Re: PGSQL 11.4: shared_buffers and /dev/shm size

On Tue, Jul 9, 2019 at 10:15 PM Jean Louis <bugs(at)gnu(dot)support> wrote:
> * Konstantin Malanchev <hombit(at)gmail(dot)com> [2019-07-09 12:10]:
> > I have 8 GB RAM and /dev/shm size is 4GB, and there is no significant memory usage by other system processes. I surprised that Postgres uses more space in /dev/shm than sharred_buffers parameter allows, probably I don't understand what this parameter means.
> >
> > I have no opportunity to enlarge total RAM and probably this query requires too much RAM to execute. Should Postgres just use HDD as temporary storage in this case?
>
> That I cannot know. I know that /dev/shm could
> grow as much as available free RAM.

Hi,

PostgreSQL creates segments in /dev/shm for parallel queries (via
shm_open()), not for shared buffers. The amount used is controlled by
work_mem. Queries can use up to work_mem for each node you see in the
EXPLAIN plan, and for each process, so it can be quite a lot if you
have lots of parallel worker processes and/or lots of
tables/partitions being sorted or hashed in your query.

--
Thomas Munro
https://enterprisedb.com