[syslog-ng] linux kernel tuning

Hi all, What are your experiences with tuning the kernel for UDP, TCP, etc. How long does it take to see results from a single change? We are working issues with UDP drops, and have made fairly radical increases in buffer sizes with no apparent improvement, only to re-visit the system in a few days and seeing it cleaned up. "intuitively" I have always thought it should happen right away - but that is not what we are seeing here now. Thoughts? Thanks in advance! Jim