be reported, so as soon as one event must be reported, all events from all sensors can be reported.

For example, if the following sensors are activated:

  • accelerometer batched with max_report_latency = 20s
  • gyroscope batched with max_report_latency = 5s

Then the accelerometer batches can be reported at the same time the gyroscope batches are reported (every 5 seconds), even if the accelerometer and the gyroscope do not share the same FIFO.

Behavior in suspend mode

Batching is particularly beneficial when wanting to collect sensor data in the background without keeping the SoC awake. Because the sensor drivers and HAL implementation are not allowed to hold a wake-lock*, the SoC can enter the suspend mode even while sensor data is being collected.

The behavior of sensors while the SoC is suspended depends on whether the sensor is a wake-up sensor. See Wake-up sensors for some details.

When a non-wake-up FIFO fills up, it must wrap around and behave like a circular buffer, overwriting older events: the new events replace the old ones. max_report_latency has no impact on non-wake-up FIFOs while in suspend mode.

When a wake-up FIFO fills up, or when the max_report_latency of one of the wake-up sensor elapsed, the hardware must wake up the SoC and report the data.

In both cases (wake-up and non-wake-up), as soon as the SoC comes out of suspend mode, a batch is produced with the content of all FIFOs, even if max_report_latency of some sensors didn’t elapse yet. This minimizes the risk of having to wake-up the SoC again soon if it goes back to suspend. Hence, it minimizes power consumption.

*One notable exception of drivers not being allowed to hold a wake lock is when a wake-up sensor with continuous reporting mode is activated with max_report_latency < 1 second. In that case, the driver can hold a wake lock because the SoC would anyway not have the time to enter the suspend mode, as it would be awoken by a wake-up event before reaching the suspend mode.

Precautions to take when batching wake-up sensors

Depending on the device, it might take a few milliseconds for the SoC to entirely come out of suspend and start flushing the FIFO. Enough head room must be allocated in the FIFO to allow the device to entirely come out of suspend without the wake-up FIFO overflowing. No events shall be lost, and the max_report_latency must be respected.

Precautions to take when batching non-wake-up on-change sensors

On-change sensors only generate events when the value they are measuring is changing. If the measured value changes while the SoC is in suspend mode, applications expect to receive an event as soon as the SoC wakes up. Because of this, batching of non-wake-up on-change sensor events must be performed carefully if the sensor shares its FIFO with other sensors. The last event generated by each on-change sensor must always be saved outside of the shared FIFO so it can never be overwritten by other events. When the SoC wakes up, after all events from the FIFO have been reported, the last on-change sensor event must be reported.

Here is a situation we want to avoid:

  1. An application registers to the non-wake-up step counter (on-change) and the non-wake-up accelerometer (continuous), both sharing the same FIFO
  2. The application receives a step counter event “step_count=1000 steps”
  3. The SoC goes to suspend
  4. The user walks 20 steps, causing step counter and accelerometer events to be interleaved, the last step counter event being “step_count = 1020 steps”
  5. The user doesn’t move for a long time, causing accelerometer events to continue accumulating in the FIFO, eventually overwriting every step_count event in the shared FIFO
  6. SoC wakes up and all events from the FIFO are sent to the application
  7. The application receives only accelerometer events and thinks that the user didn’t walk (bad!)

By saving the last step counter event outside of the FIFO, the HAL can report this event when the SoC wakes up, even if all other step counter events were overwritten by accelerometer events. This way, the application receives “step_count = 1020 steps” when the SoC wakes up.

Implementing batching

Batching cannot be emulated in software. It must be implemented entirely in hardware, with hardware FIFOs. In particular, it cannot be implemented on the SoC, for example in the HAL implementation, as this would be counter-productive. The goal here is to save significant amounts of power. Batching must be implemented without the aid of the SoC, which should be allowed to be in suspend mode during batching.

max_report_latency can be modified at any time, in particular while the specified sensor is already enabled; and this shall not result in the loss of events.

FIFO allocation priority

On platforms in which hardware FIFO size is limited, the system designers may have to choose how much FIFO to reserve for each sensor. To help with this choice, here is a list of applications made possible when batching is implemented on the different sensors.

High value: Low power pedestrian dead reckoning

Target batching time: 1 to 10 minutes

Sensors to batch:

  • Wake-up Step detector
  • Wake-up Game rotation vector at 5Hz
  • Wake-up Barometer at 5Hz
  • Wake-up Uncalibrated Magnetometer at 5Hz

Batching this data allows performing pedestrian dead reckoning while letting the SoC go to suspend.

High value: Medium power intermittent activity/gesture recognition

Target batching time: 3 seconds

Sensors to batch: Non-wake-up Accelerometer at 50Hz

Batching this data allows periodically recognizing arbitrary activities and gestures without having to keep the SoC awake while the data is collected.

Medium value: Medium power continuous activity/gesture recognition

Target batching time: 1 to 3 minutes

Sensors to batch: Wake-up Accelerometer at 50Hz

Batching this data allows continuously recognizing arbitrary activities and gestures without having to keep the SoC awake while the data is collected.

Medium-high value: Interrupt load reduction

Target batching time: < 1 second

Sensors to batch: any high frequency sensor, usually non-wake-up.

If the gyroscope is set at 240Hz, even batching just 10 gyro events can reduce the number of interrupts from 240/second to 24/second.

Medium value: Continuous low frequency data collection

Target batching time: 1 to 10 minutes

Sensors to batch:

  • Wake-up barometer at 1Hz,
  • Wake-up humidity sensor at 1Hz
  • Other low frequency wake-up sensors at similar rates

Allows creating monitoring applications at low power.

Medium-low value: Continuous full-sensors collection

Target batching time: 1 to 10 minutes

Sensors to batch: all wake-up sensors, at high frequencies

Allows full collection of sensor data while leaving the SoC in suspend mode. Only to consider if FIFO space is not an issue.