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From "nkeywal (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HBASE-4195) Possible unconsistency in a memstore read after a reseek, possible performance improvement
Date Thu, 11 Aug 2011 22:18:29 GMT
Possible unconsistency in a memstore read after a reseek, possible performance improvement

                 Key: HBASE-4195
                 URL: https://issues.apache.org/jira/browse/HBASE-4195
             Project: HBase
          Issue Type: Bug
          Components: regionserver
    Affects Versions: 0.90.4
         Environment: all
            Reporter: nkeywal
            Priority: Critical

This follows the dicussion around HBASE-3855, and the random errors (20% failure on trunk)
on the unit test org.apache.hadoop.hbase.regionserver.TestHRegion.testWritesWhileGetting

I saw some points related to numIterReseek, used in the MemStoreScanner#getNext (line 690):

679	    protected KeyValue getNext(Iterator it) {
680	      KeyValue ret = null;
681	      long readPoint = ReadWriteConsistencyControl.getThreadReadPoint();
682	      //DebugPrint.println( " MS@" + hashCode() + ": threadpoint = " + readPoint);
684	      while (ret == null && it.hasNext()) {
685	        KeyValue v = it.next();
686	        if (v.getMemstoreTS() <= readPoint) {
687	          // keep it.
688	          ret = v;
689	        }
690	        numIterReseek--;
691	        if (numIterReseek == 0) {
692	          break;
693	         }
694	      }
695	      return ret;
696	    }

This function is called by seek, reseek, and next. The numIterReseek is only usefull for reseek.

There are some issues, I am not totally sure it's the root cause of the test case error, but
it could explain partly the randomness of the error, and one point is for sure a bug.

1) In getNext, numIterReseek is decreased, then compared to zero. The seek function sets numIterReseek
to zero before calling getNext. It means that the value will be actually negative, hence the
test will always fail, and the loop will continue. It is the expected behaviour, but it's
quite smart.

2) In "reseek", numIterReseek is not set between the loops on the two iterators. If the numIterReseek
is equals to zero after the loop on the first one, the loop on the second one will never call
seek, as numIterReseek will be negative.

3) Still in "reseek", the test to call "seek" is (kvsetNextRow == null && numIterReseek
== 0). In other words, if kvsetNextRow is not null when numIterReseek equals zero, numIterReseek
will start to be negative at the next iteration and seek will never be called.

4) You can have side effects if reseek ends with a numIterReseek > 0: the following calls
to the "next" function will decrease numIterReseek to zero, and getNext will break instead
of continuing the loop. As a result, later calls to next() may return null or not depending
on how is configured the default value for numIterReseek.

To check if the issue comes from point 4, you can set the numIterReseek to zero before returning
in reseek:

      numIterReseek = 0;
      return (kvsetNextRow != null || snapshotNextRow != null);

On my env, on trunk, it seems to work, but as it's random I am not really sure. I also had
to modify the test (I added a loop) to make it fails more often, the original test was working
quite well here.

It has to be confirmed that this totally fix (it could be partial or unrelated) org.apache.hadoop.hbase.regionserver.TestHRegion.testWritesWhileGetting
before implementing a complete solution.

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