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https://issues.apache.org/jira/browse/HBASE-15392?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15187412#comment-15187412
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ramkrishna.s.vasudevan commented on HBASE-15392:
------------------------------------------------
bq.When would a bunch of skipping be better than SEEK to the next block when
done since we'll be loading the next block anyways?
Yes. That was the intention. When any way next() is going to fetch me the next
row (even if it is the next block) why to do a seek and then do a next().
moreRowsMayExistAfter() does a smarter job now for things that does not have
stopRow but as in my 'suggest' patch the thing is that if we have a scan with
specified start and stopRow and have some explicit columns now after the
required columns are fetched for every row now on additional comparison keeps
happening. That is what Lars was saying too. If that compare is fine then your
patch should be ideal.
bq.Is this true? If a row of 1M columns and we are looking for the second
column only, SKIP'ing through the rest is what we want?
This comment of yours seems to be theoritcally right. Why to keep reading and
skipping remaning(1M-2) columns when I know I can directly go to the next row.
But Lars says from some MR experiments.
> Single Cell Get reads two HFileBlocks
> -------------------------------------
>
> Key: HBASE-15392
> URL: https://issues.apache.org/jira/browse/HBASE-15392
> Project: HBase
> Issue Type: Sub-task
> Components: BucketCache
> Reporter: stack
> Assignee: stack
> Attachments: 15392-0.98-looksee.txt, 15392.wip.patch,
> 15392v2.wip.patch, 15392v3.wip.patch, 15392v4.patch,
> HBASE-15392_suggest.patch, no_optimize.patch, no_optimize.patch, two_seeks.txt
>
>
> As found by Daniel "SystemTap" Pol, a simple Get results in our reading two
> HFileBlocks, the one that contains the wanted Cell, and the block that
> follows.
> Here is a bit of custom logging that logs a stack trace on each HFileBlock
> read so you can see the call stack responsible:
> {code}
> 2016-03-03 22:20:30,191 INFO
> [B.defaultRpcServer.handler=20,queue=2,port=16020] regionserver.StoreScanner:
> START LOOP
> 2016-03-03 22:20:30,192 INFO
> [B.defaultRpcServer.handler=20,queue=2,port=16020] regionserver.StoreScanner:
> QCODE SEEK_NEXT_COL
> 2016-03-03 22:20:30,192 INFO
> [B.defaultRpcServer.handler=20,queue=2,port=16020] hfile.HFileBlockIndex:
> STARTED WHILE
> 2016-03-03 22:20:30,192 INFO
> [B.defaultRpcServer.handler=20,queue=2,port=16020] hfile.CombinedBlockCache:
> OUT OF L2
> 2016-03-03 22:20:30,192 TRACE
> [B.defaultRpcServer.handler=20,queue=2,port=16020] bucket.BucketCache: Read
> offset=31409152, len=2103
> 2016-03-03 22:20:30,192 TRACE
> [B.defaultRpcServer.handler=20,queue=2,port=16020] bucket.FileIOEngine:
> offset=31409152, length=2103
> 2016-03-03 22:20:30,193 TRACE
> [B.defaultRpcServer.handler=20,queue=2,port=16020] hfile.HFileReaderImpl:
> From Cache [blockType=DATA, fileOffset=2055421, headerSize=33,
> onDiskSizeWithoutHeader=2024, uncompressedSizeWithoutHeader=2020,
> prevBlockOffset=2053364, isUseHBaseChecksum=true, checksumType=CRC32C,
> bytesPerChecksum=16384, onDiskDataSizeWithHeader=2053,
