Android Low Memory Killer Daemon
================================


Introduction
------------

Android Low Memory Killer Daemon (lmkd) is a process monitoring memory
state of a running Android system and reacting to high memory pressure
by killing the least essential process(es) to keep system performing
at acceptable levels.


Background
----------

Historically on Android systems memory monitoring and killing of
non-essential processes was handled by a kernel lowmemorykiller driver.
Since Linux Kernel 4.12 the lowmemorykiller driver has been removed and
instead userspace lmkd daemon performs these tasks.


Android Properties
------------------

lmkd can be configured on a particular system using the following Android
properties:

  - `ro.config.low_ram`:         choose between low-memory vs high-performance
                                 device. Default = false.

  - `ro.lmk.use_minfree_levels`: use free memory and file cache thresholds for
                                 making decisions when to kill. This mode works
                                 the same way kernel lowmemorykiller driver used
                                 to work. Default = false

  - `ro.lmk.low`:                min oom_adj score for processes eligible to be
                                 killed at low vmpressure level. Default = 1001
                                 (disabled)

  - `ro.lmk.medium`:             min oom_adj score for processes eligible to be
                                 killed at medium vmpressure level. Default = 800
                                 (non-essential processes)

  - `ro.lmk.critical`:           min oom_adj score for processes eligible to be
                                 killed at critical vmpressure level. Default = 0
                                 (all processes)

  - `ro.lmk.critical_upgrade`:   enables upgrade to critical level. Default = false

  - `ro.lmk.upgrade_pressure`:   max mem_pressure at which level will be upgraded
                                 because system is swapping too much. Default = 100
                                 (disabled)

  - `ro.lmk.downgrade_pressure`: min mem_pressure at which vmpressure event will
                                 be ignored because enough free memory is still
                                 available. Default = 100 (disabled)

  - `ro.lmk.kill_heaviest_task`: kill heaviest eligible task (best decision) vs.
                                 any eligible task (fast decision). Default = false

  - `ro.lmk.kill_timeout_ms`:    duration in ms after a kill when no additional
                                 kill will be done. Default = 0 (disabled)

  - `ro.lmk.debug`:              enable lmkd debug logs, Default = false

  - `ro.lmk.swap_free_low_percentage`: level of free swap as a percentage of the
                                 total swap space used as a threshold to consider
                                 the system as swap space starved. Default for
                                 low-RAM devices = 10, for high-end devices = 20

  - `ro.lmk.thrashing_limit`:    number of workingset refaults as a percentage of
                                the file-backed pagecache size used as a threshold
                                 to consider system thrashing its pagecache.
                                 Default for low-RAM devices = 30, for high-end
                                 devices = 100

  - `ro.lmk.thrashing_limit_decay`: thrashing threshold decay expressed as a
                                 percentage of the original threshold used to lower
                                 the threshold when system does not recover even
                                 after a kill. Default for low-RAM devices = 50,
                                 for high-end devices = 10

  - `ro.lmk.psi_partial_stall_ms`: partial PSI stall threshold in milliseconds for
                                 triggering low memory notification. Default for
                                 low-RAM devices = 200, for high-end devices = 70

  - `ro.lmk.psi_complete_stall_ms`: complete PSI stall threshold in milliseconds for
                                 triggering critical memory notification. Default =
                                 700

lmkd will set the following Android properties according to current system
configurations:

  - `sys.lmk.minfree_levels`:    minfree:oom_adj_score pairs, delimited by comma

  - `sys.lmk.reportkills`:       whether or not it supports reporting process kills
                                 to clients. Test app should check this property
                                 before testing low memory kill notification.
                                 Default will be unset.