Minimize scheduler load on Redis, closes #4882

Changes.md

@@ -2,6 +2,11 @@
 
 [Sidekiq Changes](https://github.com/mperham/sidekiq/blob/master/Changes.md) | [Sidekiq Pro Changes](https://github.com/mperham/sidekiq/blob/master/Pro-Changes.md) | [Sidekiq Enterprise Changes](https://github.com/mperham/sidekiq/blob/master/Ent-Changes.md)
 
+HEAD
+---------
+
+- Minimize scheduler load on Redis at scale [#4882]
+
 6.2.1
 ---------
 

lib/sidekiq/scheduled.rb

@@ -49,6 +49,7 @@ module Sidekiq
         @sleeper = ConnectionPool::TimedStack.new
         @done = false
         @thread = nil
+        @count_calls = 0
       end
 
       # Shut down this instance, will pause until the thread is dead.
@@ -152,8 +153,13 @@ module Sidekiq
       end
 
       def process_count
-        pcount = Sidekiq::ProcessSet.new.size
+        # The work buried within Sidekiq::ProcessSet#cleanup can be
+        # expensive at scale. Cut it down by 90% with this counter.
+        # NB: This method is only called by the scheduler thread so we
+        # don't need to worry about the thread safety of +=.
+        pcount = Sidekiq::ProcessSet.new(@count_calls % 10 == 0).size
         pcount = 1 if pcount == 0
+        @count_calls += 1
         pcount
       end