why SGA + PGA must be maximum 60% of total memory available on a Linux Server

http://blog.ronnyegner-consulting.de/2011/10/13/info-task-blocked-for-more-than-120-seconds/

By default Linux uses up to 40% of the available memory for file system caching. 
You can lower the mark for flushing the cache from 40% to 10% by setting “vm.dirty_ratio=10″ in /etc/sysctl.conf.