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From "stack (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-14221) Reduce the number of time row comparison is done in a Scan
Date Wed, 18 Nov 2015 17:36:11 GMT

    [ https://issues.apache.org/jira/browse/HBASE-14221?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15011495#comment-15011495
] 

stack commented on HBASE-14221:
-------------------------------

Did you mean an issue other than HBASE-14221 above [~ram_krish]? 14221 is this issue.

Is there a write up or class diagram of how our merge sort works anywhere? Even a listing
of participants would help (filters, fake keys).

>From your description, it sounds like the patch here is at a different level. Let me take
a look.

Thanks for digging in here [~ram_krish] I think there are some potential big wins here but
takes doing a deep dig. The original code is from another time.




> Reduce the number of time row comparison is done in a Scan
> ----------------------------------------------------------
>
>                 Key: HBASE-14221
>                 URL: https://issues.apache.org/jira/browse/HBASE-14221
>             Project: HBase
>          Issue Type: Sub-task
>          Components: Scanners
>            Reporter: ramkrishna.s.vasudevan
>            Assignee: ramkrishna.s.vasudevan
>             Fix For: 2.0.0
>
>         Attachments: 14221-0.98-takeALook.txt, HBASE-14221.patch, HBASE-14221_1.patch,
HBASE-14221_1.patch, HBASE-14221_6.patch, withmatchingRowspatch.png, withoutmatchingRowspatch.png
>
>
> When we tried to do some profiling with the PE tool found this.
> Currently we do row comparisons in 3 places in a simple Scan case.
> 1) ScanQueryMatcher
> {code}
>        int ret = this.rowComparator.compareRows(curCell, cell);
>     if (!this.isReversed) {
>       if (ret <= -1) {
>         return MatchCode.DONE;
>       } else if (ret >= 1) {
>         // could optimize this, if necessary?
>         // Could also be called SEEK_TO_CURRENT_ROW, but this
>         // should be rare/never happens.
>         return MatchCode.SEEK_NEXT_ROW;
>       }
>     } else {
>       if (ret <= -1) {
>         return MatchCode.SEEK_NEXT_ROW;
>       } else if (ret >= 1) {
>         return MatchCode.DONE;
>       }
>     }
> {code}
> 2) In StoreScanner next() while starting to scan the row
> {code}
>     if (!scannerContext.hasAnyLimit(LimitScope.BETWEEN_CELLS) || matcher.curCell == null
||
>         isNewRow || !CellUtil.matchingRow(peeked, matcher.curCell)) {
>       this.countPerRow = 0;
>       matcher.setToNewRow(peeked);
>     }
> {code}
> Particularly to see if we are in a new row.
> 3) In HRegion
> {code}
>           scannerContext.setKeepProgress(true);
>           heap.next(results, scannerContext);
>           scannerContext.setKeepProgress(tmpKeepProgress);
>           nextKv = heap.peek();
> moreCellsInRow = moreCellsInRow(nextKv, currentRowCell);
> {code}
> Here again there are cases where we need to careful for a MultiCF case.  Was trying to
solve this for the MultiCF case but is having lot of cases to solve. But atleast for a single
CF case I think these comparison can be reduced.
> So for a single CF case in the SQM we are able to find if we have crossed a row using
the code pasted above in SQM. That comparison is definitely needed.
> Now in case of a single CF the HRegion is going to have only one element in the heap
and so the 3rd comparison can surely be avoided if the StoreScanner.next() was over due to
MatchCode.DONE caused by SQM.
> Coming to the 2nd compareRows that we do in StoreScanner. next() - even that can be avoided
if we know that the previous next() call was over due to a new row. Doing all this I found
that the compareRows in the profiler which was 19% got reduced to 13%. Initially we can solve
for single CF case which can be extended to MultiCF cases.



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