Reduce number of reads in ruby reaper by caching some data

lib/sidekiq_unique_jobs/orphans/ruby_reaper.rb

@@ -37,6 +37,7 @@ def initialize(conn)
         @digests   = SidekiqUniqueJobs::Digests.new
         @scheduled = Redis::SortedSet.new(SCHEDULE)
         @retried   = Redis::SortedSet.new(RETRY)
+        check_expiry!
       end
 
       #
@@ -137,7 +138,7 @@ def enqueued?(digest)
 
       def active?(digest) # rubocop:disable Metrics/MethodLength, Metrics/CyclomaticComplexity, Metrics/PerceivedComplexity
         Sidekiq.redis do |conn|
-          procs = conn.sscan_each("processes").to_a
+          procs = processes(conn)
           return false if procs.empty?
 
           procs.sort.each do |key|
@@ -188,12 +189,30 @@ def considered_active?(time_f)
       # @yield queues one at a time
       #
       def queues(conn, &block)
-        conn.sscan_each("queues", &block)
+        check_expiry!
+        @saved_sizes["queues"] ||= conn.scard("queues")
+        if @saved_sizes["queues"] < reaper_count # never cache anything bigger than the reaper count
+          @saved_elems["queues"] ||= conn.smembers("queues")
+          @saved_elems["queues"].each(&block)
+        else
+          conn.sscan_each("queues", &block)
+        end
+      end
+
+      def processes(conn)
+        check_expiry!
+        @saved_sizes["processes"] ||= conn.scard("processes")
+        if @saved_sizes["processes"] < reaper_count # never cache anything bigger than the reaper count
+          @saved_elems["processes"] ||= conn.sscan_each("processes").to_a
+          @saved_elems["processes"]
+        else
+          conn.sscan_each("processes").to_a
+        end
       end
 
       def entries(conn, queue, &block) # rubocop:disable Metrics/MethodLength
         queue_key    = "queue:#{queue}"
-        initial_size = conn.llen(queue_key)
+        initial_size = queue_len(conn, queue_key)
         deleted_size = 0
         page         = 0
         page_size    = 50
@@ -225,6 +244,20 @@ def entries(conn, queue, &block) # rubocop:disable Metrics/MethodLength
       def in_sorted_set?(key, digest)
         conn.zscan_each(key, match: "*#{digest}*", count: 1).to_a.any?
       end
+
+      def queue_len(conn, queue_key)
+        check_expiry!
+        @saved_sizes[queue_key] ||= conn.llen(queue_key)
+      end
+
+      # a very clumsy local cache for frequently accessed data
+      def check_expiry!
+        if (Time.now - (@last_expired || 0)).to_f > 0.5 # only cache for max 500ms
+          @saved_sizes = {}
+          @saved_elems = {}
+          @last_expired = Time.now
+        end
+      end
     end
     # rubocop:enable Metrics/ClassLength
   end