Feature #15602

Eliminate recording full-width hash value for small Hash

Added by ko1 (Koichi Sasada) almost 2 years ago. Updated over 1 year ago.

Target version:



Let's shape up small hash value (1 to 8 entries) from 192B to 128B on 64bit ptr environments.

Data structure proposal

(step 1) Record only key and value pairs.

Now Ruby 2.6, 1 to 8 entry Hash objects allocate 192 byte (8B * 3 (key, value and hash value triple) * 8 entry = 192B) with ar_table (instead of st_table).
Eliminating to record hash value will reduce this allocation from 192B to 128B (8 * 2 * 8).

(step 2) 1 byte hash value

For 1 to 8 entries, full-width Hash value (8 bytes) may be too long to lookup the entry.
1 byte hash value can be generated from 8 byte hash value.
(hash_value & 0xff is most simple way to get it, but not sure it is enough)

Name 1 byte hash value as "hash hint" on my patch.

(step 3) Embed hash hint into RHash

RHash::iter_lev is used to recognize nesting level of a hash (h.each{ "h's iter_lev is 1 here" }).
However, there are only a few cases that this value is bigger than 1.
So we can put this value into flags (if it exceeds the limit, we can store this value into hidden attribute).
8 hash hints becomese 8B == sizeof(VALUE), so we can embed this value into RHash.


  • Pros.
    • We can reduce allocation size of small Hash.
    • Increase cache locality on hash lookup
      • We don't need to touch ar_table (allocate memory) if hash hints doesn't match.
      • We can access correct ar_table entry directly.
  • Cons.
    • hash hints can conflict more than full-width hash value => may increase eql? call.
      • performance down
      • incompatibility


I tested this patch and it slightly increase performance (not so big, on my micro-benchmark).
Memory consumption is reduced theoretically.


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