Running Multiple VMs? Here’s Why You Should Turn On Kernel Samepage Merging (KSM)

If you’re a developer running multiple virtual machines on your Linux system, memory can become a serious bottleneck fast. Even powerful machines can struggle when you're juggling different OS environments, databases, and app stacks all at once.

But there’s a built-in Linux feature that can make a huge difference—Kernel Samepage Merging (KSM).
And if you’re not using it yet, you’re missing out on free performance.

What KSM Actually Does

When you run several VMs, they usually end up loading a lot of the same data into RAM—think identical OS libraries, duplicate processes, or cloned environments.
Without KSM, each VM keeps its own separate copy, wasting valuable memory.

🔁 KSM finds identical memory pages across your VMs and merges them into one shared copy.
When something needs to change? No problem—Linux automatically copies the page again (thanks to copy-on-write magic).

Result: huge RAM savings, without you needing to micromanage anything.

Why Developers Should Care

🖥️ More Virtual Machines, Less Hardware Strain

Instead of watching your system crawl once you spin up your fourth or fifth VM, KSM helps you keep everything running smoother. You can test, deploy, and simulate complex environments without constantly hitting memory limits.

🚀 Faster Local Testing

Waiting for cloud VMs to spin up? Burning hours reconfiguring labs?
With KSM, you can run bigger environments right on your laptop or workstation—perfect for DevOps experiments, app validation, or even running a mini Kubernetes cluster locally.

💸 Save on Hardware and Cloud Costs

Pushing your machine harder means fewer trips to the store for RAM upgrades—and fewer bills from cloud providers when local testing is good enough.
Less waste. More value.

🛡️ No Risk to Stability

Because KSM uses copy-on-write, your VMs stay isolated and secure.
There’s no risk of data leaks or instability—just better memory efficiency under the hood.

Common Myths About KSM

❌ "It’s risky to share memory between VMs."
Not true. Thanks to copy-on-write, each VM still behaves as if it owns its memory.

❌ "KSM will make everything slower."
In reality, modern systems barely feel the overhead. For most workloads, the memory savings far outweigh any tiny CPU cost.

❌ "Only enterprise servers need KSM."
Wrong again. Even a single developer’s laptop can benefit massively when juggling multiple VMs.

How to Enable KSM

Create /etc/systemd/system/ksm.service:

[Unit]
Description=Enable Kernel Samepage Merging (KSM)
After=default.target

[Service]
Type=oneshot
ExecStart=/usr/bin/sh -c 'chown root:root /sys/kernel/mm/ksm/run && echo 1 > /sys/kernel/mm/ksm/run'
ExecStartPost=/usr/bin/sh -c 'chown root:root /sys/kernel/mm/ksm/sleep_millisecs && echo 100 > /sys/kernel/mm/ksm/sleep_millisecs'
User=root
Group=root
RemainAfterExit=yes

[Install]
WantedBy=multi-user.target

Then enable and start it:

sudo systemctl daemon-reload
sudo systemctl enable --now ksm.service