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https://issues.apache.org/jira/browse/HBASE-24637?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17159477#comment-17159477
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Lars Hofhansl commented on HBASE-24637:
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I see. SKIP is not a hint as such, though, it's the default. The hint (which
can be ignore) is the SEEK hint. Both are implemented with return codes.
Looks like the SQM is now marking transitions from Column to Column with a
SEEK-to-next-column hint, and for each row with a SEEK-to-next-row.
Also look at the numbers the optimization I mentioned is turning the vast
majority back into SKIPs (and that check is not free).
As I said, it's not wrong per se (need to look at the code more), but that does
not mean that there isn't a performance regression - as I have described in the
previous comment - that we need to fix, possibly by restoring the old behavior.
> Filter SKIP hinting regression
> ------------------------------
>
> Key: HBASE-24637
> URL: https://issues.apache.org/jira/browse/HBASE-24637
> Project: HBase
> Issue Type: Bug
> Components: Filters, Performance, Scanners
> Affects Versions: 2.2.5
> Reporter: Andrew Kyle Purtell
> Priority: Major
> Attachments: W-7665966-FAST_DIFF-FILTER_ALL.pdf,
> W-7665966-Instrument-low-level-scan-details-branch-1.patch,
> W-7665966-Instrument-low-level-scan-details-branch-2.2.patch,
> parse_call_trace.pl
>
>
> I have been looking into reported performance regressions in HBase 2 relative
> to HBase 1. Depending on the test scenario, HBase 2 can demonstrate
> significantly better microbenchmarks in a number of cases, and usually shows
> improvement in whole cluster benchmarks like YCSB.
> To assist in debugging I added methods to RpcServer for updating per-call
> metrics that leverage the fact it puts a reference to the current Call into a
> thread local and that all activity for a given RPC is processed by a single
> thread context. I then instrumented ScanQueryMatcher (in branch-1) and its
> various friends (in branch-2.2), StoreScanner, HFileReaderV2 and
> HFileReaderV3 (in branch-1) and HFileReaderImpl (in branch-2.2), HFileBlock,
> and DefaultMemStore (branch-1) and SegmentScanner (branch-2.2). Test tables
> with one family and 1, 5, 10, 20, 50, and 100 distinct column-qualifiers per
> row were created, snapshot, dropped, and cloned from the snapshot. Both 1.6
> and 2.2 versions under test operated on identical data files in HDFS. For
> tests with 1.6 and 2.2 on the server side the same 1.6 PE client was used, to
> ensure only the server side differed.
> The results for pe --filterAll were revealing. See attached.
> It appears a refactor to ScanQueryMatcher and friends has disabled the
> ability of filters to provide meaningful SKIP hints, which disables an
> optimization that avoids reseeking, leading to a serious and proportional
> regression in reseek activity and time spent in that code path. So for
> queries that use filters, there can be a substantial regression.
> Other test cases that did not use filters did not show this regression. If
> filters are not used the behavior of ScanQueryMatcher between 1.6 and 2.2 was
> almost identical, as measured by counts of the hint types returned, whether
> or not column or version trackers are called, and counts of store seeks or
> reseeks. Regarding micro-timings, there was a 10% variance in my testing and
> results generally fell within this range, except for the filter all case of
> course.
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