Add FAQ entry for MADV settings in older versions (#23429)

* add FAQ entry for MADV settings in older versions

* add changelog

* finish sentence

* Update libbeat/docs/shared-faq.asciidoc

Co-authored-by: DeDe Morton <dede.morton@elastic.co>

* Update libbeat/docs/shared-faq.asciidoc

Co-authored-by: DeDe Morton <dede.morton@elastic.co>

* Update libbeat/docs/shared-faq.asciidoc

Co-authored-by: DeDe Morton <dede.morton@elastic.co>

Co-authored-by: DeDe Morton <dede.morton@elastic.co>

CHANGELOG.next.asciidoc

@@ -230,6 +230,7 @@ https://github.com/elastic/beats/compare/v7.0.0-alpha2...master[Check the HEAD d
 - Fix typo in config docs {pull}23185[23185]
 - Fix `nested` subfield handling in generated Elasticsearch templates. {issue}23178[23178] {pull}23183[23183]
 - Fix CPU usage metrics on VMs with dynamic CPU config {pull}23154[23154]
+- Add FAQ entry for madvdontneed variable {pull}23429[23429]
 - Fix panic due to unhandled DeletedFinalStateUnknown in k8s OnDelete {pull}23419[23419]
 - Fix error loop with runaway CPU use when the Kafka output encounters some connection errors {pull}23484[23484]
 

libbeat/docs/shared-faq.asciidoc

@@ -206,3 +206,10 @@ to +{beatname_lc}-customname-*+.
 For more information, see {kibana-ref}/index-patterns.html[Creating an index
 pattern] in the {kib} docs.
 endif::no_dashboards[]
+
+[[madvdontneed-rss]]
+=== High RSS memory usage due to MADV settings
+
+In versions of {beatname_uc} prior to 7.10.2, the go runtime defaults to `MADV_FREE` by default.
+In some cases, this can lead to high RSS memory usage while the kernel waits to reclaim any pages assigned to {beatname_uc}.
+On versions prior to 7.10.2, set the `GODEBUG="madvdontneed=1"` environment variable if you run into RSS usage issues.