前言

上篇文章介绍了dalvik虚拟机启动流程，在dalvik虚拟机启动时调用了dvmGcStartup来启动堆。
本文介绍我们在日常开发使用Java时的堆创建流程。

Dalvik堆介绍

Dalvik虚拟机中，堆是由heap[0] Active堆和heap[1] Zygote堆两部分组成的。其中，Zygote堆用来管理Zygote进程在启动过程中预加载和创建的各种对象，而Active堆是在Zygote进程fork第一个子进程之前创建的。
之后无论是Zygote进程还是其子进程，都在Active堆上进行对象分配和释放。这样做的目的是使得Zygote进程和其子进程最大限度地共享Zygote堆所占用的内存。

Dalvik虚拟机管理中的重要结构包括一个Card Table、两个Heap Bitmap和一个GcMarkStack。

HeapBitmap

HeapBitmap是堆的内存分配情况的映射图，它的每一个bit位记录着堆中每8个字节的分配情况。
堆中有两个HeapBitmap，一个称为LiveHeapBitmap，用来记录上次GC之后还存活的对象；另一个称为MarkHeapBitmap，用来记录当前GC中还存活的对象。这样，上次GC后存活的但是当前GC不存活的对象，就是需要释放的对象。

GcMarkStack

Davlk虚拟机使用标记-清除（Mark-Sweep）算法进行GC。在标记阶段，通过一个Mark Stack来实现递归检查被引用的对象，即在当前GC中存活的对象。有了这个Mark Stack，就可以通过循环来模拟函数递归调用。
在垃圾回收的过程中，需要通过递归的方式去检查系统中的每个对象。但是递归太深会引起栈溢出，因此，实际采用的回收算法中用GcMarkStack来保存中间的数据。

CardTable

Card Table是为了记录在垃圾收集过程中对象的引用情况的，用在Concurrent GC第二阶段记录非垃圾收集堆对象对垃圾收集堆对象的引用。后文会分析内存回收流程，即gc流程。
Card Table和Heap Bitmap的作用是类似的。区别在于：

1. Card Table不是使用一个bit来描述一个对象，而是用一个byte来描述GC_CARD_SIZE个对象；
2. Card Table不是用来描述对象的存活，而是用来描述在Concurrent GC的过程中被修改的对象，这些对象需要进行特殊处理。

初始化zygote堆

• dalvik/vm/alloc/Alloc.cpp

// Initialize the GC universe.
bool dvmGcStartup()
{
    dvmInitMutex(&gDvm.gcHeapLock);
    pthread_cond_init(&gDvm.gcHeapCond, NULL);
    return dvmHeapStartup();
}

• dalvik/vm/alloc/Heap.cpp
  初始化堆，当heapGrowthLimit=0时，使用heapMaximumSize

// Initialize the GC heap.
bool dvmHeapStartup()
{
    GcHeap *gcHeap;

    if (gDvm.heapGrowthLimit == 0) {
        gDvm.heapGrowthLimit = gDvm.heapMaximumSize;
    }

    gcHeap = dvmHeapSourceStartup(gDvm.heapStartingSize, gDvm.heapMaximumSize, gDvm.heapGrowthLimit);
    gDvm.gcHeap = gcHeap;

    // Set up the lists we'll use for cleared reference objects.
    gcHeap->clearedReferences = NULL;
    
	// 初始化cradTable
    dvmCardTableStartup(gDvm.heapMaximumSize, gDvm.heapGrowthLimit);

    return true;
}

• dalvik/vm/alloc/HeapSource.cpp
  • dvmAllocRegion()函数来分配一块内存空间，然后把这块内存空间交给dlmalloc来管理；dvmAllocRegion()函数中使用ashmem_create_region()和mmap()函数来分配需要的内存空间，这也意味着dvmAllocRegion()分配的都是大块的内存。以下几个函数中内存分配都是在使用dvmAllocRegion()分配的内存，并没有从Dalvik的堆上分配，因为这几个对象在系统中会一直存在，不能被回收，因此，直接从系统内存中分配，不用Dalvik管理。
  • addInitialHeap()函数将创建出来的内存放到了heapSource的字段HeapSource[0]里。Dalvik并没有直接使用系统调用来自己管理动态内存，而是以“私有堆”的形式交给dlmalloc管理。
  • dvmHeapBitmapInit()函数创建了两个HeapBitmap的对象，HeapBitmap是堆的内存分配情况的映射图，它的每一个bit位记录着堆中每8个字节的分配情况。
  • allocMarkStack()函数分配了一块内存，并用它来初始化GcMarkStack结构。在垃圾回收的过程中，需要通过递归的方式去检查系统中的每个对象。但是递归太深会引起栈溢出，因此，实际采用的回收算法中用GcMarkStack来保存中间的数据。

