每个entry用单独的一块区域, 存储着当前结点的类型: 所谓的类型, 包括当前结点存储的数据是什么(二进制, 还是数值), 如何编码(如果是数值, 数值如何存储, 如果是二进制数据, 二进制数据的长度)
最后就是真实的数据了
entry的内存布局如下所示:
prevlen即是"前一个entry所占用的字节数", 它本身是一个变长字段, 规约如下:
若前一个entry占用的字节数小于 254, 则prevlen字段占一字节
若前一个entry占用的字节数等于或大于 254, 则prevlen字段占五字节: 第一个字节值为 254, 即0xfe, 另外四个字节, 以uint32_t存储着值.
encoding字段的规约就复杂了许多
若数据是二进制数据, 且二进制数据长度小于64字节(不包括64), 那么encoding占一字节. 在这一字节中, 高两位值固定为0, 低六位值以无符号整数的形式存储着二进制数据的长度. 即 00xxxxxx, 其中低六位bitxxxxxx是用二进制保存的数据长度.
若数据是二进制数据, 且二进制数据长度大于或等于64字节, 但小于16384(不包括16384)字节, 那么encoding占用两个字节. 在这两个字节16位中, 第一个字节的高两位固定为01, 剩余的14个位, 以小端序无符号整数的形式存储着二进制数据的长度, 即 01xxxxxx, yyyyyyyy, 其中yyyyyyyy是高八位, xxxxxx是低六位.
若数据是二进制数据, 且二进制数据的长度大于或等于16384字节, 但小于2^32-1字节, 则encoding占用五个字节. 第一个字节是固定值10000000, 剩余四个字节, 按小端序uint32_t的形式存储着二进制数据的长度. 这也是ziplist能存储的二进制数据的最大长度, 超过2^32-1字节的二进制数据, ziplist无法存储.
若数据是整数值, 则encoding和data的规约如下:
首先, 所有存储数值的entry, 其encoding都仅占用一个字节. 并且最高两位均是11
若数值取值范围位于[0, 12]中, 则encoding和data挤在同一个字节中. 即为1111 0001~1111 1101, 高四位是固定值, 低四位的值从0001至1101, 分别代表 0 ~ 12这十五个数值
若数值取值范围位于[-128, -1] [13, 127]中, 则encoding == 0b 1111 1110. 数值存储在紧邻的下一个字节, 以int8_t形式编码
若数值取值范围位于[-32768, -129] [128, 32767]中, 则encoding == 0b 1100 0000. 数值存储在紧邻的后两个字节中, 以小端序int16_t形式编码
若数值取值范围位于[-8388608, -32769] [32768, 8388607]中, 则encoding == 0b 1111 0000. 数值存储在紧邻的后三个字节中, 以小端序存储, 占用三个字节.
若数值取值范围位于[-2^31, -8388609] [8388608, 2^31 - 1]中, 则encoding == 0b 1101 0000. 数值存储在紧邻的后四个字节中, 以小端序int32_t形式编码
若数值取值均不在上述范围, 但位于int64_t所能表达的范围内, 则encoding == 0b 1110 0000, 数值存储在紧邻的后八个字节中, 以小端序int64_t形式编码
在大规模数值存储中, ziplist几乎不浪费内存空间, 其苟的程序到达了字节级别, 甚至对于[0, 12]区间的数值, 连data里的那一个字节也要省下来. 显然, ziplist是一种特别节省内存的数据结构, 但它的缺点也十分明显:
和intset一样, ziplist也不预留内存空间, 并且在移除结点后, 也是立即缩容, 这代表每次写操作都会进行内存分配操作.
ziplist最蛋疼的一个问题是: 结点如果扩容, 导致结点占用的内存增长, 并且超过254字节的话, 可能会导致链式反应: 其后一个结点的entry.prevlen需要从一字节扩容至五字节. 最坏情况下, 第一个结点的扩容, 会导致整个ziplist表中的后续所有结点的entry.prevlen字段扩容. 虽然这个内存重分配的操作依然只会发生一次, 但代码中的时间复杂度是o(N)级别, 因为链式扩容只能一步一步的计算. 但这种情况的概率十分的小, 一般情况下链式扩容能连锁反映五六次就很不幸了. 之所以说这是一个蛋疼问题, 是因为, 这样的坏场景下, 其实时间复杂度并不高: 依次计算每个entry新的空间占用, 也就是o(N), 总体占用计算出来后, 只执行一次内存重分配, 与对应的memmove操作, 就可以了. 蛋疼说的是: 代码特别难写, 难读. 下面放一段处理插入结点时处理链式反应的代码片断, 大家自行感受一下:
unsigned char *__ziplistInsert(unsigned char *zl, unsigned char *p, unsigned char *s, unsigned int slen) { size_t curlen = intrev32ifbe(ZIPLIST_BYTES(zl)), reqlen; unsigned int prevlensize, prevlen = 0; size_t offset; int nextdiff = 0; unsigned char encoding = 0; long long value = 123456789; /* initialized to avoid warning. Using a value that is easy to see if for some reason we use it uninitialized. */ zlentry tail; /* Find out prevlen for the entry that is inserted. */ if (p[0] != ZIP_END) { ZIP_DECODE_PREVLEN(p, prevlensize, prevlen); } else { unsigned char *ptail = ZIPLIST_ENTRY_TAIL(zl); if (ptail[0] != ZIP_END) { prevlen = zipRawEntryLength(ptail); } } /* See if the entry can be encoded */ if (zipTryEncoding(s,slen,&value,&encoding)) { /* 'encoding' is set to the appropriate integer encoding */ reqlen = zipIntSize(encoding); } else { /* 'encoding' is untouched, however zipStoreEntryEncoding will use the * string length to figure out how to encode it. */ reqlen = slen; } /* We need space for both the length of the previous entry and * the length of the payload. */ reqlen += zipStorePrevEntryLength(NULL,prevlen); reqlen += zipStoreEntryEncoding(NULL,encoding,slen); /* When the insert position is not equal to the tail, we need to * make sure that the next entry can hold this