What does "GC (Allocation Failure)" mean in my ElasticSearch 5.6 logs?

GC (Allocation Failure) is a JVM message (not an Elasticsearch-specific one) that can be a sign that there's memory pressure, but it's not catastropic to the JVM (that would cause a OutOfMemoryError log line). It can also be completely innocuous.

GC (Allocation Failure) means that the Java garbage collector tried to run, ran out of space in the heap, then tried to allocate more memory. It's not a bad sign, necessarily. If you're receiving OutOfMemoryError errors and the JVM is crashing, then you know you're in trouble.

Side note / disclaimer / full disclosure / whatever the heck you want: I work for a DBaaS company that hosts Elasticsearch clusters.