memory - HSET 与 SET 内存使用情况？

Question

我正在阅读 this article其中提到在 redis 中存储 100 万个 key 将使用 17GB 的内存。然而，当切换到散列时，将它们分 block 为 1k(例如:HSET "mediabucket:1155""1155315""939")允许它们将 1M 存储在 5GB 中，这是一个相当大的节省。

我读过 redis memory-optimization但我不太明白其中的区别。它说 HGET 不是 O(1) 但足够接近，并且在使用 hset 时提到了更多的 cpu 使用率。我不明白为什么会有更多的 cpu 使用率(确保空间交易时间。但是如何/什么？)。它提到了“编码”，但没有提到它们是如何编码的。

它也只提到了字符串，但我不知道只有字符串是什么意思。是哈希字段吗？这是否意味着哈希字段？我在 HSET 中没有看到任何相关信息.究竟要编码什么？为什么编码比使用 SET 更有效？

HSET "mediabucket:1155""1155315""939" 怎么可能比 SET "mediabucket:1155315""939" 更有效率呢？ SET 中的数据较少(使用 1155315 和 1155 而不是 1155315)。我个人会尝试使用二进制 key ，但我认为这与 HSET 更高效的原因无关。

编辑:

在 redis-db 邮件列表上的交叉发布:https://groups.google.com/d/topic/redis-db/90K3UqciAx0/discussion

Answer 1

根据 hash-max-ziplist-entries 和 hash-max-ziplist-value 参数的值，将小散列对象编码为 ziplist。这是简单的数据序列化。

一个ziplist定义如下(摘自Redis源码):

/* The ziplist is a specially encoded dually linked list that is designed
* to be very memory efficient. It stores both strings and integer values,
* where integers are encoded as actual integers instead of a series of
* characters. It allows push and pop operations on either side of the list
* in O(1) time. However, because every operation requires a reallocation of
* the memory used by the ziplist, the actual complexity is related to the
* amount of memory used by the ziplist.
*
* ----------------------------------------------------------------------------
*
* ZIPLIST OVERALL LAYOUT:
* The general layout of the ziplist is as follows:
* <zlbytes><zltail><zllen><entry><entry><zlend>
*
* <zlbytes> is an unsigned integer to hold the number of bytes that the
* ziplist occupies. This value needs to be stored to be able to resize the
* entire structure without the need to traverse it first.
*
* <zltail> is the offset to the last entry in the list. This allows a pop
* operation on the far side of the list without the need for full traversal.
*
* <zllen> is the number of entries.When this value is larger than 2**16-2,
* we need to traverse the entire list to know how many items it holds.
*
* <zlend> is a single byte special value, equal to 255, which indicates the
* end of the list.
*
* ZIPLIST ENTRIES:
* Every entry in the ziplist is prefixed by a header that contains two pieces
* of information. First, the length of the previous entry is stored to be
* able to traverse the list from back to front. Second, the encoding with an
* optional string length of the entry itself is stored.
*
* The length of the previous entry is encoded in the following way:
* If this length is smaller than 254 bytes, it will only consume a single
* byte that takes the length as value. When the length is greater than or
* equal to 254, it will consume 5 bytes. The first byte is set to 254 to
* indicate a larger value is following. The remaining 4 bytes take the
* length of the previous entry as value.
*
* The other header field of the entry itself depends on the contents of the
* entry. When the entry is a string, the first 2 bits of this header will hold
* the type of encoding used to store the length of the string, followed by the
* actual length of the string. When the entry is an integer the first 2 bits
* are both set to 1. The following 2 bits are used to specify what kind of
* integer will be stored after this header. An overview of the different
* types and encodings is as follows:
*
* |00pppppp| - 1 byte
*      String value with length less than or equal to 63 bytes (6 bits).
* |01pppppp|qqqqqqqq| - 2 bytes
*      String value with length less than or equal to 16383 bytes (14 bits).
* |10______|qqqqqqqq|rrrrrrrr|ssssssss|tttttttt| - 5 bytes
*      String value with length greater than or equal to 16384 bytes.
* |11000000| - 1 byte
*      Integer encoded as int16_t (2 bytes).
* |11010000| - 1 byte
*      Integer encoded as int32_t (4 bytes).
* |11100000| - 1 byte
*      Integer encoded as int64_t (8 bytes).
* |11110000| - 1 byte
*      Integer encoded as 24 bit signed (3 bytes).
* |11111110| - 1 byte
*      Integer encoded as 8 bit signed (1 byte).
* |1111xxxx| - (with xxxx between 0000 and 1101) immediate 4 bit integer.
*      Unsigned integer from 0 to 12. The encoded value is actually from
*      1 to 13 because 0000 and 1111 can not be used, so 1 should be
*      subtracted from the encoded 4 bit value to obtain the right value.
* |11111111| - End of ziplist.
*
* All the integers are represented in little endian byte order.
*/

散列对象中的每个项目都表示为 ziplist 中的键/值对(2 个连续条目)。键和值都可以存储为简单的字符串或整数。这种格式在内存中更紧凑，因为它节省了大量实现动态数据结构(如真正的哈希表)所需的指针(每个 8 字节)。

缺点是当应用于 ziplist 时，HSET/HGET 操作实际上是 O(N)。这就是为什么 ziplist 必须保持较小的原因。当 ziplist 数据适合 L1 CPU 缓存时，尽管具有线性复杂度，但相应的算法仍然足够快。

您可能需要引用以下链接以获取更多信息:

Redis 10x more memory usage than data

Redis Data Structure Space Requirements

这些答案涉及其他数据结构(如集合、列表或排序集合)，但它们的概念完全相同。