Oracle Database Discussions

Hello everyone

I just read this slide deck from some folks from Oracle re database consolidation.....

http://www.slideshare.net/MarkusMichalewicz/cloud-consolidation-with-oracle-rac-how-much-is-too-much

in it there is a reasonable 80%-20% suggestion as a rule of thumb when consolidating several Oracle 11g R2 instances on one physical server regarding memory allocation, 80% for Oracle DBs and 20% for the OS.

This seems to be OK for small to medium servers with up to ~64-96 GB of RAM.

Let us now look at the current generation of Intel X64 servers with 4 CPU sockets with 10 cores, = 40-cores (=80 logical CPUs, if using Hyper-Threading) and relatively affordable 512GB of RAM (8GB DIMMS are cheap now) being used for database consolidation of perhaps 10 Oracle database instances.

Given such a large DB server, how much RAM should I leave for Linux 6.x OS (with UEK) ?
Assume 2 scenarios:
(1) using HugePages (recommended by Oracle)
(2) NOT using Hugepages ?

If I use 80%-20% rule then I would need to allocate massive 128GB of RAM just for the Linux OS - just to run a bunch of Oracle database instances. This seems very excessive, no ?
I would think much of this RAM would be better used for SGA/PGA needs.
Am I wrong in this? How much memory does Linux need to support 512GB of physical RAM, worst case?

thanks
Yuri