Android分析之LowMemoryKiller

Android分析之LowMemoryKiller

linux操作系统的传统理念就是内存用的越多越好，尽可能拿来用，既然被尽量的使用，自然应该有清除机制。Android以linux为基础，自然部分继承了这个特性。Android使用lowmemorykiller在达到某个内存门限的情况下去选择进程删除来释放内存。关键的配置文件有如下两个，/sys/module/lowmemorykiller/parameters/adj和/sys/module/lowmemorykiller/parameters/minfree配置系统的相关参数。

adj文件包含队列包含oomadj队列，当对应的minfree值达到，则进程的oomadj如果大于这个值将被杀掉。

minfree包含对应oom_adj的警戒值。

参考如下：

oom_adj 内存警戒值( 以4K为单位）

0 1536

1 2048

2 4096

7 5120

14 5632

15 6144

一般情况下，默认设置在system/core/rootdir/init.rc中

# 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.SECONDARY_SERVER_ADJ 2

setprop ro.BACKUP_APP_ADJ 2

setprop ro.HOME_APP_ADJ 4

setprop ro.HIDDEN_APP_MIN_ADJ 7

setprop ro.CONTENT_PROVIDER_ADJ 14

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).

setprop ro.FOREGROUND_APP_MEM 1536

setprop ro.VISIBLE_APP_MEM 2048

setprop ro.SECONDARY_SERVER_MEM 4096

setprop ro.BACKUP_APP_MEM 4096

setprop ro.HOME_APP_MEM 4096

setprop ro.HIDDEN_APP_MEM 5120

setprop ro.CONTENT_PROVIDER_MEM 5632

setprop ro.EMPTY_APP_MEM 6144

# Write value must be consistent with the above properties.

# Note that the driver only supports 6 slots, so we have HOME_APP at the

# same memory level as services.

write /sys/module/lowmemorykiller/parameters/adj 0,1,2,7,14,15

write /sys/module/lowmemorykiller/parameters/minfree 1536,2048,4096,5120,5632,6144

# Set init its forked children's oom_adj.

write /proc/1/oom_adj -16

实际上在frameworks/base/services/java/com/android/server/am/ActivityManagerService.java中定义了两个更高的优先级。

// This is a process running a core server, such as telephony. Definitely

// don't want to kill it, but doing so is not completely fatal.

static final int CORE_SERVER_ADJ = -12;

// The system process runs at the default adjustment.

static final int SYSTEM_ADJ = -16;

根据以上分析，对于某些小内存设备，我们可以调整对应的门限值，例如：

一般调整后三个值。

echo "1536,2048,4096,15360,17920,20480">/sys/module/lowmemorykiller/parameters/minfree