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https://issues.apache.org/jira/browse/HBASE-28905?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17887285#comment-17887285
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Wellington Chevreuil commented on HBASE-28905:
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Hi [~charlesconnell], thanks for the heads up. Yes, I had seen the increased
CPU usage on flamagraphs as well, however, it didn't seem cause much impacts on
our tests. Nevertheless, this improvement is much welcomed, since there might
be some workloads that get further impacted. Even for branch-2 with
HBASE-28596, although we removed the cacheIfCompactionsOff from
HFileReaderImpl, we still need to check for references when getting blocks from
the BucketCache
[here|https://github.com/wchevreuil/hbase/blob/a23ca87bcc7a40507069e504bf368b9d8b7c3fc2/hbase-server/src/main/java/org/apache/hadoop/hbase/io/hfile/bucket/BucketCache.java#L579].
> 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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