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https://issues.apache.org/jira/browse/HBASE-4195?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13089856#comment-13089856
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nkeywal commented on HBASE-4195:
--------------------------------
For #2, yes, it seems that HBASE-2856 is addressing this type of issues.
Problem solved then :)
For #1, note that the previous implementation will completly ignore the
modifications made during the flush: the flush creates a new KV List (kvset),
and this list is not seen by the MemScanner, as it uses the previous iterator.
I don't know if it's an issue or not.
This said, I believe it's possible to have an implementation with the same
properties as the previous one, with an optimized reseek time, by keeping the
pointers to the sublists in the MemStoreScanner. The "reseek" implementation
would then become very similar to the "seek" one. I tested this approach, it
seems to work functionally as the previous one (i.e. I fail in the case #2
mentionned above). I have not tested the reality of performance improvement,
but if there is an aggreement on this approach, I can do it.
The implementation of reseek would be:
{noformat}
public synchronized boolean reseek(KeyValue key) {
// kvset and snapshot will never be empty.
// if tailSet cant find anything, SS is empty (not null).
kvTail = kvTail.tailSet(key, true);
snapshotTail = snapshotTail.tailSet(key, true);
kvsetIt = kvTail.iterator();
snapshotIt = snapshotTail.iterator();
kvsetNextRow = getNext(kvsetIt);
snapshotNextRow = getNext(snapshotIt);
KeyValue lowest = getLowest();
// has data := (lowest != null)
return lowest != null;
}
{noformat}
> 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
> 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):
> {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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