Android記憶體管理基礎，android記憶體管理

Android記憶體管理基礎，android記憶體管理
虛擬記憶體:將外儲存空間的部分空間作為記憶體的擴充，資源不足時按演算法儲存起來 當要用到資料區塊時交換回記憶體。邏輯地址=Segment Selector + Offset；線性地址物理地址Android系統記憶體配置和回收

Native層
智能指標的實現原理規避對象的產生和銷毀
Java層
相對C層做出了一定的努力，並對Android記憶體提供了管理機制

void* mmap(void* start,size_t length,int prot,int flags,int fd,off_t offset);
start：映射區的開始地址
length：映射區的長度。
prot：期望的記憶體保護標誌（操作許可權）
flags：指定程式對記憶體塊的影響。
fd：有效檔案描述詞。一般是由open()函數返回
offset：被對應檔記憶體起點。
mmap將一個檔案或者其它對象映射進記憶體。檔案被映射到多個頁上，如果檔案的大小不是所有頁的大小之和，
最後一個頁不被使用的空間將會清零。

Android Low memory killer （LMK驅動）

/**以下代碼為kernel3.8_為例*/static int __init lowmem_init(void){    task_free_register(&task_nb);    register_shrinker(&lowmem_shrinker);    return 0;}//表示某一級要處理的對應的進程數static int lowmem_adj[6] = {    0,    1,    6,    12,};static int lowmem_adj_size = 4;//頁大小//表示當剩下記憶體少於對應層級時，執行清理操作。static size_t lowmem_minfree[6] = {    3 * 512,    /* 6MB */    2 * 1024,   /* 8MB */    4 * 1024,   /* 16MB */    16 * 1024,  /* 64MB */};static int lowmem_minfree_size = 4;

3.如何知道需要被清理的進程,如下 Android定義了一系列的ADJ

    static final int EMPTY_APP_ADJ = 15;    //當前只運行了不可見Activity組件的進程    static final int HIDDEN_APP_MAX_ADJ = 15;    static int HIDDEN_APP_MIN_ADJ = 7;    //Luancher進程    static final int HOME_APP_ADJ = 6;    // 2級服務    // system/rootdir/init.rc on startup.    static final int SECONDARY_SERVER_ADJ = 5;    // 承載backup相關操作進程    static final int BACKUP_APP_ADJ = 4;    // 重量級應用程式進程    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.  Value set in    // system/rootdir/init.rc on startup.    static final int PERCEPTIBLE_APP_ADJ = 2;    // 前台可見進程    static final int VISIBLE_APP_ADJ = 1;    // 前台運行與使用者互動的進程    static final int FOREGROUND_APP_ADJ = 0;    // 核心進程 如telephony進程    static final int CORE_SERVER_ADJ = -12;    // 系統進程    static final int SYSTEM_ADJ = -16;

4.可以通過檔案修改某個進程對應ADJ值。也可以在application標籤中添加android:persistent=true屬性，
此屬性是將程式設定為常駐記憶體.

