if using 1.2.*, Bloom filters are in native memory so not pressuring your heap, how many data do you have per node ? If this value is big, you have samples index in the heap consuming a lot of memory, for sure, and growing as your data per node grow.

Solutions : increase the heap if < 8GB and / or reduce sampling index_interval: 128 to a bigger value (256 - 512) and /or wait for 2.0.* which, of the top of my head, should move the sampling in native memory allowing heap size to be independent from the data size per node.

This should alleviate things. Yet these are only guesses since I know almost nothing about your cluster...

Hope this help somehow.

2013/8/12 Robert Coli <rcoli@eventbrite.com>
On Mon, Aug 12, 2013 at 11:14 AM, Paul Ingalls <paulingalls@gmail.com> wrote:
I don't really need exact numbers, just a rough cost would be sufficient.  I'm running into memory problems on my cluster, and I'm trying to decide if reducing the number of column families would be worth the effort.  Looking at the rule of thumb from the wiki entry made it seem like reducing the number of tables would make a big impact, but I'm running 1.2.8 so not sure if it is still true.

Is there a new rule of thumb?
If you want a cheap/quick measure of how much space partially full memtables are taking, just nodetool flush and check heap usage before and after?

If you want a cheap/quick measure of how much space empty sstables take in heap, I think you're out of luck.