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Date: Wed, 06 Nov 2013 12:29:48 -0500
From: Chris Burroughs <chris.burroughs@gmail.com>
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Subject: Re: Cass 2.0.0: Extensive memory allocation when row_cache enabled
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Both caches involve several objects per entry (What do we want?  Packed 
objects.  When do we want them? Now!).  The "size" is an estimate of the 
off heap values only and not the total size nor number of entries.

An acceptable size will depend on your data and access patterns.  In one 
case we had a cluster that at 512mb would go into a GC death spiral 
despite plenty of free heap (presumably just due to the number of 
objects) while empirically the cluster runs smoothly at 384mb.

Your caches appear on the larger size, I suggest trying smaller values 
and only increase when it produces measurable sustained gains.

On 11/05/2013 04:04 AM, Jiri Horky wrote:
> Hi there,
>
> we are seeing extensive memory allocation leading to quite long and
> frequent GC pauses when using row cache. This is on cassandra 2.0.0
> cluster with JNA 4.0 library with following settings:
>
> key_cache_size_in_mb: 300
> key_cache_save_period: 14400
> row_cache_size_in_mb: 1024
> row_cache_save_period: 14400
> commitlog_sync: periodic
> commitlog_sync_period_in_ms: 10000
> commitlog_segment_size_in_mb: 32
>
> -XX:+UseThreadPriorities -XX:ThreadPriorityPolicy=42 -Xms10G -Xmx10G
> -Xmn1024M -XX:+HeapDumpOnOutOfMemoryError
> -XX:HeapDumpPath=/data2/cassandra-work/instance-1/cassandra-1383566283-pid1893.hprof
> -Xss180k -XX:+UseParNewGC -XX:+UseConcMarkSweepGC
> -XX:+CMSParallelRemarkEnabled -XX:SurvivorRatio=8
> -XX:MaxTenuringThreshold=1 -XX:CMSInitiatingOccupancyFraction=75
> -XX:+UseCMSInitiatingOccupancyOnly -XX:+UseTLAB -XX:+UseCondCardMark
>
> We have disabled row cache on one node to see  the  difference. Please
> see attached plots from visual VM, I think that the effect is quite
> visible. I have also taken 10x "jmap -histo" after 5s on a affected
> server and plotted the result, attached as well.
>
> I have taken a dump of the application when the heap size was 10GB, most
> of the memory was unreachable, which was expected. The majority was used
> by 55-59M objects of HeapByteBuffer, byte[] and
> org.apache.cassandra.db.Column classes. I also include a list of inbound
> references to the HeapByteBuffer objects from which it should be visible
> where they are being allocated. This was acquired using Eclipse MAT.
>
> Here is the comparison of GC times when row cache enabled and disabled:
>
> prg01 - row cache enabled
>        - uptime 20h45m
>        - ConcurrentMarkSweep - 11494686ms
>        - ParNew - 14690885 ms
>        - time spent in GC: 35%
> prg02 - row cache disabled
>        - uptime 23h45m
>        - ConcurrentMarkSweep - 251ms
>        - ParNew - 230791 ms
>        - time spent in GC: 0.27%
>
> I would be grateful for any hints. Please let me know if you need any
> further information. For now, we are going to disable the row cache.
>
> Regards
> Jiri Horky
>