Recently I tested our new segmented key cache feature for MyISAM in MariaDB 5.2.2-gamma for performance gains. You can check our new features in MariaDB 5.2 in our Ask Monty Knowledge Base
You will also find the details about the segmented key cache feature in our Knowledge Base at:
We wrote a test in LUA for SysBench v0.5 called select_random_points.lua, to figure out the performance gain of splitting the key cache’s global mutex into several mutex under multi user load.
You can find all the details about the benchmark in our Knowledge Base article here: