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From Viktor Jevdokimov <>
Subject RE: Reduce Cassandra GC
Date Tue, 16 Apr 2013 09:21:57 GMT
For a >40GB of data 1GB of heap is too low.

Best regards / Pagarbiai
Viktor Jevdokimov
Senior Developer

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From: Joel Samuelsson []
Sent: Tuesday, April 16, 2013 10:47
Subject: Reduce Cassandra GC


We have a small production cluster with two nodes. The load on the nodes is very small, around
20 reads / sec and about the same for writes. There are around 2.5 million keys in the cluster
and a RF of 2.

About 2.4 million of the rows are skinny (6 columns) and around 3kb in size (each). Currently,
scripts are running, accessing all of the keys in timeorder to do some calculations.

While running the scripts, the nodes go down and then come back up 6-7 minutes later. This
seems to be due to GC. I get lines like this in the log:
INFO [ScheduledTasks:1] 2013-04-15 14:00:02,749 (line 122) GC for ParNew:
338798 ms for 1 collections, 592212416 used; max is 1046937600

However, the heap is not full. The heap usage has a jagged pattern going from 60% up to 70%
during 5 minutes and then back down to 60% the next 5 minutes and so on. I get no "Heap is
X full..." messages. Every once in a while at one of these peaks, I get these stop-the-world
GC for 6-7 minutes. Why does GC take up so much time even though the heap isn't full?

I am aware that my access patterns make key caching very unlikely to be high. And indeed,
my average key cache hit ratio during the run of the scripts is around 0.5%. I tried disabling
key caching on the accessed column family (UPDATE COLUMN FAMILY cf WITH caching=none;) through
the cassandra-cli but I get the same behaviour. Is the turning key cache off effective immediately?

Stop-the-world GC is fine if it happens for a few seconds but having them for several minutes
doesn't work. Any other suggestions to remove them?

Best regards,
Joel Samuelsson

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