admin-guide/mm/ksm.rst

8 KSM is a memory-saving de-duplication feature, enabled by CONFIG_KSM=y,
19 content which can be replaced by a single write-protected page (which
46 unmerges whatever it merged in that range.  Note: this unmerging call
47 may suddenly require more memory than is available - possibly failing
48 with EAGAIN, but more probably arousing the Out-Of-Memory killer.
54 the range for whenever the KSM daemon is started; even if the range
56 MADV_UNMERGEABLE is applied to a range which was never MADV_MERGEABLE.
60 will exceed ``vm.max_map_count`` (see Documentation/admin-guide/sysctl/vm.rst).
63 the user address space: they will report ENOMEM if the specified range
84         been set to scan-time.
137         deduplication limit to avoid high latency for virtual memory
164         enabled, pages that have previously not been de-duplicated get
166         de-duplication has already been tried and failed. By default this
172         supported: none and scan-time. The default is none. By setting
173         ``advisor_mode`` to scan-time, the scan time advisor is enabled.
223 A high ratio of ``pages_sharing`` to ``pages_shared`` indicates good
224 sharing, but a high ratio of ``pages_unshared`` to ``pages_sharing``
226 different kinds of activity, but a high proportion there would also
242 1) How to determine whether KSM save memory or consume memory in system-wide
243    range? Here is a simple approximate calculation for reference::
245 	general_profit =~ ksm_saved_pages * sizeof(page) - (all_rmap_items) *
256 	process_profit =~ ksm_saved_pages * sizeof(page) -
265 From the perspective of application, a high ratio of ``ksm_rmap_items`` to
266 ``ksm_merging_pages`` means a bad madvise-applied policy, so developers or
269 separately 32B on 32-bit CPU architecture and 64B on 64-bit CPU architecture.
270 so if the ``ksm_rmap_items/ksm_merging_pages`` ratio exceeds 64 on 64-bit CPU
271 or exceeds 128 on 32-bit CPU, then the app's madvise policy should be dropped,
281 range of madvise(, , MADV_MERGEABLE).
290 	cannot do all the locking needed to reconstitute a cross-anon_vma KSM page.
295 The number of candidate pages for KSM is dynamic. It can be often observed
303 scan-time. With none, no advisor is enabled. The default is none.
318 The ``pages_to_scan`` parameter is re-calculated after a scan has been completed.
321 --