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    SubjectRe: [PATCH 0/4] ksm - dynamic page sharing driver for linux
    Andrew Morton wrote:
    > On Tue, 11 Nov 2008 21:18:23 +0200
    > Izik Eidus <ieidus@redhat.com> wrote:
    >
    >
    >>> hm.
    >>>
    >>> There has been the occasional discussion about idenfifying all-zeroes
    >>> pages and scavenging them, repointing them at the zero page. Could
    >>> this infrastructure be used for that? (And how much would we gain from
    >>> it?)
    >>>
    >>> [I'm looking for reasons why this is more than a muck-up-the-vm-for-kvm
    >>> thing here ;) ]
    >>>
    >
    > ^^ this?
    >
    >
    >> KSM is separate driver , it doesn't change anything in the VM but adding
    >> two helper functions.
    >>
    >
    > What, you mean I should actually read the code? Oh well, OK.
    >
    Andrea i think what is happening here is my fault
    i will try to give here much more information about KSM:
    first the bad things:
    KSM shared pages are right now (we have patch that can change it but we
    want to wait with it) unswappable
    this mean that the entire memory of the guest is swappable but the pages
    that are shared are not.
    (when the pages are splited back by COW they become anonymous again with
    the help of do_wp_page()
    the reason that the pages are not swappable is beacuse the way the Linux
    Rmap is working, this not allow us to create nonlinear anonymous pages
    (we dont want to use nonlinear vma for kvm, as it will make swapping for
    kvm very slow)
    the reason that ksm pages need to have nonlinear reverse mapping is that
    for one guest identical page can be found in whole diffrent offset than
    other guest have it
    (this is from the userspace VM point of view)

    the rest is quite simple:
    it is walking over the entire guest memory (or only some of it) and scan
    for identical pages using hash table
    it merge the pages into one single write protected page

    numbers for ksm is something that i have just for desktops and just the
    numbers i gave you
    what is do know is:
    big overcommit like 300% is possible just when you take into account
    that some of the guest memory will be free
    we are sharing mostly the DLLs/ KERNEL / ZERO pages, for the DLLS and
    KERNEL PAGEs this pages likely will never break
    but ZERO pages will be break when windows will allocate them and will
    come back when windows will free the memory.
    (i wouldnt suggest 300% overcommit for servers workload, beacuse you can
    end up swapping in that case,
    but for desktops after runing in production and passed some seiroes qa
    tress tests it seems like 300% is a real number that can be use)

    i just ran test on two fedora 8 guests and got that results (using GNOME
    in both of them)
    9959 root 15 0 730m 537m 281m S 8 3.4 0:44.28
    kvm

    9956 root 15 0 730m 537m 246m S 4 3.4 0:41.43 kvm
    as you can see the physical sharing was 281mb and 246mb (kernel pages
    are counted as shared)
    there is small lie in this numbers beacuse pages that was shared across
    two guests and was splited by writing from guest number 1 will still
    have 1 refernce count to it
    and will still be kernel page (untill the other guest (num 2) will write
    to it as well)


    anyway i am willing to make much better testing or everything that
    needed for this patchs to be merged.
    (just tell me what and i will do it)

    beside that you should know that patch 4 is not a must, it is just nice
    optimization...

    thanks.



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