When 1500 processes share memory, page tables can eat 878 GB
Page Table Memory Consumption
Page tables are usually under 1% of mapped memory, but that ratio breaks when many processes share the same pages. Fernando Simões traces the problem from Linus Torvalds' 2003 defense of tree page tables to real incidents: Andrea Arcangeli's 64 GiB x86 machines running out of lowmem, Khalid Aziz's 512 GB Oracle server crashing with 1500 clients on a 300 GB SGA, and Postgres and ClickHouse deployments where page tables ballooned to tens of GiB. Huge pages help but aren't free, and NUMA adds a new trade-off between remote table walks and replication.
In the worst case, where every process maps the whole SGA, PTEs alone would take 878 GB.