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https://issues.apache.org/jira/browse/IMPALA-10391?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Joe McDonnell resolved IMPALA-10391.
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Fix Version/s: Impala 4.0
Resolution: Fixed
> 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
> Assignee: Joe McDonnell
> Priority: Major
> Fix For: Impala 4.0
>
>
> 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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