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From "ramkrishna.s.vasudevan (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-14221) Reduce the number of time row comparison is done in a Scan
Date Tue, 17 Nov 2015 10:50:11 GMT

    [ https://issues.apache.org/jira/browse/HBASE-14221?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15008465#comment-15008465

ramkrishna.s.vasudevan commented on HBASE-14221:

In order to proceed with HBASE-14221, I did some analysis in this JIRa with the patches provided
and the benchmarks given. What Matt says is true, reseeks with LoserTree is faster whereas
next() with LoserTree is slower. This is surely because of the number of operations and comparisons
that we perform. 
In case of next() with KVHeap, we always have do a peek() to the next KVScanner and with that
we compare the current KV if we really to add the scanner back to the KVHeap.  So here there
is no change to the heap. and so no comparison. Where as in case of LoserTree we need to fetch
the next key and always keep adjusting the tree. 
In case of reseek() the case with KVHeap is that we do lot of changes to the heap, first we
simply add the current scanner to the heap, then do a poll and again based on the need we
again add the current scanner to the heap. But in case of LoserTree these many comparisons
are avoided and it is similar to the next() call with LoserTree.
Am not very sure which one would be better here because both next() and reseek() is going
to be important when we have filters along with scans. 
Coming to this patch- this patch is more to do with the comparisons in the StoreScanner level
and Hregion level and it is mainly in comparing the rows. So this patch can be seen as a different
area than where the KVHeap would come in. So it may be worth pursuing? Am still doing some
more analysis on the LoserTree and its impl and see how we can benefit from them. 

> 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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