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From Nicholas Knight <>
Subject Re: Config Maximum heap size for Cassandra
Date Thu, 28 Oct 2010 02:15:19 GMT
Presumably you're on a 32-bit architecture (or at least a 32-bit JVM). 32-bit processes won't
be able to address more than "X" amount of memory, where X would usually be >= 2GB, and
< 4GB.

The reason you can't use a full 4GB is that part of the address space is necessarily reserved
by the OS kernel. Exactly how much is reserved depends on the OS, version thereof, and/or
configuration, but at least 0.5-1GB is usually a safe bet, so the behavior you see is exactly
as expected.

If your hardware is 64-bit, make sure you're running a 64-bit OS and a 64-bit JVM. If you're
stuck on 32-bit hardware that just happens to have lots of RAM, you could run multiple Cassandra
instances on each box..


On Oct 28, 2010, at 10:04 AM, JKnight JKnight wrote:

> Hi all, 
> When I config Maximum heap size -Xmx4G, the memory will consume to 3.5G. When I call
Perform GC (jconsole), the used memory reduce to 1G. 
> When I config Maximum heap size -Xmx2G, Cassandra system run well. 
> Is that Casandra problem? 
> I want Cassandra use memory more effective. How can I do that?
> Thank a lot for support. 
> -- 
> Best regards,
> JKnight

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