On a 7.5 GB host (Ubuntu 16.04.5 LTS), I have allocated 3 GB to shared_buffers:
postgres=# SHOW shared_buffers;
shared_buffers
----------------
3GB
(1 row)
with:
postgres@my-host:~$ cat /etc/postgresql/10/main/postgresql.conf | grep shared_buffers
shared_buffers = 3072MB # min 128kB, valid units are kB, MB, GB, TB
I have also configured my host with HP as:
postgres@my-host:~$ cat /proc/meminfo | grep ^Huge
HugePages_Total: 1600
HugePages_Free: 1600
HugePages_Rsvd: 0
HugePages_Surp: 0
Hugepagesize: 2048 kB
With a Hugepagesize at 2048 kB, that works out as 1600*2048/1024 = 3200 MB of HP. When I then try to turn:
postgres@raeflightcachemaster0-nonprod:~$ cat /etc/postgresql/10/main/postgresql.conf | grep huge_pages
huge_pages = on
PostgreSQL (10.5) is reporting:
2018-09-27 09:31:54.397 BST [16739] FATAL: could not map anonymous shared memory: Cannot allocate memory
2018-09-27 09:31:54.397 BST [16739] HINT: This error usually means that PostgreSQL's request for a shared memory segment exceeded available memory, swap space, or huge pages. To reduce the request size (currently 3523592192 bytes), reduce PostgreSQL's shared memory usage, perhaps by reducing shared_buffers or max_connections.
2018-09-27 09:31:54.397 BST [16739] LOG: database system is shut down
pg_ctl: could not start server
Examine the log output.
and the DB server fails to start.
My question is, where is it getting the number 3523592192 bytes from? This value does not match the shared_buffers = 3072MB setting in postgresql.conf as it works out as 3523592192/1024/1024 = 3360.359375 MB. I get around the problem by increasing HugePages_Total slightly to 1700 so that we have 3400 MB of HP but I don't understand the discrepancy between the reported value of shared_bufferes and the configured value. Is this a units issue?
