Class Name                                                                                                  | Shallow Heap | Retained Heap
-------------------------------------------------------------------------------------------------------------------------------------------
                                                                                                            |              |              
java.nio.HeapByteBuffer @ 0x7806a0848                                                                       |           48 |            80
'- name org.apache.cassandra.db.Column @ 0x7806424e8                                                        |           32 |           112
   |- [338530] java.lang.Object[540217] @ 0x57d62f560 Unreachable                                           |    2,160,888 |     2,160,888
   |- [338530] java.lang.Object[810325] @ 0x591546540                                                       |    3,241,320 |     7,820,328
   |  '- elementData java.util.ArrayList @ 0x75e8424c0                                                      |           24 |     7,820,352
   |     |- list org.apache.cassandra.db.ArrayBackedSortedColumns$SlicesIterator @ 0x5940e0b18              |           48 |           128
   |     |  '- val$filteredIter org.apache.cassandra.db.filter.SliceQueryFilter$1 @ 0x5940e0b48             |           32 |     7,820,568
   |     |     '- val$iter org.apache.cassandra.db.filter.QueryFilter$2 @ 0x5940e0b68 Unreachable           |           24 |     7,820,592
   |     |- this$0, parent java.util.ArrayList$SubList @ 0x5940e0bb8                                        |           40 |            40
   |     |  '- this$1 java.util.ArrayList$SubList$1 @ 0x5940e0be0                                           |           40 |            80
   |     |     '- currentSlice org.apache.cassandra.db.ArrayBackedSortedColumns$SlicesIterator @ 0x5940e0b18|           48 |           128
   |     |        '- val$filteredIter org.apache.cassandra.db.filter.SliceQueryFilter$1 @ 0x5940e0b48       |           32 |     7,820,568
   |     |           '- val$iter org.apache.cassandra.db.filter.QueryFilter$2 @ 0x5940e0b68 Unreachable     |           24 |     7,820,592
   |     |- columns org.apache.cassandra.db.ArrayBackedSortedColumns @ 0x5b0a33488                          |           32 |            56
   |     |  '- val$cf org.apache.cassandra.db.filter.SliceQueryFilter$1 @ 0x5940e0b48                       |           32 |     7,820,568
   |     |     '- val$iter org.apache.cassandra.db.filter.QueryFilter$2 @ 0x5940e0b68 Unreachable           |           24 |     7,820,592
   |     '- Total: 3 entries                                                                                |              |              
   |- [338530] java.lang.Object[360145] @ 0x7736ce2f0 Unreachable                                           |    1,440,600 |     1,440,600
   '- Total: 3 entries                                                                                      |              |              

Are you doing large slices or do could you have a lot of tombstones on the rows ? 

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.
The default row cache is of the JVM heap, have you changed to the ConcurrentLinkedHashCacheProvider ? 

One way the SerializingCacheProvider could impact GC is if the CF takes a lot of writes. The SerializingCacheProvider invalidates the row when it is written to and had to read the entire row and serialise it on a cache miss.

-XX:+UseThreadPriorities -XX:ThreadPriorityPolicy=42 -Xms10G -Xmx10G
-Xmn1024M -XX:+HeapDumpOnOutOfMemoryError
You probably want the heap to be 4G to 8G in size, 10G will encounter longer pauses. 
Also the size of the new heap may be too big depending on the number of cores. I would recommend trying 800M


prg01.visual.vm.png
Shows the heap growing very quickly. This could be due to wide reads or a high write throughput. 

Hope that helps. 

 

-----------------
Aaron Morton
New Zealand
@aaronmorton

Co-Founder & Principal Consultant
Apache Cassandra Consulting
http://www.thelastpickle.com

On 7/11/2013, at 6:29 am, Chris Burroughs <chris.burroughs@gmail.com> wrote:

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