num_os_semaphores

Definition PG 18 manual

Reports the number of semaphores that are needed for the server based on the configured number of allowed connections (max_connections), allowed autovacuum worker processes (autovacuum_max_workers), allowed WAL sender processes (max_wal_senders), allowed background processes (max_worker_processes), etc.

How it works

Shows the number of semaphores required for the server. It is read-only and cannot be changed with SET or configuration-file edits.

It is computed from the startup process and connection configuration so administrators can plan System V semaphore capacity.

Interpret it with shared_memory_size, shared_memory_size_in_huge_pages, huge_pages. Applications and operations tooling may read num_os_semaphores for capability or environment detection, but cannot SET it; cross-cluster comparisons must account for build, initdb, and current-database differences.

Operational considerations

Trying to change read-only num_os_semaphores with SET or a configuration file.

Assuming every cluster, database, or vendor build reports the same value.

Ignoring the value in file-format or feature-compatibility checks.

Treating reported metadata as a performance target instead of an environment fact.

Workload guidance

OLAP: Not tunable. Use num_os_semaphores to verify format, build, or resource assumptions on analytical nodes and keep every read/write candidate compatible.

OLTP: Not tunable. Record num_os_semaphores as a deployment/capability fact; if it is unexpected, fix the binary, initdb, or database-creation process rather than trying SET.

SMALL: Not tunable. Preserve it in inventory and incident reports; do not custom-build or rebuild merely to change it without a demonstrated compatibility need.