Rewrite ProcessSet#each to use pipelining to avoid lots of roundtrips in large Sidekiq installations.

lib/sidekiq/api.rb

@@ -447,13 +447,22 @@ module Sidekiq
       procs = Sidekiq.redis { |conn| conn.smembers('processes') }
 
       to_prune = []
+      sorted = procs.sort
       Sidekiq.redis do |conn|
-        procs.sort.each do |key|
-          info, busy, at_s = conn.hmget(key, 'info', 'busy', 'beat')
+        # We're making a tradeoff here between consuming more memory instead of
+        # making more roundtrips to Redis, but if you have hundreds or thousands of workers,
+        # you'll be happier this way
+        result = conn.pipelined do
+          sorted.each do |key|
+            conn.hmget(key, 'info', 'busy', 'beat')
+          end
+        end
+
+        result.each_with_index do |(info, busy, at_s), i|
           # the hash named key has an expiry of 60 seconds.
           # if it's not found, that means the process has not reported
           # in to Redis and probably died.
-          (to_prune << key; next) if info.nil?
+          (to_prune << sorted[i]; next) if info.nil?
           hash = Sidekiq.load_json(info)
           yield hash.merge('busy' => busy.to_i, 'beat' => at_s.to_f)
         end