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From Peter Schuller <>
Subject Re: Garbage collection freezes cassandra node
Date Tue, 20 Dec 2011 05:35:38 GMT
>  During the garbage collections, Cassandra freezes for about ten seconds. I observe the
following log entries:
> “GC for ConcurrentMarkSweep: 11597 ms for 1 collections, 1887933144 used; max is 8550678528”

Ok, first off: Are you certain that it is actually pausing, or are you
assuming that due to the log entry above? Because the log entry in no
way indicates a 10 second pause; it only indicates that CMS took 10
seconds - which is entirely expected, and most of CMS is concurrent
and implies only short pauses. A full pause can happen, but that log
entry is expected and is not in and of itself indicative of a
stop-the-world 10 second pause.

It is fully expected using the CMS collector that you'll have a
sawtooth pattern as young gen is being collected, and then a sudden
drop as CMS does its job concurrently without pausing the application
for a long period of time.

I will second the recommendation to run with -XX:+DisableExplicitGC
(or -XX:+ExplicitGCInvokesConcurrent) to eliminate that as a source.

I would also run with -XX:+PrintGC -XX:+PrintGCDetails
-XX:+PrintGCTimeStamps -XX:+PrintGCDateStamps and report back the
results (i.e., the GC log around the time of the pause).

Your graph is looking very unusual for CMS. It's possible that
everything is as it otherwise should and CMS is kicking in too late,
but I am kind of skeptical towards that even the extremely smooth look
of your graph.

/ Peter Schuller (@scode,

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