Resolving PostgreSQL `shared_buffers` Memory Allocation Crash on macOS

Troubleshoot and fix PostgreSQL startup failures on macOS caused by excessive `shared_buffers` configuration exceeding system or kernel memory limits.

Introduction

Running PostgreSQL on a local macOS development environment is a common practice. However, if you've been optimizing your postgresql.conf for performance by increasing the shared_buffers parameter, you might encounter a persistent issue where PostgreSQL refuses to start, emitting cryptic memory allocation errors. This typically occurs when the configured shared_buffers value exceeds the system's kernel-imposed limits for shared memory segments, which are often conservative by default on macOS. This guide will walk you through diagnosing and resolving this specific memory limit configuration crash.

Symptom & Error Signature

When attempting to start PostgreSQL, either manually via pg_ctl or using brew services, the server will fail to initialize. The most common symptom is a "FATAL" error message in the PostgreSQL logs, indicating a failure to create a shared memory segment.

You might see output similar to this in your terminal when attempting to start via brew services:

$ brew services start postgresql
Bootstrap failed: 5: Input/output error
Try again: brew services --debug start postgresql
Error: Failure while executing; `/bin/launchctl bootstrap gui/501 /Users/youruser/Library/LaunchAgents/homebrew.mxcl.postgresql.plist` exited with 5.

The underlying cause will be logged in PostgreSQL's server log file (often found in /usr/local/var/log/postgres.log or similar for Homebrew installations). The critical error signature typically looks like this:

2026-08-14 10:30:05.123 PDT [12345] FATAL:  could not create shared memory segment: Cannot allocate memory
2026-08-14 10:30:05.123 PDT [12345] DETAIL:  Failed system call was shmget(KEY, SIZE_IN_BYTES, 0600).
2026-08-14 10:30:05.123 PDT [12345] HINT:  This error usually means that PostgreSQL's request for a shared memory segment exceeded available RAM or macOS's SHMMAX parameter. You might need to reduce the effective shared_buffers parameter (currently NGB).

Root Cause Analysis

The "FATAL: could not create shared memory segment: Cannot allocate memory" error arises because PostgreSQL requires a contiguous block of shared memory for its shared_buffers parameter. Unix-like operating systems, including macOS, provide System V Inter-Process Communication (IPC) shared memory facilities, which are governed by kernel parameters.

On macOS, the key kernel parameters controlling shared memory are:

• kern.sysv.shmmax: The maximum size, in bytes, of a single shared memory segment. PostgreSQL's shared_buffers value must be less than or equal to this limit.
• kern.sysv.shmall: The maximum total amount of shared memory that can be allocated system-wide, specified in "pages" (where a page is typically 4096 bytes or 4KB on macOS).
• kern.sysv.shmseg: The maximum number of shared memory segments per process.
• kern.sysv.shmmni: The maximum number of shared memory identifiers (segments) system-wide.

The default values for kern.sysv.shmmax and kern.sysv.shmall on macOS are often set quite low for a development machine. When a user configures shared_buffers in postgresql.conf to a value (e.g., 2GB, 4GB) that exceeds these kernel limits, PostgreSQL's attempt to allocate the required memory segment fails, leading to the crash.

While increasing kernel shared memory limits can resolve this specific issue, carelessly setting shared_buffers too high can starve other applications of memory or lead to excessive swapping, degrading overall system performance. Always balance shared_buffers with your total available RAM and other system processes.

Step-by-Step Resolution

Follow these steps to diagnose and resolve the PostgreSQL shared_buffers memory limit crash on macOS.

1. Identify the Current shared_buffers Configuration

First, locate your PostgreSQL configuration file (postgresql.conf) and determine the value set for shared_buffers.

# Find the postgresql.conf file (common Homebrew path example)
POSTGRES_CONF_PATH=$(find /usr/local -name postgresql.conf 2>/dev/null | head -n 1)
echo "PostgreSQL config path: $POSTGRES_CONF_PATH"

# Check the shared_buffers setting
grep -E '^shared_buffers' "$POSTGRES_CONF_PATH"

You'll see an output like:

shared_buffers = 2GB
#shared_buffers = 128MB          # min 128kB

Note the uncommented shared_buffers value (e.g., 2GB). This is the target value we need to accommodate.

2. Check Current macOS Shared Memory Limits

Next, examine the current kernel limits for shared memory on your macOS system.

sysctl kern.sysv.shmmax
sysctl kern.sysv.shmall
sysctl kern.sysv.shmseg
sysctl kern.sysv.shmmni

Example output:

kern.sysv.shmmax: 4194304
kern.sysv.shmall: 1024
kern.sysv.shmseg: 32
kern.sysv.shmmni: 32

In this example:

• shmmax is 4MB (4194304 bytes). If your shared_buffers is, say, 2GB, this is clearly the bottleneck.
• shmall is 1024 pages. Given a 4KB page size, this means a total of 4MB (1024 * 4096 bytes).

3. Calculate Required Shared Memory Limits

To allow PostgreSQL to start with your desired shared_buffers value, you need to set kern.sysv.shmmax and kern.sysv.shmall appropriately.

• kern.sysv.shmmax: This should be at least equal to your shared_buffers value, plus a small buffer. If shared_buffers = 2GB, then shmmax should be at least 2 * 1024 * 1024 * 1024 bytes. It's often recommended to set it slightly higher than your shared_buffers for flexibility. For 2GB, setting it to 2147483648 (2GB) or 4294967296 (4GB) might be appropriate.

