[lab] csapp-malloc

malloc lib

该lab要求实现 mm_init, mm_malloc, mm_free, mm_realloc 接口

准备工作

这次的lab没有step-by-step的提示, 要求你自己找到解决方案, 选择一种并实现.

首先是测试用例文件, 从官网上下载下来没有包含完整的, 只有两个简单的用例, 我找到 utah 大学的课程网站上的用例文件, 可以参考, 但这里的也缺少了 realloc 的用例.

其次是如何实现, 首先基于第9章已有的实现, 然后参考 utah 大学课程的 slide

思路

初步实现思路为

• 已分配块 只使用头部
• 使用显式空闲链表

得到结果:

Results for mm malloc:
trace  valid  util     ops      secs  Kops
 0       yes   89%    5694  0.001513  3764
 1       yes   91%    5848  0.001202  4864
 2       yes   94%    6648  0.002109  3152
 3       yes   97%    5380  0.001928  2790
 4       yes   80%   14402  0.001270 11345
 5       yes   88%    4800  0.003394  1414
 6       yes   88%    4800  0.003437  1397
 7       yes   55%   12000  0.006543  1834
 8       yes   51%   24000  0.007400  3243
Total          81%   83572  0.028796  2902

Perf index = 49 (util) + 40 (thru) = 89/100

不足:

最后两个用例效果不好, 可以观察到两个用例都是先将一整块(512), 分成两部分malloc(64+448), 最后将一部分free(448), 然后请求分配一整块(512), 对于我的实现, 不重新利用free掉的内存, 从而内存利用率下降. 一个比较好的方案是 Segregated Free List, 将不同大小的块分别放在不同的空闲链表中, 比如当请求分配一块大小为64的时候, 从(32,64]的空闲链表中取出一个块, 如果没有, 则分配一个新的块, 如果有, 则直接使用. 这里我实现时参考了博客.

然后还要对小内存块分配进行优化, 考虑到小内存块和大内存块交替分配的场景, 就算使用了Segregated Free List还是没有解决内存碎片, 可以开辟连续的空间, 加载到单独的链表上, 专门给小内存分配使用, 这样大段内存和小段内存分开分配, 可以减轻内存碎片现象

加入优化

• Segregated Free List
• 单独的内存 chunk 只分配小内存

trace  valid  util     ops      secs  Kops
 0       yes   99%    5694  0.001857  3066
 1       yes   96%    5848  0.001976  2959
 2       yes  100%    6648  0.002472  2689
 3       yes  100%    5380  0.003225  1668
 4       yes   80%   14402  0.002151  6697
 5       yes   96%    4800  0.009382   512
 6       yes   95%    4800  0.004393  1093
 7       yes   97%   12000  0.002544  4717
 8       yes   52%   24000  0.003696  6494
Total          90%   83572  0.031695  2637

Perf index = 54 (util) + 40 (thru) = 94/100

性能的分数已经满了, 可惜空间利用率还差一点.

对最后一个情况还是不行, 因为最后一个两次分配都是小内存, 我认为还要优化的话可以采用随机化思路, first random fit, 给定一个较小的概率, 在遇到合适的内存块时根据这个概率拒绝, 由于 debug 过于痛苦, 这个思路就暂时不去实现了.

代码

代码我将 list_node 的一些操作移到了单独的文件里, 实现比较简单, 这里只给出头文件

// 64 will let a better performance for binary2, but worse performace for binary
#define SMALL_BLOCK_SIZE 128

typedef struct list_node {
 struct list_node *next;
 struct list_node *prev;
} list_node, *list_ptr;

// add to list, choose list by get_free_list_idx.
void add_to_free_list(list_ptr bp, size_t size);
// choose the free list header for given size.
list_ptr get_free_list_idx(size_t size);
// list add.
void add_to_list(list_ptr bp, list_ptr list);
// list remove.
void remove_from_free_list(list_ptr node);

然后是mm.c

/* single word (4) or double word (8) alignment */
#define ALIGNMENT 8

/* rounds up to the nearest multiple of ALIGNMENT */
#define ALIGN(size) (((size) + (ALIGNMENT-1)) & ~0x7)

#define WSIZE 4
#define DSIZE 8
#define MIN_BLOCK 16 // 2 pointers + HEADER + 4 for align
#define CHUNKSIZE (1 << 12)

#define MAX(x, y) ((x) > (y) ? (x) : (y))

#define PACK(size, prev_alloc ,alloc) ((size) | (prev_alloc << 1) |(alloc))

#define GET(P) (*(size_t*)(P))
#define PUT(P, val) (*(size_t*)(P) = (val))

#define GET_SIZE(P) (GET(P) & ~0x7)
#define GET_ALLOC(P) (GET(P) & 0x1)
#define GET_PREV_ALLOC(P) ((GET(P) & 0x2) >> 1)

/* Given block ptr bp, compute address of its header and footer */
#define HDRP(bp) ((char*)bp - WSIZE)
#define FTRP(bp) ((char*)bp + GET_SIZE(HDRP(bp)) - DSIZE)

#define NEXT_BLKP(bp) ((char*)(bp) + GET_SIZE(((char*)(bp)-WSIZE)))
#define PREV_BLKP(bp) ((char*)(bp) - GET_SIZE(((char*)(bp)-DSIZE)))

static void* heap_listp;
void* find_fit(size_t size);
void place(void* bp, size_t size);
static void* extend_heap(size_t words, int prev_alloced);
static void* coalesce(void* bp);

static void update_next(void* bp, int alloc);
static void* find_for_small(size_t size);

static int m_verbose = 0;

extern list_ptr free_list_base;
static void* free_list_end, *free_list_small_end;

#define ENABLE_SMALL_OPMIZATION

void print_free_list(const char* msg) {
    void* list = free_list_base;
    list_ptr cur;
    size_t i = 0;
if (m_verbose != 2) {
    return;
}
    puts("==============");
    if (msg) {
        puts(msg);
    }
    while (list != (free_list_end + WSIZE)) {
        printf("FreeList[%d]:\n", i);
        for (cur = ((list_ptr)list)->next; cur != NULL; cur = cur->next) {
            printf("%p 0x%x\t", cur, GET(HDRP(cur)));
        }
        list += WSIZE; 
        i++;
        puts("");
    }
    puts("==============");
}

list_ptr get_free_list_idx(size_t size) {
    size_t i;
#ifdef ENABLE_SMALL_OPMIZATION
    if (size < SMALL_BLOCK_SIZE) {
        i = 5;
    } else if (size < 512) {
        i = 2;
    } else if (size < 1024) {
        i = 3;
    } else {
        i = 4;
    }
#else // ENABLE_SMALL_OPMIZATION
    if (size < 32) {
        i = 0;
    } else if (size < 128) {
        i = 1;
    } else if (size < 512) {
        i = 2;
    } else if (size < 1024) {
        i = 3;
    } else {
        i = 4;
    } 
#endif // ENABLE_SMALL_OPMIZATION

    return (list_ptr)((unsigned int*)free_list_base + i);
}

/* 
 * mm_init - initialize the malloc package.
 */
int mm_init(void)
{
    mem_init();
    if ((heap_listp = mem_sbrk(8 * WSIZE)) == (void*)-1)
        return -1;

    PUT(heap_listp, 0);           //block size <= 32   
    PUT(heap_listp+(1*WSIZE), 0); //block size <= 128
    PUT(heap_listp+(2*WSIZE), 0); //block size <= 512
    PUT(heap_listp+(3*WSIZE), 0); //block size <= 1024
    PUT(heap_listp+(4*WSIZE), 0); //block size > 1024
    PUT(heap_listp+(5*WSIZE), 0); //small block alloc 
    PUT(heap_listp+(6*WSIZE), PACK(0, 1, 1));
    PUT(heap_listp+(7*WSIZE), PACK(0, 1, 1));
    free_list_base = (list_ptr)heap_listp;
    free_list_end = heap_listp + (5*WSIZE);
    free_list_small_end = heap_listp + (2*WSIZE);
    heap_listp += (6*WSIZE);

    if (extend_heap(CHUNKSIZE / WSIZE, 1) == NULL)
        return -1;
    
if (m_verbose) {
    printf("INFO: mm_init finished\n");
}
    return 0;
}

/* 
 * mm_malloc - Allocate a block by incrementing the brk pointer.
 *     Always allocate a block whose size is a multiple of the alignment.
 */
void *mm_malloc(size_t size)
{
print_free_list("before malloc");
    size_t asize;
    char* bp;

    if (size == 0)
        return NULL;

    if (size <= 2 * WSIZE)
        asize = MIN_BLOCK;
    else
        asize = DSIZE * ((size + WSIZE + (DSIZE - 1)) / DSIZE);

#ifdef ENABLE_SMALL_OPMIZATION
    if (asize < SMALL_BLOCK_SIZE) {
        bp = find_for_small(asize);

if (m_verbose) {
printf("malloc %p %d\n", bp, asize);
}
        return bp;
    }
#endif // ENABLE_SMALL_OPMIZATION
 
    if ((bp = find_fit(asize)) != NULL) {
        place(bp, asize);

if (m_verbose) {
printf("malloc %p %d\n", bp, asize);
}
       return bp;
    }
    
    if ((bp = extend_heap(MAX(asize, CHUNKSIZE) / WSIZE, 0)) == NULL)
        return NULL;

if (m_verbose) {
printf("malloc %p %d\n", bp, asize);
}
    place(bp, asize);
    return bp;
}


/*
 * mm_free - Freeing a block does nothing.
 */
void mm_free(void *bp)
{
print_free_list("before free");
    size_t size = GET_SIZE(HDRP(bp));
    size_t prev_alloc = GET_PREV_ALLOC(HDRP(bp));

    PUT(HDRP(bp), PACK(size, prev_alloc, 0));
    PUT(FTRP(bp), PACK(size, prev_alloc, 0));
    
    update_next(bp, 0);
    coalesce(bp);

if (m_verbose) {
printf("freed %p %d\n", bp, size);
}
}

/*
 * mm_realloc - Implemented simply in terms of mm_malloc and mm_free
 */
void *mm_realloc(void *ptr, size_t size)
{
    void *newptr = ptr;
    size_t copySize = GET_SIZE(HDRP(ptr));
    if (copySize >= size) {
        // Reuse current memory.
        place(ptr, copySize);
    } else {
        newptr = mm_malloc(size);
        if (newptr == NULL)
           return NULL;
        memcpy(newptr, ptr, copySize);
        mm_free(ptr);
    }
    return newptr;
}

static void* coalesce(void* bp) {
    void* prev_block = PREV_BLKP(bp);
    void* next_block = NEXT_BLKP(bp);
    size_t prev_alloc = GET_PREV_ALLOC(HDRP(bp));
    size_t next_alloc = GET_ALLOC(HDRP(next_block));
    size_t size = GET_SIZE(HDRP(bp));
    void* new_bp = bp;
    if (prev_alloc && next_alloc) {
        add_to_free_list(bp, size);
        update_next(bp, 0);
        return bp;
    } 
     if (!next_alloc) {
        // Merge with next block.
        // putchar('1');
        remove_from_free_list((list_ptr)next_block);

        size += GET_SIZE(HDRP(next_block));
    } 
     if (!prev_alloc) {
        // Merge with previout block.
        // putchar('2');

        remove_from_free_list((list_ptr)prev_block);

        size += GET_SIZE(HDRP(prev_block));
        // bp = prev_block;
        new_bp = prev_block;
    } 
    PUT(HDRP(new_bp), PACK(size, 1, 0));
    PUT(FTRP(new_bp), PACK(size, 1, 0));
    add_to_free_list(new_bp, size);
    update_next(new_bp, 0);
    return new_bp;
}

static void* extend_heap(size_t words, int prev_alloced) {
    char* bp;
    size_t size;

    if (words % 2) {
        words += 1;
    }

    size = words * WSIZE;
    if ((bp = mem_sbrk(size)) == (void*)-1)
        return NULL;
    if (!prev_alloced) {
        prev_alloced = GET_PREV_ALLOC(HDRP(bp));
    }
    PUT(HDRP(bp), PACK(size, prev_alloced, 0));
    PUT(FTRP(bp), PACK(size, prev_alloced, 0));
    // No size, prev not alloced, self alloced to disable merge. 
    PUT(HDRP(NEXT_BLKP(bp)), PACK(0, 0, 1));

    return coalesce(bp);
}


void* find_first_fit(size_t size) {
    list_ptr bp;
    void* free_list = get_free_list_idx(size);
    void* list_end = free_list_end;
// #ifdef ENABLE_SMALL_OPMIZATION
//     if (size < SMALL_BLOCK_SIZE) {
//         list_end = free_list_small_end;
//     }
// #endif // ENABLE_SMALL_OPMIZATION
    for (; free_list < list_end; free_list += WSIZE) {
        for (bp = ((list_ptr)free_list)->next; bp != NULL ; bp = bp->next) {
            if (size <= GET_SIZE(HDRP(bp))) {
                return bp;
            }
        }
    }
    return NULL;
}

void* find_best_fit(size_t size) {
    list_ptr bp = NULL;
    list_ptr best_bp = NULL;
    void* free_list = get_free_list_idx(size);
    void* list_end = free_list_end;
// #ifdef ENABLE_SMALL_OPMIZATION
//     if (size < SMALL_BLOCK_SIZE) {
//         list_end = free_list_small_end;
//     }
// #endif // ENABLE_SMALL_OPMIZATION
    for (; free_list < list_end && best_bp == NULL; free_list += WSIZE) {
        for (bp = ((list_ptr)free_list)->next; bp != NULL ; bp = bp->next) {
            if (GET_SIZE(HDRP(bp)) >= size && 
                (best_bp == NULL || GET_SIZE(HDRP(bp)) < GET_SIZE(HDRP(best_bp)))) {
                // return bp;
                best_bp = bp;
            }
        }
    }
    return best_bp;
}


static void* find_for_small(size_t size) {
    list_ptr bp;
    list_ptr small_block_list;
    size_t asize;
    void* next_bp;
    small_block_list = (list_ptr)free_list_end;

print_free_list("before find small");

    for (bp = small_block_list->next; bp != NULL ; bp = bp->next) {
        if (GET_SIZE(HDRP(bp)) >= size) {
            break;            
        }
    }

    if (bp == NULL) {
        if ((bp = extend_heap(MAX(size, CHUNKSIZE) / WSIZE, 0)) == NULL)
            return NULL;
        remove_from_free_list((list_ptr)bp);
        // add to free list
        add_to_list((list_ptr)bp, small_block_list);
    } 
    asize = GET_SIZE(HDRP(bp));
    remove_from_free_list((list_ptr)bp);

    if (asize - size < MIN_BLOCK) {
        // not split
        PUT(HDRP(bp), PACK(asize, 1, 1));

        update_next(bp, 1);
    } else {
        PUT(HDRP(bp), PACK(size, 1, 1));
        // Split out a new block.
        if (asize - size > CHUNKSIZE) {
            // Split again if new block is larger than chunksize
            void* nn_bp;
            next_bp = NEXT_BLKP(bp);
            PUT(HDRP(next_bp), PACK(CHUNKSIZE, 1, 0));
            PUT(FTRP(next_bp), PACK(CHUNKSIZE, 1, 0));
            nn_bp = NEXT_BLKP(next_bp);
            PUT(HDRP(nn_bp), PACK(asize - size - CHUNKSIZE, 0, 0));
            PUT(FTRP(nn_bp), PACK(asize - size - CHUNKSIZE, 0, 0));
            add_to_free_list(nn_bp, GET_SIZE(HDRP(nn_bp)));
        } else {
            next_bp = NEXT_BLKP(bp);
            PUT(HDRP(next_bp), PACK(asize - size, 1, 0));
            PUT(FTRP(next_bp), PACK(asize - size, 1, 0));
        }
        add_to_list(next_bp, small_block_list);
    }

print_free_list("after find small");

    return bp;
}

void* find_fit(size_t size) {
    return find_best_fit(size);
    // return find_first_fit(size);
} 

void place(void* bp, size_t size) {
    size_t asize;
    void* next_bp;

    asize = GET_SIZE(HDRP(bp));
    remove_from_free_list((list_ptr)bp);
    if (asize - size < MIN_BLOCK) {
        // not split
        PUT(HDRP(bp), PACK(asize, GET_PREV_ALLOC(HDRP(bp)), 1));

        update_next(bp, 1);
    } else {
        PUT(HDRP(bp), PACK(size, GET_PREV_ALLOC(HDRP(bp)), 1));
        // Split out a new block.
        next_bp = NEXT_BLKP(bp);
        PUT(HDRP(next_bp), PACK(asize - size, 1, 0));
        PUT(FTRP(next_bp), PACK(asize - size, 1, 0));
        add_to_free_list(next_bp, asize - size);
    }
}

/**
 *  When bp is alloced or freed, update `prev_alloc` flag for the block next to bp
 */
static void update_next(void* bp, int alloc) {
    char* next_block = HDRP(NEXT_BLKP(bp));
    size_t next_alloc = GET_ALLOC(next_block);
    size_t next_size = GET_SIZE(next_block);
    PUT(next_block, PACK(next_size, alloc, next_alloc));
    if (!next_alloc) {
        PUT(FTRP(NEXT_BLKP(bp)), PACK(next_size, alloc, next_alloc));
    }
}

其实要干的就两件事, 维护 free list, 维护 header, 首先区分好函数影响的范围, 然后去做维护, 这样代码就更加有条理.

比如 free, 首先要更新我 header 的 alloc , footer 和 next block 的 prev_alloc, 然后交给 coalesce 去合并和更新 free list.

malloc
  find_for_small
  find_fit
  extend_heap
    coalesce
      add_to_free_list
      remove_to_free_list
  place
    add_to_free_list

free
  coalesce
    add_to_free_list
    remove_to_free_list

一些总结和技巧

• pointer size : 32位编译, 4字节
• pointer add : 指针运算是按类型走的, list_ptr + 4 就会直接越界, 使用 void* 就是按字节加.
• 熟悉块的内存结构
  • |header|payload| allocated
  • |header|next, prev|footer| unallocated
• header = size | prev_alloc << 1 | alloc
  • 因为size是8字节对齐的, 余出3个bit 可以塞flag
  • 有了 prev_alloc , 已分配的内存块就不需要 footer, 因为合并我们知道他已经分配, 就不需要在看 header 信息.
• header 中的 size 是带上 overhead 的已分配 size, 用户 malloc 所期望返回的是一个大于等于 target-size 的内存块, 我们在它上面做了什么它并不关心, 因此在代码内部的 size 永远是块的真正 size(从header开始算, 而不是payload开始, 这里有时候会迷, 因为代码是以payload为基准).
• 为内存块设置结构体(header, free list), 这样的好处是你可以方便的从gdb里看到它的内容, 而不是手动去看内存. 你可以不按结构体存, 继续使用宏定义操作它, 需要用到结构体时直接将它转成对应结构体指针就好(类似于直接当成栈来用).
• 关于 debug, 可以尝试 vscode remote + gdb, 比 gdb 的 gui 模式方便许多.
• debug 2, mdriver 自带了一个 alloc 检查, 可以检查 overlapping / align / out of boundary. 后面我发现写一个 print_free_list, 在调试小用例时会很有帮助.