Redis成本优化-版本升级-3.hashtable优化

before 7.0
hash-max-ziplist-entries:512
hash-max-ziplist-value:64

after 7.0
hash-max-listpack-entries 512
hash-max-listpack-value 64
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来源：Significant memory savings in case of many hash or zset keys (#9228)
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typedef struct dictht {
    dictEntry **table;
    unsigned long size;
    unsigned long sizemask;
    unsigned long used;
} dictht;

typedef struct dict {
    dictType *type;
    void *privdata;
    dictht ht[2];
    long rehashidx;
    int16_t pauserehash;
} dict;
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struct dict {
    dictType *type;
    dictEntry **ht_table[2];
    unsigned long ht_used[2];
    long rehashidx;
    int16_t pauserehash;
    signed char ht_size_exp[2]; 
};
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#define DICTHT_SIZE(exp) ((exp) == -1 ? 0 : (unsigned long)1<<(exp))
#define DICTHT_SIZE_MASK(exp) ((exp) == -1 ? 0 : (DICTHT_SIZE(exp))-1)
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10000个hash * 4KB 约等于 40MB
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Redis:6.2.15> debug HTSTATS-KEY hash-c25f6e05-8cd7-4c00-9776-d1d53e3e3b00
Hash table 0 stats (main hash table):
 table size: 512
 number of elements: 509
 different slots: 325
 max chain length: 6
 avg chain length (counted): 1.57
 avg chain length (computed): 1.57
 Chain length distribution:
   0: 187 (36.52%)
   1: 200 (39.06%)
   2: 83 (16.21%)
   3: 30 (5.86%)
   4: 8 (1.56%)
   5: 3 (0.59%)
   6: 1 (0.20%)
Hash table 1 stats (rehashing target):
 table size: 1024
 number of elements: 5
 different slots: 4
 max chain length: 2
 avg chain length (counted): 1.25
 avg chain length (computed): 1.25
 Chain length distribution:
   0: 1020 (99.61%)
   1: 3 (0.29%)
   2: 1 (0.10%)
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Hash table 0 stats (main hash table):
 table size: 1024
 number of elements: 514
 different slots: 395
 max chain length: 4
 avg chain length (counted): 1.30
 avg chain length (computed): 1.30
 Chain length distribution:
   0: 629 (61.43%)
   1: 296 (28.91%)
   2: 81 (7.91%)
   3: 16 (1.56%)
   4: 2 (0.20%)
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void hashTypeConvertZiplist(robj *o, int enc) {
    serverAssert(o->encoding == OBJ_ENCODING_ZIPLIST);
   if (enc == OBJ_ENCODING_HT) {
        hashTypeIterator *hi;
        dict *dict;
        int ret;
        hi = hashTypeInitIterator(o);
        dict = dictCreate(&hashDictType, NULL);
        while (hashTypeNext(hi) != C_ERR) {
            sds key, value;
            key = hashTypeCurrentObjectNewSds(hi,OBJ_HASH_KEY);
            value = hashTypeCurrentObjectNewSds(hi,OBJ_HASH_VALUE);
            ret = dictAdd(dict, key, value);
            if (ret != DICT_OK) {
                //忽略内容
            }
        }
        hashTypeReleaseIterator(hi);
        zfree(o->ptr);
        o->encoding = OBJ_ENCODING_HT;
        o->ptr = dict;
    }
}
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Redis:6.2.15> debug htstats-key hash-aa51a2cd-21d9-4ce1-9440-77b4ef5863c6
Hash table 0 stats (main hash table):
 table size: 1024
 number of elements: 1023
 different slots: 654
 max chain length: 5
 avg chain length (counted): 1.56
 avg chain length (computed): 1.56
 Chain length distribution:
   0: 370 (36.13%)
   1: 397 (38.77%)
   2: 166 (16.21%)
   3: 73 (7.13%)
   4: 15 (1.46%)
   5: 3 (0.29%)
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Redis:7.0.12> debug htstats-key hash-ed298442-e907-4736-bf0b-57fb0578b12e
Hash table 0 stats (main hash table):
 table size: 1024
 number of elements: 1023
 different slots: 639
 max chain length: 5
 avg chain length (counted): 1.60
 avg chain length (computed): 1.60
 Chain length distribution:
   0: 385 (37.60%)
   1: 364 (35.55%)
   2: 191 (18.65%)
   3: 60 (5.86%)
   4: 23 (2.25%)
   5: 1 (0.10%)
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