it is only effective for pages merged after the change.
Default: 0 (normal KSM behaviour as in earlier releases)
+max_page_sharing - Maximum sharing allowed for each KSM page. This
+ enforces a deduplication limit to avoid the virtual
+ memory rmap lists to grow too large. The minimum
+ value is 2 as a newly created KSM page will have at
+ least two sharers. The rmap walk has O(N)
+ complexity where N is the number of rmap_items
+ (i.e. virtual mappings) that are sharing the page,
+ which is in turn capped by max_page_sharing. So
+ this effectively spread the the linear O(N)
+ computational complexity from rmap walk context
+ over different KSM pages. The ksmd walk over the
+ stable_node "chains" is also O(N), but N is the
+ number of stable_node "dups", not the number of
+ rmap_items, so it has not a significant impact on
+ ksmd performance. In practice the best stable_node
+ "dup" candidate will be kept and found at the head
+ of the "dups" list. The higher this value the
+ faster KSM will merge the memory (because there
+ will be fewer stable_node dups queued into the
+ stable_node chain->hlist to check for pruning) and
+ the higher the deduplication factor will be, but
+ the slowest the worst case rmap walk could be for
+ any given KSM page. Slowing down the rmap_walk
+ means there will be higher latency for certain
+ virtual memory operations happening during
+ swapping, compaction, NUMA balancing and page
+ migration, in turn decreasing responsiveness for
+ the caller of those virtual memory operations. The
+ scheduler latency of other tasks not involved with
+ the VM operations doing the rmap walk is not
+ affected by this parameter as the rmap walks are
+ always schedule friendly themselves.
+
+stable_node_chains_prune_millisecs - How frequently to walk the whole
+ list of stable_node "dups" linked in the
+ stable_node "chains" in order to prune stale
+ stable_nodes. Smaller milllisecs values will free
+ up the KSM metadata with lower latency, but they
+ will make ksmd use more CPU during the scan. This
+ only applies to the stable_node chains so it's a
+ noop if not a single KSM page hit the
+ max_page_sharing yet (there would be no stable_node
+ chains in such case).
+
The effectiveness of KSM and MADV_MERGEABLE is shown in /sys/kernel/mm/ksm/:
pages_shared - how many shared pages are being used
pages_volatile - how many pages changing too fast to be placed in a tree
full_scans - how many times all mergeable areas have been scanned
+stable_node_chains - number of stable node chains allocated, this is
+ effectively the number of KSM pages that hit the
+ max_page_sharing limit
+stable_node_dups - number of stable node dups queued into the
+ stable_node chains
+
A high ratio of pages_sharing to pages_shared indicates good sharing, but
a high ratio of pages_unshared to pages_sharing indicates wasted effort.
pages_volatile embraces several different kinds of activity, but a high
proportion there would also indicate poor use of madvise MADV_MERGEABLE.
+The maximum possible page_sharing/page_shared ratio is limited by the
+max_page_sharing tunable. To increase the ratio max_page_sharing must
+be increased accordingly.
+
+The stable_node_dups/stable_node_chains ratio is also affected by the
+max_page_sharing tunable, and an high ratio may indicate fragmentation
+in the stable_node dups, which could be solved by introducing
+fragmentation algorithms in ksmd which would refile rmap_items from
+one stable_node dup to another stable_node dup, in order to freeup
+stable_node "dups" with few rmap_items in them, but that may increase
+the ksmd CPU usage and possibly slowdown the readonly computations on
+the KSM pages of the applications.
+
Izik Eidus,
Hugh Dickins, 17 Nov 2009
git.samba.org / sfrench / cifs-2.6.git / blobdiff