Document that we don't recommend multiple judgedaemons on a single machine

Closes: #2277

Author: eldering

doc/manual/config-advanced.rst

@@ -338,8 +338,14 @@ systemd, then edit the ``domjudge-judgehost.target`` unit file and add
 more judgedaemons there.
 
 Although each judgedaemon process will be bound to one single CPU
-core, shared use of other resources such as disk I/O might
-still have effect on run times.
+core, shared use of other resources such as disk I/O typically
+still has an effect on run times.
+Therefore we recommend running a single judgedaemon per physical
+machine for contests where precise timing is important.
+
+Finally, running too many judgehost VMs on a single machine can also cause
+`Linux kernel issues <https://github.com/DOMjudge/domjudge/issues/2277>`_.
+
 
 Multi-site contests
 -------------------

doc/manual/install-judgehost.rst

@@ -19,6 +19,8 @@ System requirements
 * A TCP/IP network which connects the DOMserver and the judgehosts.
   The machines only need HTTP(S) access to the DOMserver.
 
+We recommend running a single judgedaemon per physical machine.
+For more details see the section :ref:`multiple-judgedaemons`.
 
 .. _judgehost_software: