Tcmalloc源码简单分析(10)

Static::InitStaticVars()最后将调用Sampler::InitStatics()函数，此函数主要初始化sampler的采样间隔之类的。到此Static::InitStaticVars介绍完毕，我们返回ThreadCache::InitModule()，接着static::InitStaticVars开始初始化threadcache_allocator，threadcache_allocator的定义是PageHeapAllocator，他的初始化函数也是PageHeapAllocator::init，此函数在上面已经解释。初始化结束后，设置phinited为1防止多次初始化。

完成InitModule的解释，我们返回函数inline ThreadCache* ThreadCache::GetCache，由于ptr为null，那么调用:CreateCacheIfNecessary创建一个threadcache。代码分析如下

ThreadCache* ThreadCache::CreateCacheIfNecessary() {

  // Initialize per-thread data if necessary

  ThreadCache* heap = NULL;

  {

    SpinLockHolder h(Static::pageheap_lock());

//首先获取整个pageheap的锁

    // Early on in glibc's life, we cannot even call pthread_self()

    pthread_t me;

    if (!tsd_inited_) {

      memset(&me, 0, sizeof(me));//由于尚未初始化不能callll pthread_self，因此将me置为0

    } else {

      me = pthread_self();//如果已经初始化完毕，那么直接调用pthread_self获得tid

    }

 

    // This may be a recursive malloc call from pthread_setspecific()

    // In that case, the heap for this thread has already been created

    // and added to the linked list.  So we search for that first.

    for (ThreadCache* h = thread_heaps_; h != NULL; h = h->next_) {

      if (h->tid_ == me) {

        heap = h;

        break;

      }

    }

//通过对thread_heaps_列表进行遍历，判断本thread的cache是否已经创建

    if (heap == NULL) heap = NewHeap(me);

  }

//如果没有找到，那么调用NewHeap分配一个新的heap

//下面部分没怎么看懂，应该是对pthread_setspecific调用进行特殊处理，防止对malloc的递归call方面。

  // We call pthread_setspecific() outside the lock because it may

  // call malloc() recursively.  We check for the recursive call using

  // the "in_setspecific_" flag so that we can avoid calling

  // pthread_setspecific() if we are already inside pthread_setspecific().

  if (!heap->in_setspecific_ && tsd_inited_) {

    heap->in_setspecific_ = true;

    perftools_pthread_setspecific(heap_key_, heap);

#ifdef HAVE_TLS

    // Also keep a copy in __thread for faster retrieval

    threadlocal_heap_ = heap;

#endif

    heap->in_setspecific_ = false;

  }

  return heap;

}

下面我们分析一下NewHeap函数，NewHeap函数通过threadcache_allocator.New分配一个threadCache，并调用ThreadCache.init进行初始化，并将新生成的heap加入thread_heaps_列表，这里有一个特殊的变量next_memory_steal，用来标识当当一个thread的thread_Cache超过max_size_时从哪个cache偷内存，next_memory_steal采用Round-robin方式在各个thread cache中转移。

ThreadCache* ThreadCache::NewHeap(pthread_t tid) {

  // Create the heap and add it to the linked list

  ThreadCache *heap = threadcache_allocator.New();

  heap->Init(tid);

  heap->next_ = thread_heaps_;

  heap->prev_ = NULL;

  if (thread_heaps_ != NULL) {

    thread_heaps_->prev_ = heap;

  } else {

    // This is the only thread heap at the momment.

    ASSERT(next_memory_steal_ == NULL);

    next_memory_steal_ = heap;

  }

  thread_heaps_ = heap;

  thread_heap_count_++;

  return heap;

}

在NewHeap中关键的函数是threadcache_allocator.New，threadcache_allocator是一个PageHeapAllocator的对象，他的New函数在前面我们已经分析过了，主要是从threadcache_allocator的free_list里面切出一个ThreadCache对象并赋值给heap。