> getOnDiskSizeWithHeader=2057, totalChecksumBytes=4, isUnpacked=true,
> buf=[org.apache.hadoop.hbase.nio.SingleByteBuff@e19fbd54],
> dataBeginsWith=\x00\x00\x00)\x00\x00\x01`\x00\x16user995139035672819231,
> fileContext=[usesHBaseChecksum=true, checksumType=CRC32C,
> bytesPerChecksum=16384, blocksize=65536, encoding=NONE, includesMvcc=true,
> includesTags=false, compressAlgo=NONE, compressTags=false,
> cryptoContext=[cipher=NONE keyHash=NONE]]]
> 2016-03-03 22:20:30,193 TRACE
> [B.defaultRpcServer.handler=20,queue=2,port=16020] hfile.HFileReaderImpl:
> Cache hit return [blockType=DATA, fileOffset=2055421, headerSize=33,
> onDiskSizeWithoutHeader=2024, uncompressedSizeWithoutHeader=2020,
> prevBlockOffset=2053364, isUseHBaseChecksum=true, checksumType=CRC32C,
> bytesPerChecksum=16384, onDiskDataSizeWithHeader=2053,
> getOnDiskSizeWithHeader=2057, totalChecksumBytes=4, isUnpacked=true,
> buf=[org.apache.hadoop.hbase.nio.SingleByteBuff@e19fbd54],
> dataBeginsWith=\x00\x00\x00)\x00\x00\x01`\x00\x16user995139035672819231,
> fileContext=[usesHBaseChecksum=true, checksumType=CRC32C,
> bytesPerChecksum=16384, blocksize=65536, encoding=NONE, includesMvcc=true,
> includesTags=false, compressAlgo=NONE, compressTags=false,
> cryptoContext=[cipher=NONE keyHash=NONE]]]
> java.lang.Throwable
> at
> org.apache.hadoop.hbase.io.hfile.HFileReaderImpl.readBlock(HFileReaderImpl.java:1515)
> at
> org.apache.hadoop.hbase.io.hfile.HFileBlockIndex$CellBasedKeyBlockIndexReader.loadDataBlockWithScanInfo(HFileBlockIndex.java:324)
> at
> org.apache.hadoop.hbase.io.hfile.HFileReaderImpl$HFileScannerImpl.seekTo(HFileReaderImpl.java:831)
> at
> org.apache.hadoop.hbase.io.hfile.HFileReaderImpl$HFileScannerImpl.reseekTo(HFileReaderImpl.java:812)
> at
> org.apache.hadoop.hbase.regionserver.StoreFileScanner.reseekAtOrAfter(StoreFileScanner.java:288)
> at
> org.apache.hadoop.hbase.regionserver.StoreFileScanner.reseek(StoreFileScanner.java:198)
> at
> org.apache.hadoop.hbase.regionserver.NonLazyKeyValueScanner.doRealSeek(NonLazyKeyValueScanner.java:54)
> at
> org.apache.hadoop.hbase.regionserver.KeyValueHeap.generalizedSeek(KeyValueHeap.java:321)
> at
> org.apache.hadoop.hbase.regionserver.KeyValueHeap.requestSeek(KeyValueHeap.java:279)
> at
> org.apache.hadoop.hbase.regionserver.StoreScanner.reseek(StoreScanner.java:806)
> at
> org.apache.hadoop.hbase.regionserver.StoreScanner.seekAsDirection(StoreScanner.java:795)
> at
> org.apache.hadoop.hbase.regionserver.StoreScanner.next(StoreScanner.java:624)
> at
> org.apache.hadoop.hbase.regionserver.KeyValueHeap.next(KeyValueHeap.java:153)
> at
> org.apache.hadoop.hbase.regionserver.HRegion$RegionScannerImpl.populateResult(HRegion.java:5703)
> at
> org.apache.hadoop.hbase.regionserver.HRegion$RegionScannerImpl.nextInternal(HRegion.java:5849)
> at
> org.apache.hadoop.hbase.regionserver.HRegion$RegionScannerImpl.nextRaw(HRegion.java:5622)
> at
> org.apache.hadoop.hbase.regionserver.HRegion$RegionScannerImpl.next(HRegion.java:5598)
> at
> org.apache.hadoop.hbase.regionserver.HRegion$RegionScannerImpl.next(HRegion.java:5584)
> at
> org.apache.hadoop.hbase.regionserver.RSRpcServices.get(RSRpcServices.java:2187)
> at
> org.apache.hadoop.hbase.regionserver.RSRpcServices.get(RSRpcServices.java:2123)
