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    SubjectRe: What to expect with the 2.6 VM
    On Tue, 1 Jul 2003, Andrea Arcangeli wrote:

    > On Tue, Jul 01, 2003 at 02:39:47AM +0100, Mel Gorman wrote:
    > > Reverse Page Table Mapping
    > > ==========================
    > >
    > > <rmap stuff snipped>
    >
    > you mention only the positive things, and never the fact that's the most
    > hurting piece of kernel code in terms of performance and smp scalability
    > until you actually have to swapout or pageout.
    >

    You're right, I was commenting only on the positive side of things. I
    didn't pay close enough attention to the development of the 2.5 series so
    right now I can only comment on whats there and only to a small extent on
    what it means or why it might be a bad thing. Here goes a more balanced
    view...

    Based on what I can decipher from this thread and other rmap related
    threads, I've added this to the end of the rmap section. I'm still working
    on the non-linear issues. It'll probably take me another day or two to get
    that together.

    --Begin Extract--
    There are two main benefits, both page-out related, with the introduction
    of reverse mapping. The first is with the management of page tables. In
    2.4, anonymous shared pages had to be placed in a swap cache until all
    references has been found. This could result in a large number of minor
    faults as page adjacent in virtual space were moved to the swap cache
    resulting in unnecessary page table updates. The second benefit is with
    the actual paging out mechanism. 2.6 is much better at selecting the
    correct page and atomically swapping it out from each virtual address
    space.

    Reverse mapping is not free though. The first obvious penalty is the space
    requirements for PTE chains. The space requirements will obviously depend
    on the number of shared pages, especially anonymous pages, in the system.
    There was patches submitted for the sharing of page tables but it was not
    merged into the mainline kernel. The second penalty is the CPU time
    required to maintain the mappings but no figures are available to measure
    how severe this is. The last point to note is that reverse mapping is only
    of benefit when page-outs are frequent which is mainly the case with
    lower-end machines. With large memory machines or with workloads that do
    not cause much swapping, there is only costs to reverse mapping but no
    benefits.
    --End Extract--

    --
    Mel Gorman
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