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From Jedd Rashbrooke <>
Subject Re: Dazed and confused with Cassandra on EC2 ...
Date Fri, 08 Oct 2010 09:54:08 GMT
On 7 October 2010 20:49, Peter Schuller <> wrote:
> ... if you "waste" 10-15 gigs of RAM on the JVM heap for a
> Cassandra instances which could live with e.g. 1 GB, you're actively
> taking away those 10-15 gigs of RAM from the operating system to use
> for the buffer cache. Particularly if you're I/O bound on reads then,
> this could have very detrimental effects (assuming the data set is

 Peter - in my defense, I was doing this on 68GB mega-boxes :)  But,
 yeah, we're seeing far more *predictable* results from the OS's ability
 to cache file system access, than Cassandra-with-Java's ability to
 manage its own memory.  My own sys-admin experience previously
 had been on real hardware, usually 16GB, occasionally 32GB boxes,
 but with *much* larger file systems than we're using with Cassandra,
 hence my earlier optimism at leaving a 'mere' 30-40GB for FS cache.

On 8 October 2010 02:05, Matthew Dennis <> wrote:
> Also, in general, you probably want to set Xms = Xmx (regardless of the
> value you eventually decide on for that).

 Matthew - we'd just about reached that conclusion!  Is it as big an
 issue for clusters that are up for days at a time, with maybe Xmx:3G?
 I'd kind of assumed that it'd get to its max pretty quickly.  I haven't
 been watching that with jconsole, but might watch it on the next startup.

 As to heap size determination ... the equation offered:

 MemtableThroughputInMB * 3 * (number of ColumnFamilies)
      + 1G + (size of internal caches)

 -- has (for us, at least) 3 constants and 2 quite fuzzy values.

 The experiments continue!


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