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Android Low Memory Killer - AngelDevil

时间:2013年05月23日信息来源:不详 点击: 【字体:

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:

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; 

  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:


/*
 * 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 *);

  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:

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);

  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:

Android Low Memory Killer - AngelDevilAndroid Low Memory Killer - AngelDevil
static int lowmem_shrink(int nr_to_scan, gfp_t gfp_mask)
{
        struct task_struct *p;
        struct task_struct *selected = NULL;
        int rem = 0;
        int tasksize;
        int i;
        int min_adj = OOM_ADJUST_MAX + 1;
        int selected_tasksize = 0;
        int selected_oom_adj;
        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;
                }
        }
        if (nr_to_scan > 0)
                lowmem_print(3, "lowmem_shrink %d, %x, ofree %d %d, ma %d\n",
                             nr_to_scan, gfp_mask, other_free, other_file,
                             min_adj);
        rem = global_page_state(NR_ACTIVE_ANON) +
                global_page_state(NR_ACTIVE_FILE) +
                global_page_state(NR_INACTIVE_ANON) +
                global_page_state(NR_INACTIVE_FILE);
        if (nr_to_scan <= 0 || min_adj == OOM_ADJUST_MAX + 1) {
                lowmem_print(5, "lowmem_shrink %d, %x, return %d\n",
                             nr_to_scan, gfp_mask, rem);
                return rem;
        }
        selected_oom_adj = min_adj;

        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;
                if (oom_adj < min_adj) {
                        task_unlock(p);
                        continue;
                }
                tasksize = get_mm_rss(mm);
                task_unlock(p);
                if (tasksize <= 0)
                        continue;
                if (selected) {
                        if (oom_adj < selected_oom_adj)
                                continue;
                        if (oom_adj == selected_oom_adj &&
                            tasksize <= selected_tasksize)
                                continue;
                }
                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);
        }
        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;
        }
        lowmem_print(4, "lowmem_shrink %d, %x, return %d\n",
                     nr_to_scan, gfp_mask, rem);
        read_unlock(&tasklist_lock);
        return rem;
}
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:

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;

  遍历所有进程找到oom_adj>min_adj并且占用内存大的进程:

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);
}

  如果选择出了符合条件的进程,发送SIGNAL信号Kill掉:

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;
}

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:

/**
 * 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_SAVE_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;

  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:

// 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 HEAVY_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;

  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:

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.HEAVY_WEIGHT_APP_ADJ) {
        return ActivityManager.RunningAppProcessInfo.IMPORTANCE_CANT_SAVE_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;
    }
}

  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。

// 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;

  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。

 

相关链接:

lowermemorykiller.txt

lowermemorykiller.c

shrinker_list

(作者:佚名 编辑:admin)
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