Use at least 2 runners regardless of the number of schedulers

Author: José Valim

lib/elixir/lib/kernel/parallel_compiler.ex

@@ -32,7 +32,7 @@ defmodule Kernel.ParallelCompiler do
 
   defp spawn_compilers(files, path, callback) do
     Code.ensure_loaded(Kernel.ErrorHandler)
-    schedulers = :erlang.system_info(:schedulers_online)
+    schedulers = max(:erlang.system_info(:schedulers_online), 2)
     spawn_compilers(files, path, callback, [], [], schedulers, [])
   end
 

lib/elixir/lib/kernel/parallel_require.ex

@@ -12,7 +12,7 @@ defmodule Kernel.ParallelRequire do
   can be optionally given as argument.
   """
   def files(files, callback // default_callback) do
-    schedulers = :erlang.system_info(:schedulers_online)
+    schedulers = max(:erlang.system_info(:schedulers_online), 2)
     spawn_requires(files, [], callback, schedulers, [])
   end