JVM Heap usage didn't decline after clearing field data cache by clear cache API

Hi,
While I cleared field data cache using Clear Cache API:
$curl -XPOST
'http://localhost:9200/_cache/clear?field_data=true'
field data cahced size decreased, indeed, but JVM heap usage didn't
declined.
What's the reasons ?

--
You received this message because you are subscribed to the Google Groups "elasticsearch" group.
To unsubscribe from this group and stop receiving emails from it, send an email to elasticsearch+unsubscribe@googlegroups.com.
To view this discussion on the web visit https://groups.google.com/d/msgid/elasticsearch/2f60ef42-36a6-4faa-a817-c48ebe36c725%40googlegroups.com.
For more options, visit https://groups.google.com/d/optout.