• kern.sysv.shmall: This is the total shared memory in pages. You need to convert your shared_buffers value to pages. Assuming a 4KB page size: Required_shmall = (shared_buffers_in_bytes / 4096). For 2GB (2147483648 bytes): 2147483648 / 4096 = 524288 pages. It's good practice to set shmall to cover a larger portion of your system's total RAM in pages, typically 25% of your system's physical RAM, converted to pages. For a system with 16GB RAM: (16 * 1024 * 1024 * 1024) / 4096 = 4194304 pages. So, 524288 pages for 2GB shared_buffers is well within a 16GB system's capacity.

The general recommendation for shared_buffers is not to exceed 25% of your total system RAM on a dedicated database server, or even less on a development machine that runs other applications. For a typical macOS development setup, 128MB-1GB is usually sufficient unless you're specifically performance testing large datasets.

4. Temporarily Adjust macOS Shared Memory Limits

You can temporarily adjust these limits using sudo sysctl -w. This is useful for testing before making permanent changes.

# Example: Set shmmax to 4GB (4294967296 bytes) and shmall to 4194304 pages (16GB total shared memory capacity)
sudo sysctl -w kern.sysv.shmmax=4294967296
sudo sysctl -w kern.sysv.shmall=4194304

# Verify the changes
sysctl kern.sysv.shmmax
sysctl kern.sysv.shmall

5. Make macOS Shared Memory Limits Permanent

Temporary changes made with sysctl -w will be lost on reboot. To make them permanent, you need to create or modify a sysctl configuration file.

# Create or open a new sysctl configuration file for PostgreSQL
# It's good practice to use a specific file in /etc/sysctl.d/
sudo nano /etc/sysctl.d/99-postgresql.conf

Add the following lines to the file, using the calculated values from Step 3. Adapt these values to your specific shared_buffers setting and available RAM.

# /etc/sysctl.d/99-postgresql.conf
# PostgreSQL shared memory limits for macOS
kern.sysv.shmmax=4294967296  # Example: 4GB (adjust based on your RAM and shared_buffers)
kern.sysv.shmall=4194304     # Example: 16GB in pages (4096 * 4194304 = 17179869184 bytes total shared memory)

Save the file and exit the editor (Ctrl+X, Y, Enter for nano).

To apply these changes without rebooting, load the new configuration:

sudo sysctl -p /etc/sysctl.d/99-postgresql.conf

6. Adjust PostgreSQL shared_buffers (If Necessary)

If you find that even with adjusted kernel parameters, PostgreSQL still struggles or if you simply don't have enough RAM to allocate the desired shared_buffers, you might need to reduce the shared_buffers value itself.

Open your postgresql.conf file:

sudo nano "$POSTGRES_CONF_PATH"

Find the shared_buffers line and adjust it to a more conservative value, uncommenting it if it's commented out:

# postgresql.conf
shared_buffers = 512MB          # Example: A more reasonable value for a local dev environment

While reducing shared_buffers will often solve the crash, setting it too low can negatively impact PostgreSQL's performance, especially for I/O-intensive workloads. Aim for a value that is stable and performs adequately for your development needs.

7. Restart PostgreSQL

After making the necessary kernel parameter and/or postgresql.conf changes, attempt to restart PostgreSQL.

# For Homebrew installations
brew services stop postgresql
brew services start postgresql

Verify that PostgreSQL is now running successfully:

brew services info postgresql

If it started, the status should indicate started and you should be able to connect to your database. Check the PostgreSQL logs again to confirm there are no new errors:

# Find the log file path
PG_LOG_PATH=$(brew services info postgresql | grep 'Log' | awk '{print $NF}')
echo "PostgreSQL log path: $PG_LOG_PATH"

# Tail the log file to see recent entries
tail -n 50 "$PG_LOG_PATH"

You should see startup messages without the "FATAL: could not create shared memory segment" error.

(Optional) 8. Linux Server Equivalent for Shared Memory Configuration

While this guide focuses on macOS, the principles for shared memory configuration are similar on Linux-based hosting environments. On systems running Ubuntu/Debian or other distributions, the relevant sysctl parameters are:

• kernel.shmmax
• kernel.shmall
• kernel.shmmni

These are also configured in /etc/sysctl.conf or a file within /etc/sysctl.d/.

# /etc/sysctl.d/99-postgresql.conf (on Linux)
# PostgreSQL shared memory limits
kernel.shmmax = 17179869184 # Example: 16GB
kernel.shmall = 4194304     # Example: 16GB in pages (4096 * 4194304 = 17179869184 bytes)

After modifying on Linux, apply with:

sudo sysctl -p

For systems using Docker, PostgreSQL's shared_buffers typically uses shared memory provided by the container runtime. The Docker daemon and docker-compose can be configured with shm_size to provide adequate shared memory within the container, overriding the host's default /dev/shm size.

# docker-compose.yml
services:
  db:
    image: postgres:16
    shm_size: '2gb' # Allocate 2GB for shared memory inside the container
    environment:
      POSTGRES_DB: mydb
      POSTGRES_USER: user
      POSTGRES_PASSWORD: password

This ensures that the container has enough shared memory for PostgreSQL's shared_buffers without hitting host kernel limits directly.