First of all, are you using new memory config files inside containers? You cannot use old RHEL5 config files for RHEL6 nodes. At least that is what I have found. You must use new config files that are designed for new memory management technique.

Files are located in /etc/sysconfig/vz-scripts on the Node. You should have sample conf files called ve-vswap-512m.conf-sample. You need to use those. If you use the old beancounter oriented config files such as ve-vps.basic.conf-sample on RHEL6 nodes then wierd things happen. At least that is what I have found.

Also, do you understand how memory caching works? It will try take advantage of all available memory by gradually filling up over time as the system is used. Do not confuse that with unavailable memory. All cached memory is available to the system if it needs it so it is the same as free memory.

The system is just using it as temporary storage until such time as the data is requested again or it needs to allocate it for new use.