Only spawn tasks if more than one core

Author: josevalim

lib/ecto/repo.ex

@@ -1319,8 +1319,9 @@ defmodule Ecto.Repo do
       are already loaded. By setting this option to true, any existing
       association will be discarded and reloaded.
     * `:in_parallel` - If the preloads must be done in parallel. It can
-      only be performed when we have more than one preload and the
-      repository is not in a transaction. Defaults to `true`.
+      only be performed when we have more than one preload, the repository
+      is not in a transaction, and the machine has more than one scheduler.
+      Defaults to `true`.
     * `:prefix` - the prefix to fetch preloads from. By default, queries
       will use the same prefix as the first struct in the given collection.
       This option allows the prefix to be changed.

lib/ecto/repo/preloader.ex

@@ -151,7 +151,7 @@ defmodule Ecto.Repo.Preloader do
   # Then we execute queries in parallel
   defp maybe_pmap(preloaders, _repo_name, {adapter_meta, opts}) do
     if match?([_, _ | _], preloaders) and not adapter_meta.adapter.checked_out?(adapter_meta) and
-         Keyword.get(opts, :in_parallel, true) do
+         Keyword.get(opts, :in_parallel, true) and System.schedulers_online() > 1 do
       # We pass caller: self() so the ownership pool knows where
       # to fetch the connection from and set the proper timeouts.
       # Note while the ownership pool uses '$callers' from pdict,