On Mon, Jun 3, 2019 at 5:56 AM Daulat Ram <Daulat.Ram@xxxxxxxxxxxxxxx> wrote: > Cause: org.postgresql.util.PSQLException: ERROR: could not resize shared memory segment "/PostgreSQL.1946998112" to 8388608 bytes: No space left on device > shm on /dev/shm type tmpfs (rw,context="system_u:object_r:container_file_t:s0:c127,c569",nosuid,nodev,noexec,relatime,size=65536k) I don't use Docker myself but I believe you can either tell it to expose the host's /dev/shm or you can start it with something like --shm-size="4096m". It's surprisingly difficult to know how much shared memory PostgreSQL really needs, today. Without parallel query, it's complicated but roughly: you can use work_mem for each executor node, and that is (at a first approximation) linked to how many joins there are in your query. With the advent of parallel query, it's multiplied by the number of workers. If any of the joins happen to be parallel hash join, then the memory comes out of shared memory instead of private memory. It's not fundamentally different in terms of the amount needed, but Docker users are forced to confront the question so they can set --shm-size. One argument is that you should set it sky-high, since private memory has no such limit, and from a virtual memory point of view it's all the same in the end, it's just memory. The difficulty with choosing a limit here is that --shm-size is a system-wide limit, but PostgreSQL not only has no system-wide memory budgeting, it doesn't even have a per-query memory budget. It just has this "per operation" (usually executor node) thing. The total amount of memory you need to run PostgreSQL queries is a function of work_mem * number of concurrent queries you expect to run * number of tables you expect them to join * number of parallel workers you expect to run. The amount of it that happens to be in /dev/shm on a Linux system (rather than private memory) is controlled by what fraction of your joins are parallel hash joins. Making our memory limits better is really hard. -- Thomas Munro https://enterprisedb.com
