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From Tamar Fraenkel <ta...@tok-media.com>
Subject Heap size question
Date Tue, 21 Aug 2012 09:13:06 GMT
Hi!
I have a question regarding Cassandra heap size.
Cassandra calculates heap size in cassandra-env.sh according to the
following algorythm
    # set max heap size based on the following
    # max(min(1/2 ram, 1024MB), min(1/4 ram, 8GB))
    # calculate 1/2 ram and cap to 1024MB
    # calculate 1/4 ram and cap to 8192MB
    # pick the max

So, for
system_memory_in_mb=7468
half_system_memory_in_mb=3734
quarter_system_memory_in_mb=1867
This will result in
max(min(3734,1024), min(1867,8000)) = max(1024,1867)=*1867MB* or in other
words 1/4 of RAM.

In http://www.datastax.com/docs/1.0/operations/tuning it says: "Cassandra's
default configuration opens the JVM with a heap size of 1/4 of the
available system memory (or a minimum 1GB and maximum of 8GB for systems
with a very low or very high amount of RAM). Heapspace should be a minimum
of 1/2 of your RAM, but a maximum of 8GB. The vast majority of deployments
do not benefit from larger heap sizes because (in most cases) the ability
of Java 6 to gracefully handle garbage collection above 8GB quickly
diminishes."
*If I understand this correctly, this means it is better if my heap size
will be 1/2 of RAM, 3734MB.*
I am running on EC2 m1.large instance (7.5 GB memory, 4 EC2 Compute Units
(2 virtual cores with 2 EC2 Compute Units each)).
My system seems to be suffering from lack of memory, and I should probably
increase heap or (and?) reduce key cache size.

Would you recommend changing the heap to half RAM?

If yes, should I hard-code it in acassandra-env.sh?

Thanks!

*Tamar Fraenkel *
Senior Software Engineer, TOK Media

[image: Inline image 1]

tamar@tok-media.com
Tel:   +972 2 6409736
Mob:  +972 54 8356490
Fax:   +972 2 5612956

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