[DOCS] Add known issue for zero memory on Debian 8 (#1545)

Co-authored-by: David Roberts <[email protected]>

Author: lcawl

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

@@ -12,6 +12,7 @@ answers for frequently asked questions.
 * <<ml-mappingclash>>
 * <<ml-jobnames>>
 * <<ml-upgradedf>>
+* <<ml-debian8-memory>>
 
 [[ml-rollingupgrade]]
 === Machine learning features unavailable after rolling upgrade
@@ -113,4 +114,39 @@ If you do not fix or delete the {dfeed} before the upgrade, in order to successf
 start the failing nodes you must downgrade the nodes then fix the problem per
 above.
 
-See also {stack-ref}/upgrading-elastic-stack.html[Upgrading the Elastic Stack].
+See also {stack-ref}/upgrading-elastic-stack.html[Upgrading the Elastic Stack].
+
+[[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.
+
+--