They said 3k+ databases per host, not 3k instances per server. One of the slides said 8 hosts in 3 continents, so each "host" probably has multiple servers.

Some config parameters that will affect memory usage:

  Advanced options:
    --osv-memory-count-limit=OSV_MEMORY_COUNT_LIMIT
                        Force a limit on the maximum number of records kept in
                        the virtual osv_memory tables. The default is False,
                        which means no count-based limit.
    --osv-memory-age-limit=OSV_MEMORY_AGE_LIMIT
                        Force a limit on the maximum age of records kept in
                        the virtual osv_memory tables. This is a decimal value
                        expressed in hours, and the default is 1 hour.
    --max-cron-threads=MAX_CRON_THREADS
                        Maximum number of threads processing concurrently cron
                        jobs (default 2).

  Multiprocessing options:
    --workers=WORKERS   Specify the number of workers, 0 disable prefork mode.
    --limit-memory-soft=LIMIT_MEMORY_SOFT
                        Maximum allowed virtual memory per worker, when
                        reached the worker be reset after the current request
                        (default 671088640 aka 640MB).
    --limit-memory-hard=LIMIT_MEMORY_HARD
                        Maximum allowed virtual memory per worker, when
                        reached, any memory allocation will fail (default
                        805306368 aka 768MB).
    --limit-request=LIMIT_REQUEST
                        Maximum number of request to be processed per worker
                        (default 8192).

Try and play with those.