> at
> org.apache.hadoop.hbase.protobuf.generated.ClientProtos$ClientService$2.callBlockingMethod(ClientProtos.java:33512)
> at org.apache.hadoop.hbase.ipc.RpcServer.call(RpcServer.java:2229)
> at org.apache.hadoop.hbase.ipc.CallRunner.run(CallRunner.java:109)
> at
> org.apache.hadoop.hbase.ipc.RpcExecutor.consumerLoop(RpcExecutor.java:136)
> at org.apache.hadoop.hbase.ipc.RpcExecutor$1.run(RpcExecutor.java:111)
> at java.lang.Thread.run(Thread.java:745)
> 2016-03-03 22:20:30,193 INFO
> [B.defaultRpcServer.handler=20,queue=2,port=16020] hfile.HFileBlockIndex:
> READ [blockType=DATA, fileOffset=2055421, headerSize=33,
> onDiskSizeWithoutHeader=2024, uncompressedSizeWithoutHeader=2020,
> prevBlockOffset=2053364, isUseHBaseChecksum=true, checksumType=CRC32C,
> bytesPerChecksum=16384, onDiskDataSizeWithHeader=2053,
> getOnDiskSizeWithHeader=2057, totalChecksumBytes=4, isUnpacked=true,
> buf=[org.apache.hadoop.hbase.nio.SingleByteBuff@e19fbd54],
> dataBeginsWith=\x00\x00\x00)\x00\x00\x01`\x00\x16user995139035672819231,
> fileContext=[usesHBaseChecksum=true, checksumType=CRC32C,
> bytesPerChecksum=16384, blocksize=65536, encoding=NONE, includesMvcc=true,
> includesTags=false, compressAlgo=NONE, compressTags=false,
> cryptoContext=[cipher=NONE keyHash=NONE]]]
> 2016-03-03 22:20:30,193 INFO
> [B.defaultRpcServer.handler=20,queue=2,port=16020] hfile.HFileBlockIndex:
> DATA BLOCK IS TRUE BREAK
> 2016-03-03 22:20:30,193 INFO
> [B.defaultRpcServer.handler=20,queue=2,port=16020] hfile.HFileBlockIndex:
> RETURNING
> 2016-03-03 22:20:30,194 INFO
> [B.defaultRpcServer.handler=20,queue=2,port=16020] regionserver.StoreScanner:
> QCODE INCLUDE
> 2016-03-03 22:20:30,194 INFO
> [B.defaultRpcServer.handler=20,queue=2,port=16020] regionserver.StoreScanner:
> QCODE SKIP
> 2016-03-03 22:20:30,194 INFO
> [B.defaultRpcServer.handler=20,queue=2,port=16020] hfile.CombinedBlockCache:
> OUT OF L2
> 2016-03-03 22:20:30,194 TRACE
> [B.defaultRpcServer.handler=20,queue=2,port=16020] bucket.BucketCache: Read
> offset=30932992, len=2103
> 2016-03-03 22:20:30,194 TRACE
> [B.defaultRpcServer.handler=20,queue=2,port=16020] bucket.FileIOEngine:
> offset=30932992, length=2103
> 2016-03-03 22:20:30,194 TRACE
> [B.defaultRpcServer.handler=20,queue=2,port=16020] hfile.HFileReaderImpl:
> From Cache [blockType=DATA, fileOffset=2057478, headerSize=33,
> onDiskSizeWithoutHeader=2024, uncompressedSizeWithoutHeader=2020,
> prevBlockOffset=2055421, isUseHBaseChecksum=true, checksumType=CRC32C,
> bytesPerChecksum=16384, onDiskDataSizeWithHeader=2053,
> getOnDiskSizeWithHeader=2057, totalChecksumBytes=4, isUnpacked=true,
> buf=[org.apache.hadoop.hbase.nio.SingleByteBuff@6063ac95],
> dataBeginsWith=\x00\x00\x00)\x00\x00\x01`\x00\x16user995698996184959679,
> fileContext=[usesHBaseChecksum=true, checksumType=CRC32C,
> bytesPerChecksum=16384, blocksize=65536, encoding=NONE, includesMvcc=true,
> includesTags=false, compressAlgo=NONE, compressTags=false,
> cryptoContext=[cipher=NONE keyHash=NONE]]]
> 2016-03-03 22:20:30,194 TRACE
> [B.defaultRpcServer.handler=20,queue=2,port=16020] hfile.HFileReaderImpl:
> Cache hit return [blockType=DATA, fileOffset=2057478, headerSize=33,
> onDiskSizeWithoutHeader=2024, uncompressedSizeWithoutHeader=2020,
