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From "nkeywal (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-4195) Possible inconsistency in a memstore read after a reseek, possible performance improvement
Date Thu, 25 Aug 2011 07:20:29 GMT

    [ https://issues.apache.org/jira/browse/HBASE-4195?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13090840#comment-13090840

nkeywal commented on HBASE-4195:

With the current implementation, it can fails in TestHRegion at assert "assertEquals("i="
+ i, expectedCount, result.size());" when there is a mess up on the lists. It should not occur
with the new implementation.

The issue in the flush is shown by the assert just next to this one. It's (log added in the
new patch)
              LOG.warn("These two KV should have the same value." +
                " Previous KV:" +
                previousKV + "(memStoreTS:" + previousKV.getMemstoreTS() + ")" +
                ", New KV: " +
                kv + "(memStoreTS:" + kv.getMemstoreTS() + ")"
              assertEquals(previousKV.getValue(), thisValue);

With these modified values on testWritesWhileGetting:
    int testCount = 1000; // more iterations. Increase it more if necessary.
    // [...]
    int flushInterval = 2; // more flush
    int compactInterval = 1000 * flushInterval; // no compact (should have no impact, but...)

I produced this error on the trunk + patch proposed:

put iteration = 1927
put iteration = 2021
These two KV should have the same value. Previous KV:row0/family4:qual99/1942/Put/vlen=4(memStoreTS:993),
New KV: row0/family5:qual0/1944/Put/vlen=4(memStoreTS:0)
Time: 19.051
There was 1 failure:
1) testWritesWhileGetting(org.apache.hadoop.hbase.regionserver.TestHRegion)
junit.framework.AssertionFailedError: expected:<1942> but was:<1944>
	at org.apache.hadoop.hbase.HBaseTestCase.assertEquals(HBaseTestCase.java:704)
	at org.apache.hadoop.hbase.regionserver.TestHRegion.testWritesWhileGetting(TestHRegion.java:2781)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)

May be there are more memory allocations on 0.90 and as a consequence more random interruptions
caused by the GC? Note that I am doing the tests outside of Maven, but with JUnit, without
specific java parameters. 

If worth checking that on .90 it is as well an error when the family changes (qualifier equals
qual0 or qual1). May be it's something different on .90.

Unfortunatly  (or not :-)) I am on vacations for a week, so I won't be able to give a hand
the next days, but I will be back middle of next week.

> Possible inconsistency 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
>            Assignee: nkeywal
>            Priority: Critical
>             Fix For: 0.90.5
>         Attachments: 20110824_4195_MemStore.patch, 20110824_4195_TestHRegion.patch
> 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
> {noformat}679	    protected KeyValue getNext(Iterator it) {
> 680	      KeyValue ret = null;
> 681	      long readPoint = ReadWriteConsistencyControl.getThreadReadPoint();
> 682	      //DebugPrint.println( " MS@" + hashCode() + ": threadpoint = " + readPoint);
> 683	 
> 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	    }{noformat}
> 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:
> {noformat}      numIterReseek = 0;
>       return (kvsetNextRow != null || snapshotNextRow != null);
>     }{noformat}
> 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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