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