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From Joel Samuelsson <samuelsson.j...@gmail.com>
Subject Re: Reduce Cassandra GC
Date Tue, 16 Apr 2013 09:51:37 GMT
How do you calculate the heap / data size ratio? Is this a linear ratio?

Each node has slightly more than 12 GB right now though.


2013/4/16 Viktor Jevdokimov <Viktor.Jevdokimov@adform.com>

>  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 [mailto:samuelsson.joel@gmail.com]
> *Sent:* Tuesday, April 16, 2013 10:47
> *To:* user@cassandra.apache.org
> *Subject:* Reduce Cassandra GC****
>
> ** **
>
> Hi,****
>
> ** **
>
> 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 GCInspector.java (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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