No matter how I look at it this doesnt seem right:

First - why and how the heck are you able to "smash" ab against spawn-fcgi (or I do misunderstood something)? Because spawn-fcgi isnt a permanent process nor does it handle any HTTP requests (or acts as fcgi gateway to handle the requests from nginx) .. spawn-fcgi is only an executable provided with lighttpd to spawn PHP master process (with setuid/setgid feature when the adaptive spawning on lighty was basically disabled). You can fully do the same just with php executable (built as fastcgi) eg provide port/socket and other settings from command line (and some from ENV variables).

So I can't really get how can spawn-fcgi eat up ram because it shouldnt be on the process list at all..

Second - MySQL ram usage on the other hand is unbelievable small.. but thats another discussion :)

rr