QEMU leverages the KSM mechanism offered by the kernel:

- per VM basis
- by invoking the syscall madvise(..., MADV_MERGEABLE)

By default, KSM is enabled for all VMs, to turn it off, users can
specify vm-merge=off as part of the machine features:

$ qemu-system-x86_64 -machine q35,mem-merge=off -nographic -m 256M
-enable-kvm

On Ubuntu/Debian, the package disables KSM systemwide (for all the 
applications, not only QEMU VMs):
 - if inside container.
 - if KSM_ENABLED is not set to 1 or "AUTO"

By default, on Ubuntu, KSM_ENABLED is set to AUTO

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https://bugs.launchpad.net/bugs/2033565

Title:
  ksmd should be opt-in rather than opt-out

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