Add known issue for zero memory on Debian 8

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

@@ -188,3 +188,40 @@ 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. This breaks down when the
+amount of memory on a node cannot be accurately determined, and then jobs are
+assigned simply by balancing numbers of jobs per {ml} node. This could lead to
+a situation where all the big jobs are on one node and all the small jobs on
+another.
+
+Elasticsearch will fail 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 Elasticsearch on Debian 8 with an old version of Java and
+have not already modified the Cgroups setup then you should do one of the
+following:
+
+1. Upgrade Java to version 15
+2. Upgrade to the latest Java update for the version of Java you are running
+3. Enable the "memory" Cgroup by editing `/etc/default/grub` and adding:
+
+```
+GRUB_CMDLINE_LINUX_DEFAULT="quiet cgroup_enable=memory swapaccount=1"
+```
+
+Then update your GRUB configuration by running:
+
+```
+sudo update-grub
+```
+
+And finally reboot the machine.
+