entry's length in * its prevlen field. */ int forcelarge = 0; nextdiff = (p[0] != ZIP_END) ? zipPrevLenByteDiff(p,reqlen) : 0; if (nextdiff == -4 && reqlen < 4) { nextdiff = 0; forcelarge = 1; } /* Store offset because a realloc may change the address of zl. */ offset = p-zl; zl = ziplistResize(zl,curlen+reqlen+nextdiff); p = zl+offset; /* Apply memory move when necessary and update tail offset. */ if (p[0] != ZIP_END) { /* Subtract one because of the ZIP_END bytes */ memmove(p+reqlen,p-nextdiff,curlen-offset-1+nextdiff); /* Encode this entry's raw length in the next entry. */ if (forcelarge) zipStorePrevEntryLengthLarge(p+reqlen,reqlen); else zipStorePrevEntryLength(p+reqlen,reqlen); /* Update offset for tail */ ZIPLIST_TAIL_OFFSET(zl) = intrev32ifbe(intrev32ifbe(ZIPLIST_TAIL_OFFSET(zl))+reqlen); /* When the tail contains more than one entry, we need to take * "nextdiff" in account as well. Otherwise, a change in the * size of prevlen doesn't have an effect on the *tail* offset. */ zipEntry(p+reqlen, &tail); if (p[reqlen+tail.headersize+tail.len] != ZIP_END) { ZIPLIST_TAIL_OFFSET(zl) = intrev32ifbe(intrev32ifbe(ZIPLIST_TAIL_OFFSET(zl))+nextdiff); } } else { /* This element will be the new tail. */ ZIPLIST_TAIL_OFFSET(zl) = intrev32ifbe(p-zl); } /* When nextdiff != 0, the raw length of the next entry has changed, so * we need to cascade the update throughout the ziplist */ if (nextdiff != 0) { offset = p-zl; zl = __ziplistCascadeUpdate(zl,p+reqlen); p = zl+offset; } /* Write the entry */ p += zipStorePrevEntryLength(p,prevlen); p += zipStoreEntryEncoding(p,encoding,slen); if (ZIP_IS_STR(encoding)) { memcpy(p,s,slen); } else { zipSaveInteger(p,value,encoding); } ZIPLIST_INCR_LENGTH(zl,1); return zl; } unsigned char *__ziplistCascadeUpdate(unsigned char *zl, unsigned char *p) { size_t curlen = intrev32ifbe(ZIPLIST_BYTES(zl)), rawlen, rawlensize; size_t offset, noffset, extra; unsigned char *np; zlentry cur, next; while (p[0] != ZIP_END) { zipEntry(p, &cur); rawlen = cur.headersize + cur.len; rawlensize = zipStorePrevEntryLength(NULL,rawlen); /* Abort if there is no next entry. */ if (p[rawlen] == ZIP_END) break; zipEntry(p+rawlen, &next); /* Abort when "prevlen" has not changed. */ if (next.prevrawlen == rawlen) break; if (next.prevrawlensize < rawlensize) { /* The "prevlen" field of "next" needs more bytes to hold * the raw length of "cur". */ offset = p-zl; extra = rawlensize-next.prevrawlensize; zl = ziplistResize(zl,curlen+extra); p = zl+offset; /* Current pointer and offset for next element. */ np = p+rawlen; noffset = np-zl; /* Update tail offset when next element is not the tail element. */ if ((zl+intrev32ifbe(ZIPLIST_TAIL_OFFSET(zl))) != np) { ZIPLIST_TAIL_OFFSET(zl) = intrev32ifbe(intrev32ifbe(ZIPLIST_TAIL_OFFSET(zl))+extra); } /* Move the tail to the back. */ memmove(np+rawlensize, np+next.prevrawlensize, curlen-noffset-next.prevrawlensize-1); zipStorePrevEntryLength(np,rawlen); /* Advance the cursor */ p += rawlen; curlen += extra; } else { if (next.prevrawlensize > rawlensize) { /* This would result in shrinking, which we want to avoid. * So, set "rawlen" in the available bytes. */ zipStorePrevEntryLengthLarge(p+rawlen,rawlen); } else { zipStorePrevEntryLength(p+rawlen,rawlen); } /* Stop here, as the raw length of "next" has not changed. */ break; } } return zl; }这种代码的特点就是: 最好由作者去维护, 最好一次性写对. 因为读起来真的费劲, 改起来也很费劲.
2.7 quicklist