Untracked memory is really anything that isn't explicitly tracked by query execution. We track all of the large amounts of memory used by query execution - buffers for reading from disk, the actual row data, hash tables in joins, etc, etc. The untracked memory should be small relative to that - it's mostly overhead for control structures like the runtime profile and things like that. That's usually small but it could add up easily when there are lots of queries being left open. It looks like something is unhealthy there. For one, there are a lot of queries that are still hanging around. I'd guess that there's probably a client that is misbehaving and not closing queries once it is finished with them. Those queries look like they were probably cancelled (or had all the results fetched) but were not closed by the client. One workaround for problems like that is to set an idle session timeout to periodically clear out user sessions that are not active: https://www.cloudera.com/documentation/enterprise/latest/topics/impala_timeouts.html
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