Postgresql in Ubuntu distribution

Ubuntu has default limitation for shared memory about 32 MB. This is why (I guess) packaged Postgres has shared_buffers parameter set to modest 24 MB.

This is quite low value for a large DB and for modern hardware. There are numerous recommendations on how big this value should be. It makes sense to try setting this value and run your benchmarks.

To increase kernel shared memory limitation edit /etc/sysctl.conf and add or replace the following (about 110 MB in this example):

kernel.shmmax = 110000000

Then run

sudo sysctl -p

to make the settings be effective immediately.