[IMPALA-4465] Don't hold process wide lock while serializing Runtime Profile in GetRuntimeProfileStr()

• Type: Improvement
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Affects Version/s: Impala 2.7.0
• Fix Version/s: Impala 2.8.0
• Component/s: Backend
• Labels: performance
• Assignee: Sailesh Mukil
• Reporter: Sailesh Mukil
• Created: 10/Nov/16 18:29
• Updated: 15/Nov/16 18:51
• Resolved: 15/Nov/16 18:51

Description

In ImpalaServer::GetRuntimeProfileStr(), we hold the query_exec_state_map_lock_ while serializing the runtime profile.
https://github.com/apache/incubator-impala/blob/master/be/src/service/impala-server.cc#L568

The SerializeToArchiveString() is a slow operation and is called at least once per query. We don't need to hold the map lock, we can instead take a shared_ptr reference and hold the individual exec_state lock while serializing the runtime profile.

If the query gets unregistered during the serialization operation, it will wait on the exec_state lock in QueryExecState::Done() until the serialization is complete.
https://github.com/apache/incubator-impala/blob/master/be/src/service/query-exec-state.cc#L526