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From "Karl Mueller (JIRA)" <j...@apache.org>
Subject [jira] [Created] (CASSANDRA-7507) OOM creates unreliable state - die instantly better
Date Mon, 07 Jul 2014 21:57:34 GMT
Karl Mueller created CASSANDRA-7507:
---------------------------------------

             Summary: OOM creates unreliable state - die instantly better
                 Key: CASSANDRA-7507
                 URL: https://issues.apache.org/jira/browse/CASSANDRA-7507
             Project: Cassandra
          Issue Type: New Feature
            Reporter: Karl Mueller
            Priority: Minor


I had a cassandra node run OOM. My heap had enough headroom, there was just something which
either was a bug or some unfortunate amount of short-term memory utilization. This resulted
in the following error:

 WARN [StorageServiceShutdownHook] 2014-06-30 09:38:38,251 StorageProxy.java (line 1713) Some
hints were not written before shutdown.  This is not supposed to happen.  You should (a) run
repair, and (b) file a bug report

There are no other messages of relevance besides the OOM error about 90 minutes earlier.

My (limited) understanding of the JVM and Cassandra says that when it goes OOM, it will attempt
to signal cassandra to shut down "cleanly". The problem, in my view, is that with an OOM situation,
nothing is guaranteed anymore. I believe it's impossible to reliably "cleanly shut down" at
this point, and therefore it's wrong to even try. 

Yes, ideally things could be written out, flushed to disk, memory messages written, other
nodes notified, etc. but why is there any reason to believe any of those steps could happen?
Would happen? Couldn't bad data be written at this point to disk rather than good data? Some
network messages delivered, but not others?

I think Cassandra should have the option to (and possibly default) to kill itself immediately
upon the OOM condition happening in a hard way, and not rely on the java-based clean shutdown
process. Cassandra already handles recovery from unclean shutdown, and it's not a big deal.
My node, for example, kept in a sort-of alive state for 90 minutes where who knows what it
was doing or not doing.

I don't know enough about the JVM and options for it to know the best exact implementation
of "die instantly on OOM", but it should be something that's possible either with some flags
or a C library (which doesn't rely on java memory to do something which it may not be able
to get!)

Short version: a kill -9 of all C* processes in that instance without needing more java memory,
when OOM is raised



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