Memory footprint diff between 9.5 and 12

[Tory M Blue <tmblue@xxxxxxxxx>]

I hadn't noticed this until today, but a running 9.5 system with buffers at 10GB starts and has been running years without issues. (15GB available)

Postgres 12 will not start with that configuration, complaining about memory availability.  So Postgres12 won't start until shared buffers is 6GB, but even with that, my DB servers , postgres queries started complaining about being unable to allocate memory "unable to allocate".

So dropping them to 4GB (on a 15GB system), may help, but did I miss a huge note about significant memory changes between 9.5 to 12?

Is there something else I'm missing that on busy systems is important, something introduced in 10 or 11 as again I'm not seeing anything noted in 12.

Thanks

Tory