lmkd: Add lmkd README file

Add README file providing lmkd overview and description for its
properties.

Bug: 77299493
Bug: 75322373
Change-Id: I30236dd4ccaa2b48c31dfb80b63f0b82b49b5744
Merged-In: I30236dd4ccaa2b48c31dfb80b63f0b82b49b5744
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
(cherry picked from commit 7e5dea82a5a2f86117722ff11c93bc672ffb7903)

README.md

@@ -0,0 +1,60 @@
+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.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