About MyISAM's key cache

i have one questions about myisam’s key cache
1.In mysql’s document,it said “A buffer that is being updated causes sessions that need to use it to wait until the
update is complete”,My question is “does the update set an global mutex to block all sessions but itself or just set
an mutex on related cache index”

thanks in advance!

This blog post, and similar on the same blog, have some good reading.

http://www.mysqlperformanceblog.com/2008/11/26/using-multipl e-key-caches-for-myisam-scalability/