Redis源码分析：内存管理

源码版本： redis 2.4.4

redis内存相关函数都放在zmalloc.h zmalloc.c中

redis中可以使用tcmalloc、jemalloc
Makefile：

ifeq ($(USE_TCMALLOC),yes)
  ALLOC_DEP=
  ALLOC_LINK=-ltcmalloc
  ALLOC_FLAGS=-DUSE_TCMALLOC
endif

ifeq ($(USE_JEMALLOC),yes)
  ALLOC_DEP=../deps/jemalloc/lib/libjemalloc.a
  ALLOC_LINK=$(ALLOC_DEP) -ldl
  ALLOC_FLAGS=-DUSE_JEMALLOC -I../deps/jemalloc/include
endif

zmalloc.c中

#if defined(USE_TCMALLOC)
#define malloc(size) tc_malloc(size)
#define calloc(count,size) tc_calloc(count,size)
#define realloc(ptr,size) tc_realloc(ptr,size)
#define free(ptr) tc_free(ptr)

#elif defined(USE_JEMALLOC)
#define malloc(size) je_malloc(size)
#define calloc(count,size) je_calloc(count,size)
#define realloc(ptr,size) je_realloc(ptr,size)
#define free(ptr) je_free(ptr)
#endif

简单封装
void *zmalloc(size_t size);
void *zcalloc(size_t size);
void *zrealloc(void *ptr, size_t size);
void zfree(void *ptr);
分别对malloc、calloc、realloc、free做了简单封装
对zmalloc和zfree做分析

void *zmalloc(size_t size) {
    void *ptr = malloc(size+PREFIX_SIZE);

    if (!ptr) zmalloc_oom(size);
#ifdef HAVE_MALLOC_SIZE
    update_zmalloc_stat_alloc(zmalloc_size(ptr),size);
    return ptr;
#else
    *((size_t*)ptr) = size;
    update_zmalloc_stat_alloc(size+PREFIX_SIZE,size);
    return (char*)ptr+PREFIX_SIZE;
#endif
}
void zfree(void *ptr) {
#ifndef HAVE_MALLOC_SIZE
    void *realptr;
    size_t oldsize;
#endif

    if (ptr == NULL) return;
#ifdef HAVE_MALLOC_SIZE
    update_zmalloc_stat_free(zmalloc_size(ptr));
    free(ptr);
#else
    realptr = (char*)ptr-PREFIX_SIZE;
    oldsize = *((size_t*)realptr);
    update_zmalloc_stat_free(oldsize+PREFIX_SIZE);
    free(realptr);
#endif
}

除了分配给指定大小的内存之外，还分配 了PREFIX_SIZE
内存格局如下：

申请内存返回Real_ptr

#define update_zmalloc_stat_alloc(__n,__size) do { \
    size_t _n = (__n); \
    if (_n&(sizeof(long)-1)) _n += sizeof(long)-(_n&(sizeof(long)-1)); \
    if (zmalloc_thread_safe) { \
        pthread_mutex_lock(&used_memory_mutex);  \
        used_memory += _n; \
        pthread_mutex_unlock(&used_memory_mutex); \
    } else { \
        used_memory += _n; \
    } \
} while(0)

#define update_zmalloc_stat_free(__n) do { \
    size_t _n = (__n); \
    if (_n&(sizeof(long)-1)) _n += sizeof(long)-(_n&(sizeof(long)-1)); \
    if (zmalloc_thread_safe) { \
        pthread_mutex_lock(&used_memory_mutex);  \
        used_memory -= _n; \
        pthread_mutex_unlock(&used_memory_mutex); \
    } else { \
        used_memory -= _n; \
    } \
} while(0)

used_memory记录当前分配的总内存
在定义了 int vm_max_threads; /* Max number of I/O threads running at the same time */
    if (server.vm_max_threads != 0)
        zmalloc_enable_thread_safeness(); /* we need thread safe zmalloc() */
在使用Threaded Virtual Memory I/O的时候，需要安全的zmalloc

size_t zmalloc_used_memory(void) {
    size_t um;

    if (zmalloc_thread_safe) pthread_mutex_lock(&used_memory_mutex);
    um = used_memory;
    if (zmalloc_thread_safe) pthread_mutex_unlock(&used_memory_mutex);
    return um;
}

zmalloc_used_memory返回进程当前使用的内存，用以做相应的清内存操作，eg：

zmalloc_used_memory() > server.maxmemory //和配置的最大内存比较
....

zmalloc_get_rss()用来计算 进程实际使用物理内存