use_presence: Default to False

Tracking issue with use_presence's performance problem:
matrix-org#3971.

Signed-off-by: rht <rhtbot@protonmail.com>

INSTALL.md

@@ -581,6 +581,12 @@ This also requires the optional `lxml` python dependency to be  installed. This
 in turn requires the `libxml2` library to be available - on  Debian/Ubuntu this
 means `apt-get install libxml2-dev`, or equivalent for your OS.
 
+### Presence
+
+By default, presence is disabled because because it consumes a lot CPU. If you
+want to turn it on anyway, you can set `use_presence: true`. The tracking issue
+is https://github.com/matrix-org/synapse/issues/3971.
+
 ### Troubleshooting Installation
 
 `pip` seems to leak *lots* of memory during installation. For instance, a Linux

changelog.d/9251.misc

@@ -0,0 +1,2 @@
+use_presence: Default to False.
+Wait until https://github.com/matrix-org/synapse/issues/3971 is fixed.

synapse/config/server.py

@@ -168,7 +168,8 @@ def read_config(self, config, **kwargs):
             self.public_baseurl += "/"
 
         # Whether to enable user presence.
-        self.use_presence = config.get("use_presence", True)
+        # TODO https://github.com/matrix-org/synapse/issues/3971
+        self.use_presence = config.get("use_presence", False)
 
         # Whether to update the user directory or not. This should be set to
         # false only if we are updating the user directory in a worker