For z Systems optimizations are essential to keep the consolidation ratio high and make the best use of the hardware. As more and more open source software is ported to Linux on z / LinuxONE Kevin Shum has posted his excellent "BM z Systems Processor Optimization Primer" on developerworks.
There is a long version with all the details and a shorter summary presented at last SHARE.
This is a "must read" for everyone working on compilers and inline assembly on z.