Add known issue for zero memory on Debian 8

docs/en/stack/ml/anomaly-detection/ml-troubleshooting.asciidoc

@@ -188,3 +188,39 @@ POST _ml/set_upgrade_mode?enabled=false&timeout=10m
 DELETE temp_ml_config
 --------------------------------------------------
 // TEST[skip:TBD]
+
+[discrete]
+[[ml-debian8-memory]]
+== Suboptimal job assignment on Debian 8
+
+Where possible, {ml} jobs are assigned to nodes based on the memory requirement
+of the job and the memory available on the node. However, in certain cases, the
+amount of memory on a node cannot be accurately determined and jobs are assigned
+by balancing the number of jobs per {ml} node. It may lead to a situation where
+all the jobs with high memory requirements are on one node and the less
+memory-intensive jobs on another.
+
+One particular case of this problem is that {es} fails to determine the amount
+of memory on a machine that is running Debian 8 with the default Cgroups setup
+and certain updates of Java versions earlier than Java 15. For example, Java
+8u271 is known to be affected while Java 8u272 is not. Java 15 was fixed from
+its initial release.
+
+If you are running {es} on Debian 8 with an old version of Java and have not
+already modified the Cgroups setup then it is recommended to do one of the
+following:
+
+* Upgrade Java to version 15.
+* Upgrade to the latest Java update for the version of Java you are running.
+* Enable the "memory" Cgroup by editing `/etc/default/grub` and adding:
++
+--
+[source,sh]
+--------------------------------------------------
+GRUB_CMDLINE_LINUX_DEFAULT="quiet cgroup_enable=memory swapaccount=1"
+--------------------------------------------------
+
+Update your GRUB configuration by running `sudo update-grub`, then reboot the
+machine.
+
+--