Use hiredis to increase performance of reading large sets

hiredis-rb uses the hiredis C client library to optimize read/writes to Redis. Note that it merely replaces the underlying read/write/connect calls of the redis-rb library (https://github.com/redis/hiredis-rb/blob/master/ext/hiredis_ext/connection.c#L505-L513), so all high-level functionality is still preserved.

For large payloads, this can increase read throughput by a modest amount. For example, in the benchmark below, we see about a 15% performance increase reading from a large SMEMBERS payload (19,667 i/s to 23,306/s)

require 'redis/connection/hiredis'
require 'redis'
require 'benchmark/ips'
require 'json'

redis = Redis.new(host: 'localhost')

string_key = 'test:string'
set_key = 'test:set'
redis.del(set_key)

data = []

5000.times do |index|
  name = "*" * 16
  redis.sadd(set_key, name)
  data << name
end

redis.set(string_key, data.to_json)

Benchmark.ips do |x|
  x.report("smembers") {
    members = redis.smembers(set_key)
  }

  x.report("get") {
    members = JSON.parse(redis.get(string_key))
  }

  x.compare!
end

Redis 3.2.1

Without hiredis

Calculating -------------------------------------
            smembers     1.946k i/100ms
                 get    85.000  i/100ms
-------------------------------------------------
            smembers     19.667k (± 8.0%) i/s -     99.246k
                 get    844.214  (± 3.6%) i/s -      4.250k

Comparison:
            smembers:    19667.0 i/s
                 get:      844.2 i/s - 23.30x slower

With hiredis

Calculating -------------------------------------
            smembers     2.301k i/100ms
                 get    76.000  i/100ms
-------------------------------------------------
            smembers     23.306k (± 7.5%) i/s -    117.351k
                 get    841.539  (± 3.7%) i/s -      4.256k

Comparison:
            smembers:    23306.2 i/s
                 get:      841.5 i/s - 27.69x slower

Redis 5.0.7

Without hiredis

Calculating -------------------------------------
            smembers     1.955k i/100ms
                 get    74.000  i/100ms
-------------------------------------------------
            smembers     21.447k (± 4.0%) i/s -    107.525k
                 get    812.842  (± 4.3%) i/s -      4.070k

Comparison:
            smembers:    21447.3 i/s
                 get:      812.8 i/s - 26.39x slower

With hiredis

Calculating -------------------------------------
            smembers     2.404k i/100ms
                 get    78.000  i/100ms
-------------------------------------------------
            smembers     24.274k (± 5.8%) i/s -    122.604k
                 get    850.175  (± 3.3%) i/s -      4.290k

Comparison:
            smembers:    24273.7 i/s
                 get:      850.2 i/s - 28.55x slower