Hello Thomas, Thank you for explanation. work_mem = 512MB and max_parallel_workers_per_gather = 2 and I run only one Postgres instance and only one query. EXPLAIN shows "Workers Planned: 2" for this query. Why it can use more than 1GB of /dev/shm?
Konstantin > On 9 Jul 2019, at 13:51, Thomas Munro <thomas.mu...@gmail.com> wrote: > > On Tue, Jul 9, 2019 at 10:15 PM Jean Louis <bugs@gnu.support> wrote: >> * Konstantin Malanchev <hom...@gmail.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
