Android Low Memory Killer

转自：http://www.cnblogs.com/angeldevil/archive/2013/05/21/3090872.html

Low Memory Killer的原理

　　在Android中，即使当用户退出应用程序之后，应用程序的进程也还是存在于系统中，这样是为了方便程序的再次启动，但是这样的话，随着打开的程序数量的增加，系统的内存会变得不足，就需要杀掉一部分进程以释放内存空间。至于是否需要杀死一些进程和哪些进程需要被杀死，是通过Low Memory Killer机制来进行判定的。

　　Android的Low Memory Killer基于Linux的OOM机制，在Linux中，内存是以页面为单位分配的，当申请页面分配时如果内存不足会通过以下流程选择bad进程来杀掉从而释放内存：

alloc_pages -> out_of_memory() -> select_bad_process() -> badness()

　　在Low Memory Killer中通过进程的oom_adj与占用内存的大小决定要杀死的进程，oom_adj越小越不容易被杀死。

　　Low Memory Killer Driver在用户空间指定了一组内存临界值及与之一一对应的一组oom_adj值，当系统剩余内存位于内存临界值中的一个范围内时，如果一个进程的oom_adj值大于或等于这个临界值对应的oom_adj值就会被杀掉。

　　可以通过修改/sys/module/lowmemorykiller/parameters/minfree与/sys/module/lowmemorykiller/parameters/adj来改变内存临界值及与之对应的oom_adj值。minfree中数值的单位是内存中的页面数量，一般情况下一个页面是4KB。

　　比如如果向/sys/module/lowmemorykiller/parameters/adj写入0,8，向/sys/module/lowmemorykiller/parameters/minfree中写入1024,4096，假设一个页面大小为4KB，这样当系统空闲内存位于1024*4~4096*4KB之间时oom_adj大于等于8的进程就会被杀掉。

　　在lowmemorykiller.c中定义了阈值表的默认值，可以通过init.rc自定义：

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[code]
static int lowmem_adj[6] = {
        0,
        1,
        6,
        12,
};
static int lowmem_adj_size = 4;
static size_t lowmem_minfree[6] = {
        3 * 512,        /* 6MB */
        2 * 1024,       /* 8MB */
        4 * 1024,       /* 16MB */
        16 * 1024,      /* 64MB */
};

static int lowmem_minfree_size = 4;　

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　　在init.rc中定义了init进程的oom_adj为-16，不可能会被杀死(init的PID是1)：

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

　　在Linux中有一个kswapd的内核线程，当linux回收内存分页的时候，kswapd线程将会遍历一张shrinker链表，并执行回调，定义如下：
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/*
 * A callback you can register to apply pressure to ageable caches.
 *
 * 'shrink' is passed a count 'nr_to_scan' and a 'gfpmask'.  It should
 * look through the least-recently-used 'nr_to_scan' entries and
 * attempt to free them up.  It should return the number of objects
 * which remain in the cache.  If it returns -1, it means it cannot do
 * any scanning at this time (eg. there is a risk of deadlock).
 *
 * The 'gfpmask' refers to the allocation we are currently trying to
 * fulfil.
 *
 * Note that 'shrink' will be passed nr_to_scan == 0 when the VM is
 * querying the cache size, so a fastpath for that case is appropriate.
*/

struct shrinker {
    int (*shrink)(int nr_to_scan, gfp_t gfp_mask);
    int seeks;      /* seeks to recreate an obj */

    /* These are for internal use */
    struct list_head list;
    long nr;        /* objs pending delete */
};
#define DEFAULT_SEEKS 2 /* A good number if you don't know better. */
extern void register_shrinker(struct shrinker *);
extern void unregister_shrinker(struct shrinker *);

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　　通过register_shrinker与unregister_shrinker向shrinker链表中添加或移除回调。当注册Shrinker后就可以在回收内存分页时按自己定义的规则释放内存。

　　Android Low Memory Killer的代码在drivers/staging/android/lowmemorykiller.c中，通过以下代码在模块初始化时注册Shrinker：

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[code]static int lowmem_shrink(int nr_to_scan, gfp_t gfp_mask);

static struct shrinker lowmem_shrinker = {
        .shrink = lowmem_shrink,
        .seeks = DEFAULT_SEEKS * 16
};

static int __init lowmem_init(void)
{
        register_shrinker(&lowmem_shrinker);
        return 0;
}

static void __exit lowmem_exit(void)
{
        unregister_shrinker(&lowmem_shrinker);
}

module_init(lowmem_init);
module_exit(lowmem_exit);

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　　这样就可以在回收内存分页时调用lowmem_shrink函数。

Low Memory Killer的实现

　　lowmem_shrink的定义如下：

View Code

　　分开来看这段代码，首先取得内存阈值表的大小，取阈值表数组大小与lowmem_adj_size，lowmem_minfree_size的较小值，然后通过globa_page_state获得当前剩余内存的大小，然后跟内存阈值表中的阈值相比较获得min_adj与selected_oom_adj：

