Key as dict entry - memory optimization for sets (#11595)
If a dict has only keys, and no use of values, then a key can be stored directly in a dict's hashtable. The key replaces the dictEntry. To distinguish between a key and a dictEntry, we only use this optimization if the key is odd, i.e. if the key has the least significant bit set. This is true for sds strings, since the sds header is always an odd number of bytes. Dict entries are used as a fallback when there is a hash collision. A special dict entry without a value (only key and next) is used so we save one word in this case too. This saves 24 bytes per set element for larges sets, and also gains some speed improvement as a side effect (less allocations and cache misses). A quick test adding 1M elements to a set using the command below resulted in memory usage of 28.83M, compared to 46.29M on unstable. That's 18 bytes per set element on average. eval 'for i=1,1000000,1 do redis.call("sadd", "myset", "x"..i) end' 0 Other...
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