KSM is a linux driver that allows dynamically sharing identical memory pages between one or more processes. Unlike traditional page sharing that is made at the allocation of the memory, ksm do it dynamically after the memory was created. Memory is periodically scanned; identical pages are identified and merged. The sharing is unnoticeable by the process that use this memory. (the shared pages are marked as readonly, and in case of write do_wp_page() take care to create new copy of the page).
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