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[code]int array_size = ARRAY_SIZE(lowmem_adj);
int other_free = global_page_state(NR_FREE_PAGES);
int other_file = global_page_state(NR_FILE_PAGES);

if (lowmem_adj_size < array_size)
        array_size = lowmem_adj_size;
if (lowmem_minfree_size < array_size)
        array_size = lowmem_minfree_size;
for (i = 0; i < array_size; i++) {
    if (other_free < lowmem_minfree[i] && other_file < lowmem_minfree[i]) {
         min_adj = lowmem_adj[i];
         break;
    }
}
selected_oom_adj = min_adj;

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　　遍历所有进程找到oom_adj>min_adj并且占用内存大的进程：

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[code]read_lock(&tasklist_lock);
for_each_process(p) {
    struct mm_struct *mm;
    int oom_adj;

    task_lock(p);
    mm = p->mm;
    if (!mm) {
        task_unlock(p);
        continue;
    }
    oom_adj = mm->oom_adj;
    //获取task_struct->struct_mm->oom_adj，如果小于警戒值min_adj不做处理
    if (oom_adj < min_adj) {
        task_unlock(p);
        continue;
    }
    //如果走到这里说明oom_adj>=min_adj，即超过警戒值
    //获取内存占用大小，若<=0，不做处理
    tasksize = get_mm_rss(mm);
    task_unlock(p);
    if (tasksize <= 0)
        continue;
    //如果之前已经先择了一个进程，比较当前进程与之前选择的进程的oom_adj与内存占用大小，如果oom_adj比之前选择的小或相等而内存占用比之前选择的进程小，不做处理。
    if (selected) {
        if (oom_adj < selected_oom_adj)
            continue;
        if (oom_adj == selected_oom_adj &&
            tasksize <= selected_tasksize)
            continue;
    }
    //走到这里表示当前进程比之前选择的进程oom_adj大或相等但占用内存大，选择当前进程
    selected = p;
    selected_tasksize = tasksize;
    selected_oom_adj = oom_adj;
    lowmem_print(2, "select %d (%s), adj %d, size %d, to kill\n",
                 p->pid, p->comm, oom_adj, tasksize);
}

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如果选择出了符合条件的进程，发送SIGNAL信号Kill掉：

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[code]if (selected) {
    lowmem_print(1, "send sigkill to %d (%s), adj %d, size %d\n",
                 selected->pid, selected->comm,
                 selected_oom_adj, selected_tasksize);
    force_sig(SIGKILL, selected);
    rem -= selected_tasksize;
}

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oom_adj与上层Process Importance的关系

我们知道，在上层进程按重要性可以分为：Foreground process，Visible process，Service process，Background process与Empty process，那么这些重要性怎么与Low Memory Killer中的oom_adj对应起来的呢？

在ActivityManager.RunningAppProcessInfo中我们可以看到如下关于importance的定义：

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/**

* Constant for {@link #importance}: this is a persistent process.

* Only used when reporting to process observers.

* @hide

*/

public static final int IMPORTANCE_PERSISTENT = 50;

/**

* Constant for {@link #importance}: this process is running the

* foreground UI.

*/

public static final int IMPORTANCE_FOREGROUND = 100;

/**

* Constant for {@link #importance}: this process is running something

* that is actively visible to the user, though not in the immediate

* foreground.

*/

public static final int IMPORTANCE_VISIBLE = 200;

/**

* Constant for {@link #importance}: this process is running something

* that is considered to be actively perceptible to the user. An

* example would be an application performing background music playback.

*/

public static final int IMPORTANCE_PERCEPTIBLE = 130;

/**

* Constant for {@link #importance}: this process is running an

* application that can not save its state, and thus can’t be killed

* while in the background.

* @hide

*/

public static final int IMPORTANCE_CANT_S***E_STATE = 170;

/**

* Constant for {@link #importance}: this process is contains services

* that should remain running.

*/

public static final int IMPORTANCE_SERVICE = 300;

/**

* Constant for {@link #importance}: this process process contains

* background code that is expendable.

*/

public static final int IMPORTANCE_BACKGROUND = 400;

/**

* Constant for {@link #importance}: this process is empty of any

* actively running code.

*/

public static final int IMPORTANCE_EMPTY = 500;

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　　这些常量表示了Process的Importance等级，而在ProcessList中我们会发现关于adj的一些定义：

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// This is a process only hosting activities that are not visible,

// so it can be killed without any disruption.

static final int HIDDEN_APP_MAX_ADJ = 15;

static int HIDDEN_APP_MIN_ADJ = 9;

// The B list of SERVICE_ADJ – these are the old and decrepit

// services that aren’t as shiny and interesting as the ones in the A list.

static final int SERVICE_B_ADJ = 8;

// This is the process of the previous application that the user was in.

