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From Joel Samuelsson <samuelsson.j...@gmail.com>
Subject Re: Reduce Cassandra GC
Date Tue, 16 Apr 2013 12:18:20 GMT
Indeed!
What I meant was:
If 1GB heap is too low for >40GB data, how can I know what an appropiate
heap is for various data sizes?


2013/4/16 Tomàs Núnez <tomas.nunez@groupalia.com>

> 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 Size Less than 2GB1/2 of system memory2GB to 4GB1GB Greater
> 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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