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quicklist

quicklist是由ziplist构成的双向链表。ziplist需要可能对内存进行复制,在长度较长的时候,性能不佳。quicklist存储多个小ziplist,对除headtail外的节点还进行了压缩,保证了push和pop性能的同时,又减少了内存的占用。

结构

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typedef struct quicklistNode {
struct quicklistNode *prev;
struct quicklistNode *next;
unsigned char *zl;
unsigned int sz; /* ziplist size in bytes */
unsigned int count : 16; /* count of items in ziplist */
unsigned int encoding : 2; /* RAW==1 or LZF==2 */
unsigned int container : 2; /* NONE==1 or ZIPLIST==2 */
unsigned int recompress : 1; /* was this node previous compressed? */
unsigned int attempted_compress : 1; /* node can't compress; too small */
unsigned int extra : 10; /* more bits to steal for future usage */
} quicklistNode;

typedef struct quicklistLZF {
unsigned int sz; /* LZF size in bytes*/
char compressed[];
} quicklistLZF;

typedef struct quicklist {
quicklistNode *head;
quicklistNode *tail;
unsigned long count; /* total count of all entries in all ziplists */
unsigned long len; /* number of quicklistNodes */
int fill : 16; /* fill factor for individual nodes */
unsigned int compress : 16; /* depth of end nodes not to compress;0=off */
} quicklist;

quicklistLZF:存储压缩后的ziplist

quicklist

  • fill:存放list-max-ziplist-size参数
    • fill取正值时,表示entry个数
    • 取负值时,表示大小
  • compress:存放list-compress-depth参数

push

push时,先检查本次push是否会超过限制大小,quicklist->fill,依次按照用户定义大小、默认个数(SIZE_SAFETY_LIMIT)、用户定义个数检查。

  • 如果没有超过,那么在head的ziplist头部添加entry。
  • 如果超过,那么创建新的ziplist,并在quicklist->head前插入新node。最后压缩原来的quicklist->head节点。

quicklistPushTail也是类似的,但是在ziplist的尾部添加entry,且插入后是压缩原来的quicklist->tail节点。

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int quicklistPushHead(quicklist *quicklist, void *value, size_t sz) {
quicklistNode *orig_head = quicklist->head;
if (likely(
_quicklistNodeAllowInsert(quicklist->head, quicklist->fill, sz))) {
quicklist->head->zl =
ziplistPush(quicklist->head->zl, value, sz, ZIPLIST_HEAD);
quicklistNodeUpdateSz(quicklist->head);
} else {
quicklistNode *node = quicklistCreateNode();
node->zl = ziplistPush(ziplistNew(), value, sz, ZIPLIST_HEAD);

quicklistNodeUpdateSz(node);
_quicklistInsertNodeBefore(quicklist, quicklist->head, node);
}
quicklist->count++;
quicklist->head->count++;
return (orig_head != quicklist->head);
}