1 ======================= 2 Kernel Samepage Merging 3 ======================= 4 5 KSM is a memory-saving de-duplication feature, enabled by CONFIG_KSM=y, 6 added to the Linux kernel in 2.6.32. See ``mm/ksm.c`` for its implementation, 7 and http://lwn.net/Articles/306704/ and https://lwn.net/Articles/330589/ 8 9 The userspace interface of KSM is described in Documentation/admin-guide/mm/ksm.rst 10 11 Design 12 ====== 13 14 Overview 15 -------- 16 17 .. kernel-doc:: mm/ksm.c 18 :DOC: Overview 19 20 Reverse mapping 21 --------------- 22 KSM maintains reverse mapping information for KSM pages in the stable 23 tree. 24 25 If a KSM page is shared between less than ``max_page_sharing`` VMAs, 26 the node of the stable tree that represents such KSM page points to a 27 list of struct ksm_rmap_item and the ``page->mapping`` of the 28 KSM page points to the stable tree node. 29 30 When the sharing passes this threshold, KSM adds a second dimension to 31 the stable tree. The tree node becomes a "chain" that links one or 32 more "dups". Each "dup" keeps reverse mapping information for a KSM 33 page with ``page->mapping`` pointing to that "dup". 34 35 Every "chain" and all "dups" linked into a "chain" enforce the 36 invariant that they represent the same write protected memory content, 37 even if each "dup" will be pointed by a different KSM page copy of 38 that content. 39 40 This way the stable tree lookup computational complexity is unaffected 41 if compared to an unlimited list of reverse mappings. It is still 42 enforced that there cannot be KSM page content duplicates in the 43 stable tree itself. 44 45 The deduplication limit enforced by ``max_page_sharing`` is required 46 to avoid the virtual memory rmap lists to grow too large. The rmap 47 walk has O(N) complexity where N is the number of rmap_items 48 (i.e. virtual mappings) that are sharing the page, which is in turn 49 capped by ``max_page_sharing``. So this effectively spreads the linear 50 O(N) computational complexity from rmap walk context over different 51 KSM pages. The ksmd walk over the stable_node "chains" is also O(N), 52 but N is the number of stable_node "dups", not the number of 53 rmap_items, so it has not a significant impact on ksmd performance. In 54 practice the best stable_node "dup" candidate will be kept and found 55 at the head of the "dups" list. 56 57 High values of ``max_page_sharing`` result in faster memory merging 58 (because there will be fewer stable_node dups queued into the 59 stable_node chain->hlist to check for pruning) and higher 60 deduplication factor at the expense of slower worst case for rmap 61 walks for any KSM page which can happen during swapping, compaction, 62 NUMA balancing and page migration. 63 64 The ``stable_node_dups/stable_node_chains`` ratio is also affected by the 65 ``max_page_sharing`` tunable, and an high ratio may indicate fragmentation 66 in the stable_node dups, which could be solved by introducing 67 fragmentation algorithms in ksmd which would refile rmap_items from 68 one stable_node dup to another stable_node dup, in order to free up 69 stable_node "dups" with few rmap_items in them, but that may increase 70 the ksmd CPU usage and possibly slowdown the readonly computations on 71 the KSM pages of the applications. 72 73 The whole list of stable_node "dups" linked in the stable_node 74 "chains" is scanned periodically in order to prune stale stable_nodes. 75 The frequency of such scans is defined by 76 ``stable_node_chains_prune_millisecs`` sysfs tunable. 77 78 Reference 79 --------- 80 .. kernel-doc:: mm/ksm.c 81 :functions: mm_slot ksm_scan stable_node rmap_item 82 83 -- 84 Izik Eidus, 85 Hugh Dickins, 17 Nov 2009
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