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From Nicholas Knight <nkni...@runawaynet.com>
Subject Re: Config Maximum heap size for Cassandra
Date Thu, 28 Oct 2010 02:51:38 GMT
Cassandra needs all the RAM you can give it so it can cache things for optimum performance.
If you need it to use less, give it less.

-NK


On Oct 28, 2010, at 10:30 AM, JKnight JKnight wrote:

> Could you tell me why Cassandra use memory more than needed?
> 
> 
> On Thu, Oct 28, 2010 at 9:15 AM, Nicholas Knight <nknight@runawaynet.com> wrote:
> 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..
> 
> -NK
> 
> 
> 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
> 
> 
> 
> 
> -- 
> Best regards,
> JKnight


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