Redis对对象的编码和大小的影响

Question

当我将一些对象写入 Redis 时，我可以获得不同的内存使用统计信息。我想了解这是如何发生的。

简单的例子：

127.0.0.1:6379> set a 1
OK
127.0.0.1:6379> MEMORY usage a
(integer) 49
127.0.0.1:6379> set a "1"
OK
127.0.0.1:6379> MEMORY usage a
(integer) 49
127.0.0.1:6379> set a \x01 <<<< Message packed of number 1
OK
127.0.0.1:6379> MEMORY usage a
(integer) 55
127.0.0.1:6379> set a '\x01' <<<< Message pack of number 1 but added '' 
OK
127.0.0.1:6379> MEMORY usage a
(integer) 55
127.0.0.1:6379> set a "\x01" <<<<< Message pack of number 1 but added "" 
OK
127.0.0.1:6379> MEMORY usage a
(integer) 52

以及每次的编码（作为 embstr）：

127.0.0.1:6379> set a 1
OK
127.0.0.1:6379> DEBUG OBJECT a
Value at:0x7f9858217ba0 refcount:2147483647 encoding:int serializedlength:2 lru:11691925 lru_seconds_idle:222
127.0.0.1:6379> set a "1"
OK
127.0.0.1:6379> DEBUG OBJECT a
Value at:0x7f9858217ba0 refcount:2147483647 encoding:int serializedlength:2 lru:11692152 lru_seconds_idle:1
127.0.0.1:6379> set a \x01
OK
127.0.0.1:6379> DEBUG OBJECT a
Value at:0x7f98563a1398 refcount:1 encoding:embstr serializedlength:5 lru:11692162 lru_seconds_idle:0
127.0.0.1:6379> set a '\x01'
OK
127.0.0.1:6379> DEBUG OBJECT a
Value at:0x7f98563a13e0 refcount:1 encoding:embstr serializedlength:5 lru:11692168 lru_seconds_idle:1
127.0.0.1:6379> set a "\x01"
OK
127.0.0.1:6379> DEBUG OBJECT a
Value at:0x7f98563a1788 refcount:1 encoding:embstr serializedlength:2 lru:11692177 lru_seconds_idle:1

更复杂一点的值（带有数字的字符串）

127.0.0.1:6379> set a "abc123"
OK
127.0.0.1:6379> DEBUG OBJECT a
Value at:0x7f985821b260 refcount:1 encoding:embstr serializedlength:7 lru:11692243 lru_seconds_idle:3
127.0.0.1:6379> MEMORY usage a
(integer) 57
127.0.0.1:6379> set a \xa6abc123
OK
127.0.0.1:6379> MEMORY usage a
(integer) 61
127.0.0.1:6379> DEBUG OBJECT a
Value at:0x7f985821b320 refcount:1 encoding:embstr serializedlength:11 lru:11692273 lru_seconds_idle:9
127.0.0.1:6379> set a "\xa6abc123"
OK
127.0.0.1:6379> MEMORY usage a
(integer) 58
127.0.0.1:6379> DEBUG OBJECT a
Value at:0x7f985821b260 refcount:1 encoding:embstr serializedlength:8 lru:11692291 lru_seconds_idle:11
127.0.0.1:6379>

还有一个大的 JSON 字符串（然后是消息打包），编码是“raw”

127.0.0.1:6379> set a '[{"id":1,"first_name":"Kyrstin","last_name":"Ifill","email":"kifill0@livejournal.com","count":93,"ip_address":"182.218.153.253"}]'
OK
127.0.0.1:6379> DEBUG OBJECT a
Value at:0x7f98582b96f0 refcount:1 encoding:raw serializedlength:128 lru:11692490 lru_seconds_idle:2
127.0.0.1:6379> MEMORY usage a
(integer) 182
127.0.0.1:6379> set a '\x91\x86\xa5count]\xaafirst_name\xa7Kyrstin\xa9last_name\xa5Ifill\xa5email\xb7kifill0@livejournal.com\xaaip_address\xaf182.218.153.253\xa2id\x01'
OK
127.0.0.1:6379> MEMORY usage a
(integer) 197
127.0.0.1:6379> DEBUG OBJECT a
Value at:0x7f98582b9700 refcount:1 encoding:raw serializedlength:143 lru:11692517 lru_seconds_idle:6
127.0.0.1:6379> set a "\x91\x86\xa5count]\xaafirst_name\xa7Kyrstin\xa9last_name\xa5Ifill\xa5email\xb7kifill0@livejournal.com\xaaip_address\xaf182.218.153.253\xa2id\x01"
OK
127.0.0.1:6379> DEBUG OBJECT a
Value at:0x7f98582b96f0 refcount:1 encoding:raw serializedlength:107 lru:11692535 lru_seconds_idle:2
127.0.0.1:6379> MEMORY usage a
(integer) 158

Answer 1

这个答案基于Redis 4.0，在64位机器上

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为了提高内存效率，Redis 通过三种方法对值字符串进行编码：

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1. Int：如果值字符串可以转换为整数，例如-2^63~2^63，Redis 会将值保存为整数。这是最有效的编码。
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2. 嵌入字符串：如果值字符串的大小小于或等于44字节，Redis 会将字符串保存在 Redis 对象本身的同一块中。这比Raw String编码的内存效率更高。此外，它对缓存更加友好。检查一下这个原因。
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3. Raw String：否则，Redis 使用原始编码。
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在你的情况下：

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1. set a 1and set a "1"：值为字符串，可转换为1。所以Redis使用Int编码，即encoding:int。
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2. set a \\x01并且set a \'\\x01\'：该值是一个包含字符\xef\xbc\x88的字符串4注意：由于该字符串用单引号引起来，因此它不会被转义\xef\xbc\x89。它无法转换为整数，并且其大小小于44。所以Redis使用Embeded String编码，即encoding:embstr.
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3. set a "\\x01"：由于字符串用双引号括起来，因此该值被转义为二进制字符串，其长度（即1字节数）小于44，并且无法转换为整数。所以Redis使用Embeded String编码，即encoding:embstr.
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4. 设置一个大的json字符串：该值是一个大小大于 的字符串44。所以Redis使用Raw String编码，即encoding:raw.
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