// Initializes the heap source; 
GcHeap* dvmHeapSourceStartup(size_t startSize, size_t maximumSize, size_t growthLimit) {
    GcHeap *gcHeap;
    HeapSource *hs;
    mspace msp;
    size_t length;
    void *base;

    // Allocate a contiguous region of virtual memory to subdivided among the heaps managed by the garbage collector. 
    length = ALIGN_UP_TO_PAGE_SIZE(maximumSize);
    base = dvmAllocRegion(length, PROT_NONE, gDvm.zygote ? "dalvik-zygote" : "dalvik-heap");
    // Create an unlocked dlmalloc mspace to use as a heap source.
    msp = createMspace(base, kInitialMorecoreStart, startSize);

    gcHeap = (GcHeap *)calloc(1, sizeof(*gcHeap));
    hs = (HeapSource *)calloc(1, sizeof(*hs));

    hs->targetUtilization = gDvm.heapTargetUtilization * HEAP_UTILIZATION_MAX;
    hs->minFree = gDvm.heapMinFree;
    hs->maxFree = gDvm.heapMaxFree;
    hs->startSize = startSize;
    hs->maximumSize = maximumSize;
    hs->growthLimit = growthLimit;
    hs->idealSize = startSize;
    hs->softLimit = SIZE_MAX;    // no soft limit at first
    hs->numHeaps = 0;
    hs->sawZygote = gDvm.zygote;
    hs->nativeBytesAllocated = 0;
    hs->nativeFootprintGCWatermark = startSize;
    hs->nativeFootprintLimit = startSize * 2;
    hs->nativeNeedToRunFinalization = false;
    hs->hasGcThread = false;
    hs->heapBase = (char *)base;
    hs->heapLength = length;
  
    // Add the initial heap. 初始化heapSource中的第一个堆
    addInitialHeap(hs, msp, growthLimit);
    // Initialize a HeapBitmap so that it points to a bitmap large enough to cover a heap at <base> of <maxSize> bytes
    dvmHeapBitmapInit(&hs->liveBits, base, length, "dalvik-bitmap-1");
    dvmHeapBitmapInit(&hs->markBits, base, length, "dalvik-bitmap-2");

    allocMarkStack(&gcHeap->markContext.stack, hs->maximumSize);
    gcHeap->markContext.bitmap = &hs->markBits;
    gcHeap->heapSource = hs;

    gHs = hs;
    return gcHeap;
}

//Add the initial heap.  
static bool addInitialHeap(HeapSource *hs, mspace msp, size_t maximumSize)
{
    if (hs->numHeaps != 0) {
        return false;
    }
    hs->heaps[0].msp = msp;
    hs->heaps[0].maximumSize = maximumSize;
    hs->heaps[0].concurrentStartBytes = SIZE_MAX;
    hs->heaps[0].base = hs->heapBase;
    hs->heaps[0].limit = hs->heapBase + maximumSize;
    hs->heaps[0].brk = hs->heapBase + kInitialMorecoreStart;
    hs->numHeaps = 1;
    return true;
}

// Initialize a HeapBitmap so that it points to a bitmap large enough to cover a heap at <base> of <maxSize> bytes, where objects are guaranteed to be HB_OBJECT_ALIGNMENT-aligned.
bool dvmHeapBitmapInit(HeapBitmap *hb, const void *base, size_t maxSize, const char *name) {
    void *bits;
    size_t bitsLen;

    bitsLen = HB_OFFSET_TO_INDEX(maxSize) * sizeof(*hb->bits);
    bits = dvmAllocRegion(bitsLen, PROT_READ | PROT_WRITE, name);
    if (bits == NULL) {
        ALOGE("Could not mmap %zd-byte ashmem region '%s'", bitsLen, name);
        return false;
    }
    hb->bits = (unsigned long *)bits;
    hb->bitsLen = hb->allocLen = bitsLen;
    hb->base = (uintptr_t)base;
    hb->max = hb->base - 1;
    return true;
}

初始化active堆

1. 直到dvmHeapStartup()函数结束，heapSource中的两个“堆”只有heaps[0]初始化了，heaps[1]仍然为NULL。因为dvmHeapStartup()的调用是在Zygote进程中进行的。
2. 在第一个应用启动前，还会继续完成Dalvik内存模块的初始化工作，但该初始化active heap只会进行一次，由gDvm.newZygoteHeapAllocated布尔变量控制，即Zygote进程只会在fork第一个子进程的时候，才会将Java堆划一分为二来管理；这么设计是因为 We create a heap for all future zygote process allocations, in an attempt to avoid touching pages in the zygote heap。
3. 在Zygote的nativeFork()函数中还会调用dvmGcPreZygoteFork()函数，其中会调用函数dvmHeapSourceStartupBeforeFork()去初始化active堆，并把该active堆放到heap数组前面，以后无论是Zygote进程，还是Zygote子进程，需要分配对象时，都在Active堆上进行。这样就可以使得Zygote堆最大限度地在Zygote进程及其子进程中共享。

