Listpack encoding for sets (#11290)

Small sets with not only integer elements are listpack encoded, by default
up to 128 elements, max 64 bytes per element, new config `set-max-listpack-entries`
and `set-max-listpack-value`. This saves memory for small sets compared to using a hashtable.

Sets with only integers, even very small sets, are still intset encoded (up to 1G
limit, etc.). Larger sets are hashtable encoded.

This PR increments the RDB version, and has an effect on OBJECT ENCODING

Possible conversions when elements are added:

    intset -> listpack
    listpack -> hashtable
    intset -> hashtable

Note: No conversion happens when elements are deleted. If all elements are
deleted and then added again, the set is deleted and recreated, thus implicitly
converted to a smaller encoding.

1 files changed, 8 insertions, 1 deletions

diff --git a/redis.conf b/redis.conf
index dfc9de308..408d0c258 100644
--- a/redis.conf
+++ b/redis.conf
@@ -1951,13 +1951,20 @@ list-max-listpack-size -2
 # etc.
 list-compress-depth 0
 
-# Sets have a special encoding in just one case: when a set is composed
+# Sets have a special encoding when a set is composed
 # of just strings that happen to be integers in radix 10 in the range
 # of 64 bit signed integers.
 # The following configuration setting sets the limit in the size of the
 # set in order to use this special memory saving encoding.
 set-max-intset-entries 512
 
+# Sets containing non-integer values are also encoded using a memory efficient
+# data structure when they have a small number of entries, and the biggest entry
+# does not exceed a given threshold. These thresholds can be configured using
+# the following directives.
+set-max-listpack-entries 128
+set-max-listpack-value 64
+
 # Similarly to hashes and lists, sorted sets are also specially encoded in
 # order to save a lot of space. This encoding is only used when the length and
 # elements of a sorted set are below the following limits: