Hi!
This could also reduce memory usage by not using memcached (as often) which, I understand, is a bigger problem.
No it is not.
First of all, our memcached and database access times not that far away - 0.7 vs 1.3 ms (again, memcached is static response time, whereas database average is impacted by calculations). On another hand, we don't store in memcached what is stored in database and we don't store in database what is stored in memcached.
Think about these as two separate systems, not as complementing each other too much. We use memcached to offload application cluster, not database cluster.
And database cluster already has over a terabyte of RAM (replicas and whatnot), whereas our memcached lives in puny 158GB arena.
I described some of fundamental differences of how we use memcached in http://dom.as/uc/workbook2007.pdf - pages 11-13. Nothing much changed since then.
Domas