// This process is kept above other things, because it is very common to

// switch back to the previous app. This is important both for recent

// task switch (toggling between the two top recent apps) as well as normal

// UI flow such as clicking on a URI in the e-mail app to view in the browser,

// and then pressing back to return to e-mail.

static final int PREVIOUS_APP_ADJ = 7;

// This is a process holding the home application – we want to try

// avoiding killing it, even if it would normally be in the background,

// because the user interacts with it so much.

static final int HOME_APP_ADJ = 6;

// This is a process holding an application service – killing it will not

// have much of an impact as far as the user is concerned.

static final int SERVICE_ADJ = 5;

// This is a process currently hosting a backup operation. Killing it

// is not entirely fatal but is generally a bad idea.

static final int BACKUP_APP_ADJ = 4;

// This is a process with a heavy-weight application. It is in the

// background, but we want to try to avoid killing it. Value set in

// system/rootdir/init.rc on startup.

static final int HE***Y_WEIGHT_APP_ADJ = 3;

// This is a process only hosting components that are perceptible to the

// user, and we really want to avoid killing them, but they are not

// immediately visible. An example is background music playback.

static final int PERCEPTIBLE_APP_ADJ = 2;

// This is a process only hosting activities that are visible to the

// user, so we’d prefer they don’t disappear.

static final int VISIBLE_APP_ADJ = 1;

// This is the process running the current foreground app. We’d really

// rather not kill it!

static final int FOREGROUND_APP_ADJ = 0;

// This is a system persistent process, such as telephony. Definitely

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

static final int PERSISTENT_PROC_ADJ = -12;

// The system process runs at the default adjustment.

static final int SYSTEM_ADJ = -16;

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　　我们可以看到：

static final int PREVIOUS_APP_ADJ = 7;

static final int HOME_APP_ADJ = 6;

　　并不是所有的Background process的等级都是相同的。

　　关于ADJ与Importance的值都找到了，那么它们是怎么对应起来的呢？Activity实际是由ActivityManagerService来管理的，在ActivityManagerService中我们可以找到以下函数：

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static int oomAdjToImportance(int adj, ActivityManager.RunningAppProcessInfo currApp) {

if (adj >= ProcessList.HIDDEN_APP_MIN_ADJ) {

if (currApp != null) {

currApp.lru = adj - ProcessList.HIDDEN_APP_MIN_ADJ + 1;

}

return ActivityManager.RunningAppProcessInfo.IMPORTANCE_BACKGROUND;

} else if (adj >= ProcessList.SERVICE_B_ADJ) {

return ActivityManager.RunningAppProcessInfo.IMPORTANCE_SERVICE;

} else if (adj >= ProcessList.HOME_APP_ADJ) {

if (currApp != null) {

currApp.lru = 0;

}

return ActivityManager.RunningAppProcessInfo.IMPORTANCE_BACKGROUND;

} else if (adj >= ProcessList.SERVICE_ADJ) {

return ActivityManager.RunningAppProcessInfo.IMPORTANCE_SERVICE;

} else if (adj >= ProcessList.HE***Y_WEIGHT_APP_ADJ) {

return ActivityManager.RunningAppProcessInfo.IMPORTANCE_CANT_S***E_STATE;

} else if (adj >= ProcessList.PERCEPTIBLE_APP_ADJ) {

return ActivityManager.RunningAppProcessInfo.IMPORTANCE_PERCEPTIBLE;

} else if (adj >= ProcessList.VISIBLE_APP_ADJ) {

return ActivityManager.RunningAppProcessInfo.IMPORTANCE_VISIBLE;

} else {

return ActivityManager.RunningAppProcessInfo.IMPORTANCE_FOREGROUND;

}

}

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　　在这个函数中实现了根据adj设置importance的功能。

　　我们还可以看到SERVICE还分为SERVICE_B_ADJ与SERVICE_ADJ，等级是不一样的，并不是所有Service的优先级都比Background process的优先级高。当调用Service的startForeground后，Service的importance就变为了IMPORTANCE_PERCEPTIBLE（在记忆中曾经将Service设置为foreground并打印出其importance的值与IMPORTANCE_PERCEPTIBLE相等），对应的adj是PERCEPTIBLE_APP_ADJ，即2，已经很难被系统杀死了。

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// This is a system persistent process, such as telephony. Definitely

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

static final int PERSISTENT_PROC_ADJ = -12;

// The system process runs at the default adjustment.

static final int SYSTEM_ADJ = -16;

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　　像电话等进程的adj为-12已基本不可能被杀死了，而在前面已经看到了，init.rc中将init进程的oom_adj设置为了-16，已经是永生进程了。