> prevBlockOffset=2055421, isUseHBaseChecksum=true, checksumType=CRC32C,
> bytesPerChecksum=16384, onDiskDataSizeWithHeader=2053,
> getOnDiskSizeWithHeader=2057, totalChecksumBytes=4, isUnpacked=true,
> buf=[org.apache.hadoop.hbase.nio.SingleByteBuff@6063ac95],
> dataBeginsWith=\x00\x00\x00)\x00\x00\x01`\x00\x16user995698996184959679,
> fileContext=[usesHBaseChecksum=true, checksumType=CRC32C,
> bytesPerChecksum=16384, blocksize=65536, encoding=NONE, includesMvcc=true,
> includesTags=false, compressAlgo=NONE, compressTags=false,
> cryptoContext=[cipher=NONE keyHash=NONE]]]
> java.lang.Throwable
> at
> org.apache.hadoop.hbase.io.hfile.HFileReaderImpl.readBlock(HFileReaderImpl.java:1515)
> at
> org.apache.hadoop.hbase.io.hfile.HFileReaderImpl$HFileScannerImpl.readNextDataBlock(HFileReaderImpl.java:906)
> at
> org.apache.hadoop.hbase.io.hfile.HFileReaderImpl$HFileScannerImpl.isNextBlock(HFileReaderImpl.java:1106)
> at
> org.apache.hadoop.hbase.io.hfile.HFileReaderImpl$HFileScannerImpl.positionForNextBlock(HFileReaderImpl.java:1100)
> at
> org.apache.hadoop.hbase.io.hfile.HFileReaderImpl$HFileScannerImpl._next(HFileReaderImpl.java:1118)
> at
> org.apache.hadoop.hbase.io.hfile.HFileReaderImpl$HFileScannerImpl.next(HFileReaderImpl.java:1139)
> at
> org.apache.hadoop.hbase.regionserver.StoreFileScanner.next(StoreFileScanner.java:152)
> at
> org.apache.hadoop.hbase.regionserver.KeyValueHeap.next(KeyValueHeap.java:114)
> at
> org.apache.hadoop.hbase.regionserver.StoreScanner.next(StoreScanner.java:628)
> at
> org.apache.hadoop.hbase.regionserver.KeyValueHeap.next(KeyValueHeap.java:153)
> at
> org.apache.hadoop.hbase.regionserver.HRegion$RegionScannerImpl.populateResult(HRegion.java:5703)
> at
> org.apache.hadoop.hbase.regionserver.HRegion$RegionScannerImpl.nextInternal(HRegion.java:5849)
> at
> org.apache.hadoop.hbase.regionserver.HRegion$RegionScannerImpl.nextRaw(HRegion.java:5622)
> at
> org.apache.hadoop.hbase.regionserver.HRegion$RegionScannerImpl.next(HRegion.java:5598)
> at
> org.apache.hadoop.hbase.regionserver.HRegion$RegionScannerImpl.next(HRegion.java:5584)
> at
> org.apache.hadoop.hbase.regionserver.RSRpcServices.get(RSRpcServices.java:2187)
> at
> org.apache.hadoop.hbase.regionserver.RSRpcServices.get(RSRpcServices.java:2123)
> at
> org.apache.hadoop.hbase.protobuf.generated.ClientProtos$ClientService$2.callBlockingMethod(ClientProtos.java:33512)
> at org.apache.hadoop.hbase.ipc.RpcServer.call(RpcServer.java:2229)
> at org.apache.hadoop.hbase.ipc.CallRunner.run(CallRunner.java:109)
> at
> org.apache.hadoop.hbase.ipc.RpcExecutor.consumerLoop(RpcExecutor.java:136)
> at org.apache.hadoop.hbase.ipc.RpcExecutor$1.run(RpcExecutor.java:111)
> at java.lang.Thread.run(Thread.java:745)
> 2016-03-03 22:20:30,195 INFO
> [B.defaultRpcServer.handler=20,queue=2,port=16020] regionserver.StoreScanner:
> QCODE DONE
> {code}
> We are in StoreScanner#next.
> Matcher does SEEK_NEXT_COL.
> We pull in the block that has our Cell in it.
> Matcher does INCLUDE
> Then Matcher does SKIP
> SKIP has us go read the next block.
> Then Matcher does DONE.
> Why ain't I getting to DONE after INCLUDE?
> Any clues?
> This code has been like this a while.
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