Decrease default concurrency, fixes #3892

More threads means more memory usage AND increased glibc memory bloat due to arena contention.  Remember to set MALLOC_ARENA_MAX=2 to minimize your bloat.

You can get the old concurrency by adding `-c 25` to your Sidekiq command line.

Changes.md

@@ -5,6 +5,7 @@
 HEAD
 -----------
 
+- **Decrease default concurrency from 25 to 10** [#3892]
 - Smoother scheduling for large Sidekiq clusters [#3889]
 - Switch Sidekiq::Testing impl from alias\_method to Module#prepend, for resiliency [#3852]
 - Update Sidekiq APIs to use SCAN for scalability [#3848, ffiller]

lib/sidekiq.rb

@@ -17,7 +17,7 @@ module Sidekiq
   DEFAULTS = {
     queues: [],
     labels: [],
-    concurrency: 25,
+    concurrency: 10,
     require: '.',
     environment: nil,
     timeout: 8,