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Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →Repair Windows errors before they cause bigger problemsFix Now →Scan for outdated or missing drivers - takes under a minuteDriver Scan →Kernel Samepage Merging (KSM) is a Linux memory-deduplication feature that can reduce physical RAM use by sharing identical pages in application-designated memory ranges. It does not scan all memory automatically: applications must opt ranges in, and savings depend on the workload.
How Kernel Samepage Merging works
The Linux kernel’s ksmd daemon periodically scans eligible memory ranges. When it finds identical pages, KSM can replace them with a single shared page. That page is write-protected; if a process tries to change its contents, the kernel creates a private copy for that process. This copy-on-write behavior lets processes share unchanged data without making their later writes affect one another. KSM was originally developed for KVM virtual machines, and can also help other applications that produce repeated data.
Which memory KSM can merge
KSM is limited to anonymous private pages, not file-backed page-cache pages. It also scans only ranges that an application has explicitly marked as mergeable, rather than indiscriminately examining all process memory. The Linux kernel documentation describes these scope limits in its Kernel Samepage Merging guide.
Applications opt ranges in and out
An application registers a range with madvise(MADV_MERGEABLE). It can withdraw that advice with madvise(MADV_UNMERGEABLE), which asks the kernel to restore separate pages. Registration can succeed even if ksmd is not running at the time; the pages may be scanned if the daemon starts later.
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What KSM requires and what it costs
The kernel must be built with CONFIG_KSM=y. Applications should mark only ranges likely to contain useful duplicates: scanning consumes CPU, and KSM’s reverse-mapping metadata for tracking scanned pages also uses memory. Unmerging can require additional memory, so it may fail or contribute to memory pressure.
There is no universal memory-saving rate. The benefit depends on how many identical pages exist in opted-in ranges, and should be weighed against scanner CPU and metadata costs. The kernel documentation provides an approximate profitability calculation and describes counters for monitoring KSM; use the guide for the running kernel because controls and defaults can vary by version: monitoring KSM profitability.
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NUMA systems: sharing versus locality
On a NUMA system, allowing pages from different nodes to be shared can increase deduplication, but may reduce the locality benefits of keeping memory close to the CPU accessing it. Restricting merging to one node may favor locality at the cost of fewer opportunities to share. The appropriate balance depends on the workload and system; there is no setting that is best for every machine.
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Best Value
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