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    SubjectRe: [patch 0/6] Guest page hinting version 7.
    Dave Hansen wrote:
    > On Fri, 2009-03-27 at 16:09 +0100, Martin Schwidefsky wrote:
    >> If the host picks one of the
    >> pages the guest can recreate, the host can throw it away instead of writing
    >> it to the paging device. Simple and elegant.
    >
    > Heh, simple and elegant for the hypervisor. But I'm not sure I'm going
    > to call *anything* that requires a new CPU instruction elegant. ;)

    I am convinced that it could be done with a guest-writable
    "bitmap", with 2 bits per page. That would make this scheme
    useful for KVM, too.

    > I don't see any description of it in there any more, but I thought this
    > entire patch set was to get rid of the idiotic triple I/Os in the
    > following scenario:

    > I don't see that mentioned at all in the current description.
    > Simplifying the hypervisor is hard to get behind, but cutting system I/O
    > by 2/3 is a much nicer benefit for 1200 lines of invasive code. ;)

    Cutting down on a fair bit of IO is absolutely worth
    1200 lines of fairly well isolated code.

    > Can we persuade the hypervisor to tell us which pages it decided to page
    > out and just skip those when we're scanning the LRU?

    The easiest "notification" points are in the page fault
    handler and the page cache lookup code.

    --
    All rights reversed.


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