How to Configure Kernel Memory Limits on FreeBSD Operating System

Introduction

FreeBSD is a powerful and versatile Unix-like operating system known for its advanced networking, performance, and security features. One of the crucial aspects of managing a FreeBSD system is configuring kernel memory limits, which ensures system stability and optimal performance by preventing excessive memory consumption by processes and system components.

This guide will provide a detailed step-by-step approach to configuring kernel memory limits on FreeBSD, including understanding various kernel memory parameters, modifying system settings, and ensuring stability.

Understanding Kernel Memory Management in FreeBSD

FreeBSD uses a virtual memory system that consists of several memory allocation zones. Key areas of memory allocation include:

• KVA (Kernel Virtual Address Space): Manages kernel memory allocation.
• UMA (Universal Memory Allocator): Handles dynamic allocation of kernel memory.
• Kernel malloc(): Allocates memory dynamically for system components.
• Kernel Stacks: Stores process control information.

To configure kernel memory limits, we need to focus on various parameters that control system memory allocation.

Checking Current Kernel Memory Usage

Before modifying any limits, it is important to check the current memory usage and kernel settings. FreeBSD provides several tools for this purpose:

1. Using sysctl to View Kernel Memory Parameters:

   sysctl -a | grep vm
   

   This command lists all kernel memory-related parameters.

2. Checking Memory Statistics with vmstat:

   vmstat -s
   

   This displays various memory statistics, including free and allocated memory.

3. Using top to Monitor Memory Usage:

   top -o size
   

   This shows real-time memory usage by processes.

4. Checking Kernel Memory Zones with sysctl vm.zone:

   sysctl vm.zone
   

   This command provides details about memory allocation zones.

Configuring Kernel Memory Limits

1. Modifying kern.maxusers

kern.maxusers is a system parameter that indirectly affects various system limits, including kernel memory allocation. Increasing this value will increase default kernel memory limits.

Check the current setting:

sysctl kern.maxusers

Modify it by adding the following line to /etc/sysctl.conf:

kern.maxusers=256

Apply the changes:

sysctl -w kern.maxusers=256

2. Adjusting Kernel Memory Parameters

Several sysctl parameters directly affect kernel memory limits:

kern.ipc.shmmax

Controls the maximum shared memory segment size:

sysctl kern.ipc.shmmax=1073741824  # Set to 1GB

To make this change permanent, add it to /etc/sysctl.conf:

kern.ipc.shmmax=1073741824

kern.ipc.shmall

Controls the total amount of shared memory:

sysctl kern.ipc.shmall=262144

vm.kmem_size

Defines the maximum size of kernel memory:

sysctl vm.kmem_size=2147483648  # Set to 2GB

3. Configuring Limits in /boot/loader.conf

For permanent changes, some kernel parameters must be set in /boot/loader.conf. Modify the file:

nano /boot/loader.conf

Add the following lines:

vm.kmem_size="2G"
vfs.zfs.arc_max="1G"
kern.ipc.shmmax="1073741824"
kern.ipc.shmall="262144"

Save the file and reboot the system:

reboot

4. Configuring Per-Process Limits

The limits command can be used to check and modify per-process memory limits. View current limits:

limits

Modify limits using ulimit:

ulimit -m 1048576  # Set to 1GB per process

For system-wide changes, edit /etc/login.conf:

default:
    :memoryuse=1024M:

Rebuild the database:

cap_mkdb /etc/login.conf

5. Configuring Swap Space

If the system runs low on memory, swap space can help prevent crashes. Check current swap usage:

swapinfo -h

To add more swap space:

dd if=/dev/zero of=/swapfile bs=1M count=2048
chmod 600 /swapfile
mdconfig -a -t vnode -f /swapfile -u 0
swapon /dev/md0

To make it persistent, add to /etc/fstab:

/dev/md0 none swap sw 0 0

Testing and Verifying Configuration

After making changes, it is important to test the system:

1. Check System Logs for Errors:

   dmesg | grep memory
   

2. Monitor Kernel Memory Usage:

   sysctl vm.kmem_size
   

3. Stress Test the System: Use tools like stress to test memory limits:

   pkg install stress
   stress --vm 2 --vm-bytes 512M --timeout 60s
   

Conclusion

Properly configuring kernel memory limits on FreeBSD ensures that the system runs efficiently and remains stable under heavy workloads. By modifying sysctl parameters, adjusting loader.conf settings, and configuring swap space, you can effectively manage memory resources. Regular monitoring and testing will help maintain system reliability while preventing memory exhaustion issues.

For further reading, consult the FreeBSD Handbook or relevant system documentation.