Android 匿名共用記憶體Anonymous Shared Momery

定義：將制定的物理地址映射到各個進程的虛擬位址空間中。

//early-init時載入ashmem.c檔案 如下調用ashmem_initstatic int __init ashmem_init(void){    //通過kmem_cache_create建立的kmem_cache    //它的類型是struct kmem_cache，表示這是一個slab緩衝區，由核心中的記憶體管理系統進行管理。    ashmem_area_cachep = kmem_cache_create("ashmem_area_cache",                      sizeof(struct ashmem_area),                      0, 0, NULL);    ashmem_range_cachep = kmem_cache_create("ashmem_range_cache",                      sizeof(struct ashmem_range),                      0, 0, NULL);    ret = misc_register(&ashmem_misc);    register_shrinker(&ashmem_shrinker);    printk(KERN_INFO "ashmem: initialized\n");    return 0;}//ashmem時一個雜項裝置（misc device），他調用ashmem_misc註冊static struct miscdevice ashmem_misc = {    .minor = MISC_DYNAMIC_MINOR,    .name = "ashmem",    .fops = &ashmem_fops,};//定義了進程間匿名共用記憶體的分配步驟ashmem_open,ashmem_ioctl,ashmem_mmap,ashmem_releasestatic struct file_operations ashmem_fops = {    .owner = THIS_MODULE,    .open = ashmem_open,    .release = ashmem_release,        .read = ashmem_read,        .llseek = ashmem_llseek,    .mmap = ashmem_mmap,    .unlocked_ioctl = ashmem_ioctl,    .compat_ioctl = ashmem_ioctl,};struct ashmem_area {      char name[ASHMEM_FULL_NAME_LEN];// 這個名字會顯示/proc/<pid>/maps檔案中，<pid>表示開啟這個共用記憶體檔案的進程ID    struct list_head unpinned_list; //  列表頭，它把這塊共用記憶體中所有被解鎖的記憶體塊串連在一起    struct file *file;      //  域file表示這個共用記憶體在臨時檔案系統tmpfs中對應的檔案    size_t size;            /* size of the mapping, in bytes */      unsigned long prot_mask;    //  表示這塊共用記憶體的訪問保護位。};//建立記憶體指定檔案，操作記憶體即操作檔案static int ashmem_open(struct inode *inode, struct file *file){    struct ashmem_area *asma;    int ret;    ret = generic_file_open(inode, file);    if (unlikely(ret))        return ret;    //從ashmem_area_cachep分配一塊ashmem_area記憶體    asma = kmem_cache_zalloc(ashmem_area_cachep, GFP_KERNEL);    if (unlikely(!asma))        return -ENOMEM;    //設定名字 許可權等    INIT_LIST_HEAD(&asma->unpinned_list);    memcpy(asma->name, ASHMEM_NAME_PREFIX, ASHMEM_NAME_PREFIX_LEN);    asma->prot_mask = PROT_MASK;    //記錄在private_data中    file->private_data = asma;    return 0;}//為每個子模組建立單獨的記憶體static long ashmem_ioctl(struct file *file, unsigned int cmd, unsigned long arg){    struct ashmem_area *asma = file->private_data;    long ret = -ENOTTY;    switch (cmd) {    case ASHMEM_SET_NAME:        ret = set_name(asma, (void __user *) arg);        break;    case ASHMEM_GET_NAME:        ret = get_name(asma, (void __user *) arg);        break;    case ASHMEM_SET_SIZE:        ret = -EINVAL;        if (!asma->file) {            ret = 0;            asma->size = (size_t) arg;        }        break;        ...        break;    }    return ret;}//執行對子模組的映射static int ashmem_mmap(struct file *file, struct vm_area_struct *vma){    struct ashmem_area *asma = file->private_data;    int ret = 0;    mutex_lock(&ashmem_mutex);//互斥鎖    /* 通過ioctl設定大小 */    if (unlikely(!asma->size)) {        ret = -EINVAL;        goto out;    }    //請求的許可權保護必須匹配我們設定的許可權值    if (unlikely((vma->vm_flags & ~calc_vm_prot_bits(asma->prot_mask)) &                        calc_vm_prot_bits(PROT_MASK))) {        ret = -EPERM;        goto out;    }    vma->vm_flags &= ~calc_vm_may_flags(~asma->prot_mask);    if (!asma->file) {//建立asm的臨時支援檔案        char *name = ASHMEM_NAME_DEF;        struct file *vmfile;        if (asma->name[ASHMEM_NAME_PREFIX_LEN] != '\0')            name = asma->name;        //由linux提供的在tmpfs中建立一個臨時檔案        vmfile = shmem_file_setup(name, asma->size, vma->vm_flags);        if (unlikely(IS_ERR(vmfile))) {            ret = PTR_ERR(vmfile);            goto out;        }        asma->file = vmfile;    }    get_file(asma->file);    if (vma->vm_flags & VM_SHARED)        shmem_set_file(vma, asma->file);//記憶體映射    else {        if (vma->vm_file)            fput(vma->vm_file);        vma->vm_file = asma->file;    }    vma->vm_flags |= VM_CAN_NONLINEAR;out:    mutex_unlock(&ashmem_mutex);    return ret;}//清楚記憶體共用區static int ashmem_release(struct inode *ignored, struct file *file){    struct ashmem_area *asma = file->private_data;    struct ashmem_range *range, *next;    mutex_lock(&ashmem_mutex);    list_for_each_entry_safe(range, next, &asma->unpinned_list, unpinned)        range_del(range);    mutex_unlock(&ashmem_mutex);    if (asma->file)        fput(asma->file);    kmem_cache_free(ashmem_area_cachep, asma);    return 0;}

MemoryFile.java

public MemoryFile(String name, int length) throws IOException {        mLength = length;        mFD = native_open(name, length);        ...}private static native FileDescriptor native_open(String name, int length) throws IOException;

android_os_MemoryFile.cpp

static jobject android_os_MemoryFile_open(JNIEnv* env, jobject clazz, jstring name, jint length){    ...    int result = ashmem_create_region(namestr, length);    ...}

\system\core\libcutils\ashmem-dev.c

int ashmem_create_region(const char *name, size_t size){    int fd, ret;    fd = open(ASHMEM_DEVICE, O_RDWR);    if (fd < 0)        return fd;    if (name) {        ...        ret = ioctl(fd, ASHMEM_SET_NAME, buf);        ...    }    ret = ioctl(fd, ASHMEM_SET_SIZE, size);    ...    return fd;error:    close(fd);    return ret;}

MemoryFile.java

public MemoryFile(String name, int length) throws IOException {          ......          mAddress = native_mmap(mFD, length, PROT_READ | PROT_WRITE);          ......  }  

android_os_MemoryFile.cpp

static jint android_os_MemoryFile_mmap(JNIEnv* env, jobject clazz, jobject fileDescriptor,          jint length, jint prot)  {      ...     jint result = (jint)mmap(NULL, length, prot, MAP_SHARED, fd, 0);      ...    return result;  }  

未完。。。

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