• dalvik/vm/native/dalvik_system_Zygote.cpp

static void Dalvik_dalvik_system_Zygote_fork(const u4* args, JValue* pResult)
{
    pid_t pid;
    dvmGcPreZygoteFork(); // 在fork前分配active堆
    setSignalHandler();
    dvmDumpLoaderStats("zygote");
    pid = fork();
    RETURN_INT(pid);
}

• dalvik/vm/alloc/Alloc.cpp

// Do any last-minute preparation before we call fork() for the first time. 
bool dvmGcPreZygoteFork() {
    return dvmHeapSourceStartupBeforeFork();
}

• dalvik/vm/alloc/HeapSource.cpp
  addNewHeap()函数主要的功能是创建了一个新的堆。
  创建的过程是将旧的heaps[0]第一页以后的内存地址空间分给了新的堆，然后对新堆的地址空间在原来地址的基础上重新执行mmap。接下来将heaps[0]指向的堆的尺寸减小为一页大小，最后将heaps[0]和heaps[1]的值交换。
  因此，两个堆都创建后，大小和以前还是一样，但是heaps[0]指向了一个新的、未分配内存的堆，而heaps[1]则包含了初始化时创建的内存对象，以后的内存分配都将在heaps[0]中进行。

/*
 * This is called while in zygote mode, right before we fork() for the
 * first time.  We create a heap for all future zygote process allocations,
 * in an attempt to avoid touching pages in the zygote heap.  (This would
 * probably be unnecessary if we had a compacting GC -- the source of our
 * troubles is small allocations filling in the gaps from larger ones.)
 */
bool dvmHeapSourceStartupBeforeFork()
{
    HeapSource *hs = gHs; // use a local to avoid the implicit "volatile"
    if (!gDvm.newZygoteHeapAllocated) {
        // Ensure heaps are trimmed to minimize footprint pre-fork.
        trimHeaps();
        // Create a new heap for post-fork zygote allocations.  We only try once, even if it fails.
        gDvm.newZygoteHeapAllocated = true;
        return addNewHeap(hs);
    }
    return true;
}


// Adds an additional heap to the heap source.  Returns false if there are too many heaps or insufficient free space to add another heap.
static bool addNewHeap(HeapSource *hs)
{
    Heap heap;
    memset(&heap, 0, sizeof(heap));

    // Heap storage comes from a common virtual memory reservation. The new heap will start on the page after the old heap.
    char *base = hs->heaps[0].brk;
    size_t overhead = base - hs->heaps[0].base;

    size_t morecoreStart = SYSTEM_PAGE_SIZE;
    heap.maximumSize = hs->growthLimit - overhead;
    heap.concurrentStartBytes = hs->minFree - CONCURRENT_START;
    heap.base = base;
    heap.limit = heap.base + heap.maximumSize;
    heap.brk = heap.base + morecoreStart;
    remapNewHeap(hs, &heap);
    heap.msp = createMspace(base, morecoreStart, hs->minFree);

    // Don't let the soon-to-be-old heap grow any further
    hs->heaps[0].maximumSize = overhead;
    hs->heaps[0].limit = base;
    mspace_set_footprint_limit(hs->heaps[0].msp, overhead);

    // Put the new heap in the list, at heaps[0]
    memmove(&hs->heaps[1], &hs->heaps[0], hs->numHeaps * sizeof(hs->heaps[0]));
    hs->heaps[0] = heap;
    hs->numHeaps++;

    return true;
}

/*
 * A helper for addNewHeap(). Remap the new heap so that it will have
 * a separate ashmem region with possibly a different name, etc. In
 * practice, this is used to give the app heap a separate ashmem
 * region from the zygote heap's.
 */
static bool remapNewHeap(HeapSource* hs, Heap* newHeap)
{
  char* newHeapBase = newHeap->base;
  size_t rem_size = hs->heapBase + hs->heapLength - newHeapBase;
  munmap(newHeapBase, rem_size);
  int fd = ashmem_create_region("dalvik-heap", rem_size);
  void* addr = mmap(newHeapBase, rem_size, PROT_READ | PROT_WRITE, MAP_PRIVATE, fd, 0);
  int ret = close(fd);
  return true;
}

参考：Dalvik虚拟机Java堆创建过程分析