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From "Christian Spriegel (JIRA)" <j...@apache.org>
Subject [jira] [Created] (CASSANDRA-7401) Memtable.maybeUpdateLiveRatio goes into an endless loop when currentOperations is zero
Date Mon, 16 Jun 2014 09:59:02 GMT
Christian Spriegel created CASSANDRA-7401:
---------------------------------------------

             Summary: Memtable.maybeUpdateLiveRatio goes into an endless loop when currentOperations
is zero
                 Key: CASSANDRA-7401
                 URL: https://issues.apache.org/jira/browse/CASSANDRA-7401
             Project: Cassandra
          Issue Type: Bug
          Components: Core
            Reporter: Christian Spriegel
            Assignee: Christian Spriegel


Hi,

I was describing an error the other day on the mailing list, where the MemoryMeter would go
into an endless loop. This happened multiple times last week, unfortunetaly I cannot reproduce
it at the moment.

The whole cassandra server got unresponsive and logged about 7000k messages per second into
the log:
{quote}
...
 INFO [MemoryMeter:1] 2014-06-14 19:24:09,488 Memtable.java (line 481) CFS(Keyspace='MDS',
ColumnFamily='ResponsePortal') liveRatio is 64.0 (just-counted was 64.0).  calculation took
0ms for 0 cells
...
{quote}

The cause for this seems to be Memtable.maybeUpdateLiveRatio(), which cannot handle currentOperations
(and liveRatioComputedAt) to be zero. The loop will iterate endlessly:
{code}
            ...
            if (operations < 2 * last) // does never break when zero: 0 < 0 is not true
                break;
            ...
{code}

One thing I cannot explain: How can the operationcount be zero when maybeUpdateLiveRatio()
gets called?

is it possible that addAndGet in resolve() increases by 0  in some cases?
{code}
currentOperations.addAndGet(cf.getColumnCount() + (cf.isMarkedForDelete() ? 1 : 0) + cf.deletionInfo().rangeCount());
// can this be zero? 
{code}

Nevertheless, the attached patch fixes the endless loop. Feel free to reassign this ticket
or create a followup ticket if currentOperations should not be zero.

kind regards,
Christian



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