The Ultra Kernel Samepage Merging feature
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/*
 * Ultra KSM. Copyright (C) 2011-2012 Nai Xia
 *
 * This is an improvement upon KSM. Some basic data structures and routines
 * are borrowed from ksm.c .
 *
 * Its new features:
 * 1. Full system scan:
 *      It automatically scans all user processes' anonymous VMAs. Kernel-user
 *      interaction to submit a memory area to KSM is no longer needed.
 *
 * 2. Rich area detection:
 *      It automatically detects rich areas containing abundant duplicated
 *      pages based. Rich areas are given a full scan speed. Poor areas are
 *      sampled at a reasonable speed with very low CPU consumption.
 *
 * 3. Ultra Per-page scan speed improvement:
 *      A new hash algorithm is proposed. As a result, on a machine with
 *      Core(TM)2 Quad Q9300 CPU in 32-bit mode and 800MHZ DDR2 main memory, it
 *      can scan memory areas that does not contain duplicated pages at speed of
 *      627MB/sec ~ 2445MB/sec and can merge duplicated areas at speed of
 *      477MB/sec ~ 923MB/sec.
 *
 * 4. Thrashing area avoidance:
 *      Thrashing area(an VMA that has frequent Ksm page break-out) can be
 *      filtered out. My benchmark shows it's more efficient than KSM's per-page
 *      hash value based volatile page detection.
 *
 *
 * 5. Misc changes upon KSM:
 *      * It has a fully x86-opitmized memcmp dedicated for 4-byte-aligned page
 *        comparison. It's much faster than default C version on x86.
 *      * rmap_item now has an struct *page member to loosely cache a
 *        address-->page mapping, which reduces too much time-costly
 *        follow_page().
 *      * The VMA creation/exit procedures are hooked to let the Ultra KSM know.
 *      * try_to_merge_two_pages() now can revert a pte if it fails. No break_
 *        ksm is needed for this case.
 *
 * 6. Full Zero Page consideration(contributed by Figo Zhang)
 *    Now uksmd consider full zero pages as special pages and merge them to an
 *    special unswappable uksm zero page.
 */

ChangeLog:

2012-05-05 The creation of this Doc
2012-05-08 UKSM 0.1.1.1 libc crash bug fix, api clean up, doc clean up.
2012-05-28 UKSM 0.1.1.2 bug fix release
2012-06-26 UKSM 0.1.2-beta1 first beta release for 0.1.2
2012-07-2  UKSM 0.1.2-beta2
2012-07-10 UKSM 0.1.2-beta3
2012-07-26 UKSM 0.1.2 Fine grained speed control, more scan optimization.
2012-10-13 UKSM 0.1.2.1 Bug fixes.
2012-12-31 UKSM 0.1.2.2 Minor bug fixes.
2014-07-02 UKSM 0.1.2.3 Fix a " __this_cpu_read() in preemptible bug".
2015-04-22 UKSM 0.1.2.4 Fix a race condition that can sometimes trigger anonying warnings.
2016-09-10 UKSM 0.1.2.5 Fix a bug in dedup ratio calculation.