Add pool_count as an example for performance (#6014)

Author: josevalim

installer/lib/phx_new/generator.ex

@@ -188,7 +188,8 @@ defmodule Phx.New.Generator do
     {adapter_app, adapter_module, adapter_config} =
       get_ecto_adapter(db, String.downcase(project.app), project.app_mod)
 
-    {web_adapter_app, web_adapter_vsn, web_adapter_module, web_adapter_docs} = get_web_adapter(web_adapter)
+    {web_adapter_app, web_adapter_vsn, web_adapter_module, web_adapter_docs} =
+      get_web_adapter(web_adapter)
 
     pubsub_server = get_pubsub_server(project.app_mod)
 
@@ -304,8 +305,16 @@ defmodule Phx.New.Generator do
     Mix.raise("Unknown database #{inspect(db)}")
   end
 
-  defp get_web_adapter("cowboy"), do: {:plug_cowboy, "~> 2.7", Phoenix.Endpoint.Cowboy2Adapter, "https://hexdocs.pm/plug_cowboy/Plug.Cowboy.html"}
-  defp get_web_adapter("bandit"), do: {:bandit, "~> 1.5", Bandit.PhoenixAdapter, "https://hexdocs.pm/bandit/Bandit.html#t:options/0"}
+  defp get_web_adapter("cowboy"),
+    do:
+      {:plug_cowboy, "~> 2.7", Phoenix.Endpoint.Cowboy2Adapter,
+       "https://hexdocs.pm/plug_cowboy/Plug.Cowboy.html"}
+
+  defp get_web_adapter("bandit"),
+    do:
+      {:bandit, "~> 1.5", Bandit.PhoenixAdapter,
+       "https://hexdocs.pm/bandit/Bandit.html#t:options/0"}
+
   defp get_web_adapter(other), do: Mix.raise("Unknown web adapter #{inspect(other)}")
 
   defp fs_db_config(app, module) do
@@ -380,6 +389,8 @@ defmodule Phx.New.Generator do
       # ssl: true,
       url: database_url,
       pool_size: String.to_integer(System.get_env("POOL_SIZE") || "10"),
+      # For machines with several cores, consider starting multiple pools of `pool_size`
+      # pool_count: 4,
       socket_options: maybe_ipv6
       """
     ]