clarify Superset different timeout config (apache#3044)

* clarify different timeout configuration for superset

docs/faq.rst

@@ -48,12 +48,21 @@ https://github.com/airbnb/superset/issues?q=label%3Aexample+is%3Aclosed
 Why are my queries timing out?
 ------------------------------
 
-If you are seeing timeouts (504 Gateway Time-out) when running queries,
-it's because the web server is timing out web requests. If you want to
-increase the default (50), you can specify the timeout when starting the
-web server with the ``-t`` flag, which is expressed in seconds.
+There are many reasons may cause long query timing out.
 
-``superset runserver -t 300``
+
+- For running long query from Sql Lab, by default Superset allows it run as long as 6 hours before it being killed by celery. If you want to increase the time for running query, you can specify the timeout in configuration. For example:
+
+  ``SQLLAB_ASYNC_TIME_LIMIT_SEC = 60 * 60 * 6``
+
+
+- Superset is running on gunicorn web server, which may time out web requests. If you want to increase the default (50), you can specify the timeout when starting the web server with the ``-t`` flag, which is expressed in seconds.
+
+  ``superset runserver -t 300``
+
+- If you are seeing timeouts (504 Gateway Time-out) when loading dashboard or explore slice, you are probably behind gateway or proxy server (such as Nginx). If it did not receive a timely response from Superset server (which is processing long queries), these web servers will send 504 status code to clients directly. Superset has a client-side timeout limit to address this issue. If query didn't come back within clint-side timeout (45 seconds by default), Superset will display warning message to avoid gateway timeout message. If you have a longer gateway timeout limit, you can change client-side timeout limit settings from ``/superset/superset/assets/javascripts/constants.js`` file and rebuild js package:
+
+  ``export const QUERY_TIMEOUT_THRESHOLD = 45000;``
 
 
 Why is the map not visible in the mapbox visualization?