1. VPP源碼分析（記憶體管理之mheap）

1.1. mheap

1.1.1. mheap_t

• first_free_elt_uoffset_by_bin: User offsets for head of doubly-linked list of free objects of this size.
• non_empty_free_elt_heads: Bitmap of non-empty free list bins.
• n_elts: Number of allocated objects.
• max_size: Maximum size (in bytes) this heap is allowed to grow to.

  

1.1.2. mheap_elt_t

• prev_n_user_data: Number of mheap_size_t words of user data in previous object. Used to find mheap_elt_t for previous object.
• prev_is_free: Set if previous object is free.
• n_user_data: Number of mheap_size_t words of user data that follow this object.
• is_free: Set if this object is on free list (and therefore following free_elt is valid).

• user_data: For allocated objects: user data follows.

  User data is allocated in units of typeof (user_data[0]).

• free_elt: For free objects, offsets of next and previous free objects of this size;

  ~0 means end of doubly-linked list.

This is stored in user data (guaranteed to be at least 8 bytes) but only for free objects.

1.1.3. mheap_alloc函數過程

從虛拟記憶體空間映射一塊size大的記憶體(am = 0x7ffff3a0ff54)

mmap_addr = mmap(0, size, PROT_READ | PROT_WRITE, flags, -1, 0);           

1. 将mmap_addr指針和頁大小對其(av = 0x7ffff3a10000)

   return (addr + mheap_page_size - 1) & ~(mheap_page_size - 1);

2. 從對齊頁的位置往前減去mheap_t的空間(ah = 0x7ffff3a0f860)

   ah = vec_aligned_header(v, sizeof (mheap_t), 16);

3. 再往後n個page，(ah + page = 0x7ffff3a10860)得到真正的mheap開始的位置【這邊類似于補運算，在申請的空間内跳過page_size - mheap_size 的空間】

4. (ah < am)

   ah += mheap_page_size; // 這樣可以保證mheap的其實位置在申請的空間内，且一定對其與page

5. 最後計算出vector的偏移

   v = mheap_vector (h); // h + sizeof(mheap_t + vec_header_t)

6. vector區域可以存放的資料大小為

   size = am + memory_size - v;

1.1.4. 從mheap中申請記憶體

void *     mheap_get_aligned(void *v, uword n_user_data_bytes, uword align, uword align_offset, uword * offset_return)           

/* Search free lists for object with given size and alignment. */     static uword     mheap_get_search_free_list(void *v, uword *n_user_bytes_arg, uword align, uword align_offset)           

/* Find bin for objects with size at least n_user_data_bytes. */     always_inline uword     user_data_size_to_bin_index(uword n_user_data_bytes)           

根據使用者請求object的size and alignment找到合适的bin(small_bin or large_bin)，再從bin中找到合适的free elt

這裡所有的操作都是基于數組的形式，elt間的雙向連結清單也是以數組的形式實作的

• 部分圖檔來源 https://blog.csdn.net/jqh9804/article/details/52873100