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From "ramkrishna.s.vasudevan (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (HBASE-14221) Reduce the number of time row comparison is done in a Scan
Date Mon, 17 Aug 2015 11:27:45 GMT

     [ https://issues.apache.org/jira/browse/HBASE-14221?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

ramkrishna.s.vasudevan updated HBASE-14221:
-------------------------------------------
    Attachment: HBASE-14221_1.patch

Updated patch correcting the failing test case and the checkstyle comments fix.  
In case of BATCH_LIMIT reached in StoreScanner we do a comparison for the row if it has moved
already. this comparison will any way as part of HRegion or in the StoreScanner while doing
next().  There is no logical mistake if we don't do it here but we do one more while loop
in the HRegion layer and that is altering the scan metrics.  Rest are all fine.  
Coming to the impact on the perf with scanRange30000 (patched PE tool to scan bigger ranges)
and with filterAll I can see clear difference with and withoutpatch on latest trunk

With patch
{code}
  Min: 379757ms   Max: 383591ms   Avg: 382071ms
  Min: 383131ms   Max: 387450ms   Avg: 385802ms
{code}

Without patch
{code}
   Min: 424281ms   Max: 428828ms   Avg: 426949ms
    Min: 419371ms   Max: 422791ms   Avg: 421344ms
{code}


> 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: HBASE-14221.patch, HBASE-14221_1.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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