[jira] [Created] (IMPALA-6679) Don't claim ideal reservation in scanner until actually processing scan ranges

["Tim Armstrong (JIRA)" <ji...@apache.org>]

Tim Armstrong created IMPALA-6679:
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             Summary: Don't claim ideal reservation in scanner until actually processing scan ranges
                 Key: IMPALA-6679
                 URL: https://issues.apache.org/jira/browse/IMPALA-6679
             Project: IMPALA
          Issue Type: Sub-task
          Components: Backend
            Reporter: Tim Armstrong
            Assignee: Tim Armstrong


One cause of the memory regression in IMPALA-4835 was that scans, particularly Parquet, claimed memory very aggressively in HdfsScanNode::Open() based on the planner's estimated ideal memory. There were two problems here:

# In many cases the ideal memory increase was not at all needed, because the total amount of data scanned in the Parquet file was actually less than the minimum reservation. We don't know this for sure until the read the Parquet footer and check the column sizes.
# HdfsScanNode::Open() may happen a long time before the first HdfsScanNode::GetNext(), particularly if the scan node is on the left side of a broadcast join. The scan node may grab a lot of reservation before other plan nodes have started running, eventually resulting in starving a lot of nodes.

It would make sense to wait until we are actually processing a scan range to increase a scanner thread's reservation from the minimum. There are various more complex things we could do here, but I suspect the simplest possible thing would help a lot. E.g. we could extend this by  releasing reservation after processing each scan range.

See https://docs.google.com/document/d/1kR0zfevNNUJom3sH1XmposacVZ-QALan7NSwnR5CkSA/edit# for some more analysis.



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