PostgreSQL "could not resize shared memory segment": Where Does Memory Run Out?

The Docker /dev/shm 64MB default limit, parallel query shared memory exhaustion, proper work_mem calculation, and ordered remediation.

Reviewed by:Senior Database Architect & SRE (dataiotech Labs)

Tested Versions: PostgreSQL 14.10, 15.6, 16.4, 17.0 (Ubuntu 24.04 LTS / Docker 26.1)

Operational Verification & Reliability Metadata

Last Technical Review: 27 Ekim 2026

Tested VersionsPostgreSQL 14.10, 15.6, 16.4, 17.0 (Ubuntu 24.04 LTS, Docker 26.1, Kernel 6.8)

Required PrivilegesSUPERUSER or pg_read_all_stats / pg_signal_backend

Risk Assessment

MediumTerminating active sessions forces application-layer transaction rollbacks

Expected OutputTotal connections drop safely below max_connections threshold in pg_stat_activity

Rollback Procedure:Reset temporary session timeouts, reload PgBouncer or revert pool sizing

PostgreSQL Docs: Connections

First 60 Seconds Emergency Protocol:

The error string is a symptom, not the root cause. Do not restart the server blindly. Run the diagnostic queries below to pinpoint the actual failure mode.

▸Symptoms

plaintext

ERROR:  could not resize shared memory segment: No space left on device
ERROR:  out of memory

The issue is almost universally Docker's default 64MB mount for /dev/shm.

▸Ordered Fix

1Adjust Docker `--shm-size`: Set --shm-size=1g or in docker-compose.yml:

yaml

   services:
     postgres:
       shm_size: 1gb

1Emergency Query Throttle: Disable parallel gathering temporarily:

sql

SET max_parallel_workers_per_gather = 0;

1Audit `work_mem` Formula:

plaintext

work_mem ≈ (RAM - shared_buffers) / (max_connections * 3)

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