CMU 15213：malloc筆記和malloc實驗

1. malloc筆記

1.1 基礎概念

1.dynamic memory allocators：應用程序運行期間申請的內存（malloc）。

2.堆：dynamic memory allocator管理的進程內存區域
3.types of allocator：隱式分配器（new and garbage collection）和顯示分配器（malloc & free）

1.2 explicit allocator

1.malloc、free、realloc
2.16-byte (x86-64) alignment on 64-bit systems
3.內部碎片產生的原因：

• 維持堆所需的數據結構開銷
• 對齊的需要
• 分配器實現的策略（比如，to return a big block to satisfy a small request）

4.外部碎片：Occurs when there is enough aggregate heap memory, but no single free block is large enough。（大塊被劃分成小塊，雖然整個空閒內存能夠放下程序，但是找不到單獨的一塊去存放）

5.keep track of free blocks：顯式、隱式鏈表、Segregated free list（Different free lists for different size classes）、Blocks sorted by size（紅黑樹，長度爲key）

1.2.1 隱式空閒鏈表

1.block structures

typedef struct block
{
    word_t header;//包含1bit的是否分配、剩下的表示payload大小	
    unsigned char payload[0];
} block_t;

2.Header access

• Getting allocated bit from header return header & 0x1;
• Getting size from header return header & ~0xfL;
• Initializing header block->header = size | alloc;

3.Traversing list

static block_t *find_next(block_t *block)
{
    return (block_t *) ((unsigned char *) block 
                        + get_size(block));
}

4. Finding a Free Block

• first fit

static block_t *find_fit(size_t asize)
{
    block_t *block;
    for (block = heap_start; block != heap_end;
         block = find_next(block)) {
    {
      if (!(get_alloc(block)) 
          && (asize <= get_size(block)))
         return block;
    }
    return NULL; // No fit found
}

• next fit
• best fit

5.Allocating in Free Block and Splitting Free Block

6.Freeing a Block and Coalescing and Bidirectional Coalescing

1.2.2 顯式空閒鏈表

1.Unordered（插入時間複雜度O(1)）

• LIFO (last-in-first-out) policy
• FIFO (first-in-first-out) policy

2. Address-ordered policy（插入時間複雜度O(n)）

1.2.3 Segregated List

1.按照不同的大小劃分不同的List

2.To allocate a block of size n:

• Search appropriate free list for block of size m > n (i.e., first fit)
• If an appropriate block is found:
  Split block and place fragment on appropriate list
  If no block is found, try next larger class
• Repeat until block is found

3.If no block is found:

• Request additional heap memory from OS (using sbrk())
• Allocate block of n bytes from this new memory
• Place remainder as a single free block in appropriate size class.

1.3 隱式內存管理（垃圾收集）

1.How does the memory manager know when memory can be freed?

• In general we cannot know what is going to be used in the future since it depends on conditionals
• But we can tell that certain blocks cannot be used if there are no pointers to them

2.Must make certain assumptions about pointers

• Memory manager can distinguish pointers from non-pointers
• All pointers point to the start of a block
• Cannot hide pointers (e.g., by coercing them to an int, and then back again)：比如c語言，可以任意的使用指針，所以c語言不能夠自動清理垃圾

3.採用有向圖標記：堆中分配的block看成節點，邊看成指針。利用dfs算法，可以標記現在或者未來依然會使用的堆中的block。

malloc實驗

看了幾個博客，用隱式鏈表的話能得個50分左右，採用顯式空閒鏈表+LIFO或者顯式空閒鏈表+地址排序能得個80分左右，採用Segregated List得分較高，有100分左右。