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From "Langston, Jim" <>
Subject Re: read ?
Date Tue, 03 Sep 2013 16:33:15 GMT
Thanks Chris,

I have about 8 heap dumps that I have been looking at. I have been trying to isolate
as to why I have be dumping heap, I've started by removing the apps that write to
cassandra and eliminating work that would entail. I am left with just the apps that
are reading the data and from the heap dumps it looks like Cassandra Column methods
being called, because there are so many objects, it is difficult to ascertain exactly what
the problem may be. That prompted my query, trying to quickly determine if Cassandra
holds objects that have been used for reading, and if so, why, and more importantly if
something can be done.


From: "Lohfink, Chris" <<>>
Reply-To: <<>>
Date: Tue, 3 Sep 2013 11:12:19 -0500
To: "<>" <<>>
Subject: RE: read ?

To get an accurate picture you should force a full GC on each node, the heap utilization can
be misleading since there can be a lot of things in the heap with no strong references.

There is a number of factors that can lead to this.  For a true comparison I would recommend
using jconsole and call dumpHeap on with the 2nd
param true (force GC).  Then open the heap dump up in a tool like yourkit and you will get
a better comparison and also it will tell you what it is that’s taking the space.


From: Langston, Jim []
Sent: Tuesday, September 03, 2013 8:20 AM
Subject: read ?

Hi all,

Quick question

I currently am looking at a 4 node cluster and I have currently stopped all writing to
Cassandra,  with the reads continuing. I'm trying to understand the utilization
of memory within the JVM. nodetool info on each of the nodes shows them all
growing in footprint, 2 of the three at a greater rate. On the restart of Cassandra
each were at about 100MB, after 2 days, each of the following are at:

Heap Memory (MB) : 798.41 / 3052.00

Heap Memory (MB) : 370.44 / 3052.00

Heap Memory (MB) : 549.73 / 3052.00

Heap Memory (MB) : 481.89 / 3052.00

Ring configuration:

Address         Rack        Status State   Load            Owns                Token
x     1d          Up     Normal  4.38 GB         25.00%              0
x   1d          Up     Normal  4.17 GB         25.00%              42535295865117307932921825928971026432
x   1d          Up     Normal  4.19 GB         25.00%              85070591730234615865843651857942052864
x   1d          Up     Normal  4.14 GB         25.00%              127605887595351923798765477786913079296

What I'm not sure of is what the growth is different between each ? and why
that growth is being created by activity that is read only.

Is Cassandra caching and holding the read data ?

I currently have caching turned off for the key/row. Also as part of the info command

Key Cache        : size 0 (bytes), capacity 0 (bytes), 0 hits, 0 requests, NaN recent hit
rate, 14400 save period in seconds
Row Cache        : size 0 (bytes), capacity 0 (bytes), 0 hits, 0 requests, NaN recent hit
rate, 0 save period in seconds



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