Add zcalloc and use it where appropriate

calloc is more effecient than malloc+memset when the system uses mmap to
allocate memory. mmap always returns zeroed memory so the memset can be
avoided.  The threshold to use mmap is 16k in osx libc and 128k in bsd
libc and glibc. The kernel can lazily allocate the pages, this reduces
memory usage when we have a page table or hash table that is mostly
empty.

This change is most visible when you start a new redis instance with vm
enabled.  You'll see no increased memory usage no matter how big your
page table is.

src/dict.c

@@ -148,14 +148,12 @@ int dictExpand(dict *d, unsigned long size)
     if (dictIsRehashing(d) || d->ht[0].used > size)
         return DICT_ERR;
 
+    /* Allocate the new hashtable and initialize all pointers to NULL */
     n.size = realsize;
     n.sizemask = realsize-1;
-    n.table = zmalloc(realsize*sizeof(dictEntry*));
+    n.table = zcalloc(realsize*sizeof(dictEntry*));
     n.used = 0;
 
-    /* Initialize all the pointers to NULL */
-    memset(n.table, 0, realsize*sizeof(dictEntry*));
-
     /* Is this the first initialization? If so it's not really a rehashing
      * we just set the first hash table so that it can accept keys. */
     if (d->ht[0].table == NULL) {

src/vm.c

@@ -86,10 +86,9 @@ void vmInit(void) {
     } else {
         redisLog(REDIS_NOTICE,"Swap file allocated with success");
     }
-    server.vm_bitmap = zmalloc((server.vm_pages+7)/8);
+    server.vm_bitmap = zcalloc((server.vm_pages+7)/8);
     redisLog(REDIS_VERBOSE,"Allocated %lld bytes page table for %lld pages",
         (long long) (server.vm_pages+7)/8, server.vm_pages);
-    memset(server.vm_bitmap,0,(server.vm_pages+7)/8);
 
     /* Initialize threaded I/O (used by Virtual Memory) */
     server.io_newjobs = listCreate();

src/zmalloc.c

@@ -89,6 +89,20 @@ void *zmalloc(size_t size) {
 #endif
 }
 
+void *zcalloc(size_t size) {
+    void *ptr = calloc(1, size+PREFIX_SIZE);
+
+    if (!ptr) zmalloc_oom(size);
+#ifdef HAVE_MALLOC_SIZE
+    increment_used_memory(redis_malloc_size(ptr));
+    return ptr;
+#else
+    *((size_t*)ptr) = size;
+    increment_used_memory(size+PREFIX_SIZE);
+    return (char*)ptr+PREFIX_SIZE;
+#endif
+}
+
 void *zrealloc(void *ptr, size_t size) {
 #ifndef HAVE_MALLOC_SIZE
     void *realptr;

src/zmalloc.h

@@ -32,6 +32,7 @@
 #define _ZMALLOC_H
 
 void *zmalloc(size_t size);
+void *zcalloc(size_t size);
 void *zrealloc(void *ptr, size_t size);
 void zfree(void *ptr);
 char *zstrdup(const char *s);