Mastering Memory Management with Policy Groups: A Step-by-Step Guide

Introduction

Memory management in Linux is a critical task, especially in multi-tenant environments or when running resource-intensive applications. The kernel's control groups (cgroups) have long been the standard for resource management, but they come with limitations, particularly for use cases beyond simple resource accounting. During the 2026 Linux Storage, Filesystem, Memory Management, and BPF Summit, Chris Li introduced a proposal called policy groups, an enhancement designed to address these shortcomings. While consensus on the final implementation remains elusive, understanding how to leverage policy groups can give you finer-grained control over memory allocation and behavior. This guide walks you through the practical steps to implement and use policy groups for memory management, assuming a supporting kernel environment.

What You Need

• A Linux system running a kernel that includes the policy groups patch (version 6.x experimental or later; check kernel config for CONFIG_CGROUP_POLICY)
• Root or sudo access to create and manage groups
• Basic familiarity with the command line and cgroup filesystem
• A test workload (e.g., a memory-bound process like stress or a web server) to observe policy effects
• Optional: cgroup-tools package for easier management

Step 1: Understand the Limitations of Standard Cgroups

Before diving into policy groups, it's essential to recognize why they are needed. Traditional cgroups work well for resource management, such as limiting CPU shares or memory usage. However, they struggle with more nuanced policies—for instance, applying different memory-reclaim strategies to different workloads, or enforcing bandwidth policies that adapt to system pressure. Policy groups are designed to fill this gap, allowing you to define custom policies that govern how the kernel handles memory pressure, page reclaim, and allocation decisions.

Step 2: Verify and Enable Policy Groups Support

First, check if your kernel has policy groups support. Run zgrep CONFIG_CGROUP_POLICY /proc/config.gz. If the output shows y, you're ready. If not, you must rebuild the kernel with the experimental patch. Closest stable patches for policy groups can be obtained from the Linux MM mailing list. After enabling, mount the cgroup filesystem with policy group controllers:

mount -t cgroup -o memory,policy none /sys/fs/cgroup

If successful, you will see a policy directory inside each cgroup hierarchy.

Step 3: Create a New Policy Group

A policy group is a child cgroup that includes a policy file. Use mkdir to create a new group:

mkdir /sys/fs/cgroup/my_policy_group

Then assign a policy by writing to the policy file. For example, to set a memory-reclaim policy that prioritizes keeping anonymous pages over file-backed pages:

echo "reclaim_anon_file_ratio = 80:20" > /sys/fs/cgroup/my_policy_group/memory.policy

Available policy parameters depend on the current patch set; check the kernel documentation for the full list.

Step 4: Attach Processes to the Policy Group

Attach a process (or a set of processes) to your policy group by writing its PID to the cgroup.procs file:

echo 1234 > /sys/fs/cgroup/my_policy_group/cgroup.procs

You can also move all threads of a process. Once attached, the kernel will apply the defined policies to memory allocations and reclaim decisions for those tasks.

Step 5: Configure Memory Limits in Tandem

Policy groups complement, not replace, traditional memory limits. Set a hard memory limit using memory.max (or memory.limit_in_bytes if using the legacy interface). For example:

echo 512M > /sys/fs/cgroup/my_policy_group/memory.max

The policy will then dictate how the kernel enforces that limit—for instance, which pages to reclaim first when the group exceeds the limit.

Step 6: Monitor and Adjust Policy Behavior

Use the memory.stat file to observe the impact of your policies. Compare metrics like pgscan_direct, pgsteal_kswapd, and workingset_refault between different policy groups. If a policy causes excessive swapping or thrashing, adjust the parameters. For example, you might change the reclaim ratio to be more aggressive on clean file-backed pages to reduce latency.

Step 7: Experiment with Multiple Policy Groups

A key advantage is the ability to assign different policies to different workloads. Create two groups—one for latency-sensitive web servers and one for batch background jobs—and apply different reclaim strategies. For instance:

# Web server: prefer to keep application memory, reclaim file cache
mkdir /sys/fs/cgroup/web_group
echo "reclaim_file_first" > /sys/fs/cgroup/web_group/memory.policy

# Batch job: be aggressive about reclaiming all types
mkdir /sys/fs/cgroup/batch_group
echo "reclaim_aggressive" > /sys/fs/cgroup/batch_group/memory.policy

This fine-grained control is what makes policy groups powerful beyond standard cgroups.

Tips for Success

• Start simple: Use one or two policies at first to understand their effects before mixing multiple policies.
• Monitor system-wide pressure: Use tools like sar, vmstat, or perf to track overall memory health. Policy groups are most useful when you have spare capacity to redistribute.
• Test with synthetic workloads: The stress-ng tool can simulate memory allocation patterns and help you validate policy behavior.
• Read the kernel documentation: Policy groups are an evolving feature; the exact parameters and interfaces may change. Always check /usr/src/linux/Documentation/admin-guide/cgroup-v2.rst or the MM mailing list for the latest updates.
• Remember backward compatibility: Standard cgroup interfaces (memory.max, memory.high) continue to work. Policy groups add an extra layer; they do not replace existing controls.
• Community consensus: As of the 2026 summit, policy groups are still under discussion. Be prepared for changes in the API when using experimental kernels.