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From Tomàs Núnez <tomas.nu...@groupalia.com>
Subject Re: Reduce Cassandra GC
Date Tue, 16 Apr 2013 11:44:16 GMT
Hi!
Reading the documentation, heap size calculation is done without counting
data size.

Extract from http://www.datastax.com/docs/1.1/operations/tuning#heap-sizing

System MemoryHeap SizeLess than 2GB1/2 of system memory2GB to 4GB1GBGreater
than 4GB1/4 system memory, but not more than 8GB



2013/4/16 Joel Samuelsson <samuelsson.joel@gmail.com>

> 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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