[jira] [Created] (IMPALA-10391) LIRS cache edge case where there is exactly one unprotected element

["Joe McDonnell (Jira)" <ji...@apache.org>]

Joe McDonnell created IMPALA-10391:
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             Summary: LIRS cache edge case where there is exactly one unprotected element
                 Key: IMPALA-10391
                 URL: https://issues.apache.org/jira/browse/IMPALA-10391
             Project: IMPALA
          Issue Type: Bug
          Components: Backend
    Affects Versions: Impala 4.0
            Reporter: Joe McDonnell


The fix for IMPALA-10127 introduced an issue in this code:
{noformat}
          // If an UNPROTECTED entry is not on the recency list, then its new reuse
          // distance is longer than any of the PROTECTED entries. So, it remains
          // an UNPROTECTED entry, but it should be added back to the recency list and
          // readded as the most recent entry on the unprotected list.
          MoveToRecencyListBack(&tstate, e, false);
          RemoveFromUnprotectedList(e);
          AddToUnprotectedList(e);{noformat}
RemoveFromUnprotectedList would detect that this is the last unprotected entry and clear unprotected_list_front_. This doesn't decrement num_unprotected_ to 0, so when AddToUnprotectedList is called, it doesn't set unprotected_list_front_ when it adds the element back.

This only applies when there is exactly one entry on the unprotected list. This can be fixed by either decrementing/incrementing the counters to match up with other locations or by skipping this code when num_unprotected_ == 1.



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