Low Memory Killer

當系統記憶體不足時，會依優先權來殺掉某些行程，以釋放記憶體。但是，Android 預設並沒有開啟這項功能，所以需要去啟用它。

<step1> Enable Kernel Module

CONFIG_ANDROID_LOW_MEMORY_KILLER=y

<step2> Setup Recycle Memory Criteria in init.rc

# Define the oom_adj values for the classes of processes that can be
# killed by the kernel.  These are used in ActivityManagerService.
setprop ro.FOREGROUND_APP_ADJ 0
setprop ro.VISIBLE_APP_ADJ 1
setprop ro.PERCEPTIBLE_APP_ADJ 2
setprop ro.HEAVY_WEIGHT_APP_ADJ 3
setprop ro.SECONDARY_SERVER_ADJ 4
setprop ro.BACKUP_APP_ADJ 5
setprop ro.HOME_APP_ADJ 6
setprop ro.HIDDEN_APP_MIN_ADJ 7
setprop ro.EMPTY_APP_ADJ 15

# Define the memory thresholds at which the above process classes will
# be killed.  These numbers are in pages (4k).
# These are currently tuned for tablets with approx 1GB RAM.
setprop ro.FOREGROUND_APP_MEM 8192
setprop ro.VISIBLE_APP_MEM 10240
setprop ro.PERCEPTIBLE_APP_MEM 12288
setprop ro.HEAVY_WEIGHT_APP_MEM 12288
setprop ro.SECONDARY_SERVER_MEM 14336
setprop ro.BACKUP_APP_MEM 14336
setprop ro.HOME_APP_MEM 14336
setprop ro.HIDDEN_APP_MEM 16384
setprop ro.EMPTY_APP_MEM 20480

# Write value must be consistent with the above properties.
# Note that the driver only supports 6 slots, so we have combined some of
# the classes into the same memory level; the associated processes of higher
# classes will still be killed first.
write /sys/module/lowmemorykiller/parameters/adj 0,1,2,4,7,15

write /proc/sys/vm/overcommit_memory 1
write /proc/sys/vm/min_free_order_shift 4
write /sys/module/lowmemorykiller/parameters/minfree 8192,10240,12288,14336,16384,20480

# Set init its forked children's oom_adj.
write /proc/1/oom_adj -16

# Tweak background writeout
write /proc/sys/vm/dirty_expire_centisecs 200
write /proc/sys/vm/dirty_background_ratio  5

　　在 system/core/init/init.c 中會去讀取 init.rc，並將以上這些參數設定給 low memory killer。另外，上層還有一支 ActivityManagerService，但不知道他和底層 low memory killer 如何溝通。