Redis笔记之快速列表

quicklist 实际上是 zipList 和 linkedList 的混合体，它将 linkedList 按段切分，每一段使用 zipList 来紧凑存储，多个 zipList 之间使用双向指针链接。

快速列表的定义

列表的节点

// quicklist.h
/* quicklistNode is a 32 byte struct describing a ziplist for a quicklist.
 * We use bit fields keep the quicklistNode at 32 bytes.
 * count: 16 bits, max 65536 (max zl bytes is 65k, so max count actually < 32k).
 * encoding: 2 bits, RAW=1, LZF=2.
 * container: 2 bits, NONE=1, ZIPLIST=2.
 * recompress: 1 bit, bool, true if node is temporary decompressed for usage.
 * attempted_compress: 1 bit, boolean, used for verifying during testing.
 * extra: 10 bits, free for future use; pads out the remainder of 32 bits */
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;

• prev: 指向链表前一个节点的指针。
• next: 指向链表后一个节点的指针。
• zl: 数据指针。如果当前节点的数据没有压缩，那么它指向一个ziplist结构；否则，它指向一个quicklistLZF结构。
• sz: 表示zl指向的ziplist的总size（包括zlbytes, zltail, zllen, zlend和各个数据项）。注意：如果ziplist被压缩了，这个sz的值仍然是压缩前的ziplist大小。
• count: 表示ziplist里面包含的数据项个数。
• encoding: 表示ziplist是否压缩了（以及用了哪个压缩算法）。目前只有两种取值。
• container: 是一个预留字段。本来设计是用来表明一个quicklist节点下面是直接存数据，还是使用ziplist存数据，或者用其它的结构来存数据（用作一个数据容器，所以叫container）。但是，在目前的实现中，这个值是一个固定的值2，表示使用ziplist作为数据容器。
• recompress: 使用例如index命令查看某一项本来压缩的数据时，需要把数据暂时解压，这时设置recompress=1做标记解压状态，后面有机会再压缩数据。
• attempted_compress: 这个值只对Redis的自动化测试程序有用。
• extra: 其它扩展字段，暂时不管。

压缩后的ziplist

// quicklist.h
/* quicklistLZF is a 4+N byte struct holding 'sz' followed by 'compressed'.
 * 'sz' is byte length of 'compressed' field.
 * 'compressed' is LZF data with total (compressed) length 'sz'
 * NOTE: uncompressed length is stored in quicklistNode->sz.
 * When quicklistNode->zl is compressed, node->zl points to a quicklistLZF */
typedef struct quicklistLZF {
    unsigned int sz; /* LZF size in bytes*/
    char compressed[];
} quicklistLZF;

quicklistLZF结构表示一个被压缩过的ziplist.

quicklist定义

// quicklist.h
/* quicklist is a 40 byte struct (on 64-bit systems) describing a quicklist.
 * 'count' is the number of total entries.
 * 'len' is the number of quicklist nodes.
 * 'compress' is: 0 if compression disabled, otherwise it's the number
 *                of quicklistNodes to leave uncompressed at ends of quicklist.
 * 'fill' is the user-requested (or default) fill factor.
 * 'bookmakrs are an optional feature that is used by realloc this struct,
 *      so that they don't consume memory when not used. */
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 : QL_FILL_BITS;              /* fill factor for individual nodes */
    unsigned int compress : QL_COMP_BITS; /* depth of end nodes not to compress;0=off */
    unsigned int bookmark_count: QL_BM_BITS;
    quicklistBookmark bookmarks[];
} quicklist;

• head: 指向头节点（左侧第一个节点）的指针。
• tail: 指向尾节点（右侧第一个节点）的指针。
• count: 所有ziplist数据项的个数总和。
• len: quicklist节点的个数。
• fill: 16bit，ziplist大小设置，存放list-max-ziplist-size的值。
• compress: 16bit，节点压缩深度值，存放list-compress-depth的值。
• bookmark_count：4bit，bookmark的计数值。
• bookmarks：bookmark列表。

bookmark的定义

// quicklist.h
/* Bookmarks are padded with realloc at the end of of the quicklist struct.
 * They should only be used for very big lists if thousands of nodes were the
 * excess memory usage is negligible, and there's a real need to iterate on them
 * in portions.
 * When not used, they don't add any memory overhead, but when used and then
 * deleted, some overhead remains (to avoid resonance).
 * The number of bookmarks used should be kept to minimum since it also adds
 * overhead on node deletion (searching for a bookmark to update). */
typedef struct quicklistBookmark {
    quicklistNode *node;
    char *name;
} quicklistBookmark;

bookmark用于加速遍历quicklist的node。它只用于大数据量结点的quicklist中，在这种情况下少量的bookmark的内存使用可以被忽略不计。

快速列表的结构图

快速列表1.PNG 快速列表2.PNG

参考文章
https://www.cnblogs.com/hunternet/p/12624691.html