@osschar @abh3 - I think we're over-thinking things. We simply need the server to still function after upgrade: we don't need to have it appropriately optimized to take advantage of new features!
There's no way to reliably determine "optimal" memory usage, especially not in Linux. Even looking at the number of the memory pages in the system can be wildly incorrect (systemd starts you up in a cgroup, after all!).
Suggestion:
- Pick a random default value. Say, 1GB RAM.
- Have a BIG LOUD WARNING message in the log if V1-style attributes are utilized and use the default value for RAM.
- If V2-style attributes are used, refuse to startup unless the admin specifies the RAM option.
This way, admins are forced to make a decision for new installs - but old installs continue to work (albeit sub-optimally). The analogy here is ext3: even though the filesystem dates back to ~RHEL3, if you didn't reformat the partition when upgrading RHEL4 -> RHEL5, it would have quite sub-optimal performance as the old data formats were still present.
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