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https://issues.apache.org/jira/browse/HBASE-28905?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17887336#comment-17887336
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Charles Connell commented on HBASE-28905:
-----------------------------------------
Gotcha.
Not sure why the PR didn't link automatically, it's
https://github.com/apache/hbase/pull/6348.
> Skip excessive evaluations of LINK_NAME_PATTERN and REF_NAME_PATTERN regular
> expressions
> ----------------------------------------------------------------------------------------
>
> Key: HBASE-28905
> URL: https://issues.apache.org/jira/browse/HBASE-28905
> Project: HBase
> Issue Type: Improvement
> Affects Versions: 2.6.0
> Reporter: Charles Connell
> Assignee: Charles Connell
> Priority: Minor
> Attachments: cpu_time_flamegraph_2.6.0.html,
> cpu_time_flamegraph_with_optimization.html,
> performance_test_query_latency_2.6.0.png,
> performance_test_query_latency_with_optimization.png
>
>
> To test if a file is a link file, HBase checks if its file name matches the
> regex
> {code:java}
> ^(?:((?:[_\p{Digit}\p{IsAlphabetic}]+))(?:\=))?((?:[_\p{Digit}\p{IsAlphabetic}][-_.\p{Digit}\p{IsAlphabetic}]*))=((?:[a-f0-9]+))-([0-9a-f]+(?:(?:_SeqId_[0-9]+_)|(?:_del))?)$
> {code}
> To test if an HFile has a "reference name," HBase checks if its file name
> matches the regex
> {code:java}
> ^([0-9a-f]+(?:(?:_SeqId_[0-9]+_)|(?:_del))?|^(?:((?:[_\p{Digit}\p{IsAlphabetic}]+))(?:\=))?((?:[_\p{Digit}\p{IsAlphabetic}][-_.\p{Digit}\p{IsAlphabetic}]*))=((?:[a-f0-9]+))-([0-9a-f]+(?:(?:_SeqId_[0-9]+_)|(?:_del))?)$)\.(.+)$
> {code}
> Matching against these big regexes is computationally expensive. HBASE-27474
> introduced (in 2.6.0) [code in a hot
> path|https://github.com/apache/hbase/blob/1602c531b245b4d455b48161757cde2ec3d1930b/hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/HFileReaderImpl.java#L1716]
> in {{HFileReaderImpl}} that checks whether an HFile is a link or reference
> file while deciding whether to cache blocks from that file. In flamegraphs
> taken at my company during performance tests, this meant that these regex
> evaulations take 2-3% of the CPU time on a busy RegionServer.
> Later, the hot-path invocation of the regexes was removed in HBASE-28596 in
> branch-2 and later, but not branch-2.6, so only the 2.6.x series suffers the
> performance regression. Nonetheless, all invocations of these regexes are
> still unnecessarily expensive and can be fast-failed easily.
> The link name pattern contains a literal "=", so any string that does not
> contain a "=" can be assumed to not match the regex. The reference name
> pattern contains a literal ".", so any string that does not contain a "." can
> be assumed to not match the regex. This optimization is mostly helpful in
> 2.6.x, but is valid in all branches.
> Running performance tests of this optimization removed the regex evaluations
> from my flamegraphs entirely, and reduced query latency by 15%. Some charts
> are attached.
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