Chrome OS Power Manager Logging

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powerd writes logs to the /var/log/power_manager directory. It creates a new file every time it starts and updates powerd.LATEST and powerd.PREVIOUS symlinks to point to the current and previous files. These files can be viewed on a live non-dev-mode system by browsing to file:///var/log/power_manager (note the triple slashes).

Any output written to stdout or stderr (including messages that are printed before the logging library has been initialized, and output from other executables that powerd runs) is redirected to /var/log/powerd.out.

Output from the powerd_suspend script, along with suspend- and resume-related kernel messages, are written to /var/log/messages.

Interpreting logs

powerd logs detailed information about what it's doing and why. Each line begins with a header similar to [1219/]:

  • The first four digits are the zero-prefixed month and day-of-month in the system's local time zone.
  • The next six digits are zero-prefixed hours, minutes, and seconds.
  • The next string, INFO, WARNING, or ERROR, describes the severity of the message.
  • The next string is the name of the file within the powerd source code that logged the message.
  • The final number in parentheses is the line in the file that logged the message.

Here are examples and explanations of some of the more-useful messages (with date/time and file/line prefixes omitted):

  • On battery at 81% (displayed as 83%), 4.242/5.248Ah at 0.483A, 8h19m9s until empty (7h54m27s until shutdown)

    powerd periodically polls sysfs for power supply information. This line includes information about the battery‘s state, whether it’s charging or discharging, and the estimated time until the battery will be empty/full or until the system will shut down automatically. If a charger is connected, additional information about it is included. The “displayed as” percentage indicates what will be displayed in the UI after the actual percentage has been adjusted to compensate for trickle charging.

  • User activity reported

    powerd periodically logs user activity (i.e. input events), which (along with video and audio activity) can affect inactivity delays.

  • Video activity reported

    Chrome attempts to detect video activity (defined as frequent updates to a fixed region of a window). When it believes a video is being played, it notifies powerd.

  • Audio activity started

    CRAS notifies powerd about changes to audio output streams. Audio activity is defined as the existence of one or more active output streams.

  • Received updated external policy: ac_dim=0s ac_screen_off=0s ac_lock=0s ac_idle_warn=0s ac_idle=30m battery_dim=0s battery_screen_off=0s battery_lock=0s battery_idle_warn=0s battery_idle=10m ac_idle=no-op battery_idle=no-op lid_closed=suspend use_audio=1 use_video=1 presentation_factor=2.0 user_activity_factor=2.0 wait_for_initial_user_activity=0 force_nonzero_brightness_for_user_activity=1 (Prefs, Playing video, Playing audio)

    This lengthy line describes a power management policy that Chrome sent to powerd:

    • The first set of values describe various inactivity delays.
    • The next ac_idle, battery_idle, and lid_closed values describe actions to be taken when the system is deemed idle on AC or battery power or its lid is closed.
    • use_audio and use_video describe whether powerd should adjust its behavior in response to audio and video activity.

    See policy.proto for descriptions of all of the fields. The final parenthesized list describes Chrome's reasoning in constructing this policy. In this case, power-management-related prefs were set, HTML5 video was playing (this is distinct from the inferred “video activity” described above), and HTML5 audio was playing (also distinct from CRAS-reported “audio activity”).

  • Updated settings: dim=0s screen_off=0s lock=0s idle_warn=0s idle=10m (no-op) lid_closed=suspend use_audio=1 use_video=1 wake_locks=screen

    This line describes the actual settings that powerd is currently using. These are based on the external policy described above, but also take into the current power source and other state. See the StateController class for the implementation.

Most other messages are hopefully self-explanatory, but more context can be found by looking at the code (a filename and line number are embedded at the beginning of each message).


powerd receives input (e.g. user/video/audio activity, lid events, etc.) and performs actions sporadically; thirty-second intervals where nothing happens are common for an idle system. Having logs of these events is essential to reconstruct the past when investigating user feedback reports.

powerd's unit tests, on the other hand, send a bunch of input very quickly. Logging all events drowns out the rest of the testing output.

To produce useful output when running in production while producing readable output from tests, powerd logs messages at the INFO level and above by default, while unit tests log WARNING and above.

Please use logging macros as follows within powerd:

VLOG(1)Debugging info that is hidden by default but can be selectively enabled by developers
LOG(INFO)Input from other systems or actions performed by powerd (i.e. things that would be useful when trying to figure out what powerd was thinking when investigating a bug report)
LOG(WARNING)Minor errors (e.g. bad input from other daemons)
LOG(ERROR)Major errors (e.g. problems communicating with the kernel)
LOG(FATAL)Critical problems that make powerd unusable or that indicate problems in the readonly system image (e.g. malformed preference defaults)
CHECK(...)Same as LOG(FATAL)