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    SubjectRe: [Lhms-devel] [PATCH 0/7] Fragmentation Avoidance V19
    On Tue, 2005-11-01 at 15:29 +0100, Ingo Molnar wrote:
    > * Dave Hansen <> wrote:
    > > > can you always, under any circumstance hot unplug RAM with these patches
    > > > applied? If not, do you have any expectation to reach 100%?
    > >
    > > With these patches, no. There are currently some very nice,
    > > pathological workloads which will still cause fragmentation. But, in
    > > the interest of incremental feature introduction, I think they're a
    > > fine first step. We can effectively reach toward a more comprehensive
    > > solution on top of these patches.
    > >
    > > Reaching truly 100% will require some other changes such as being able
    > > to virtually remap things like kernel text.
    > then we need to see that 100% solution first - at least in terms of
    > conceptual steps.

    I don't think saying "truly 100%" really even makes sense. There will
    always be restrictions of some kind. For instance, with a 10MB kernel
    image, should you be able to shrink the memory in the system below
    10MB? ;)

    There is also no precedent in existing UNIXes for a 100% solution. From , a seemingly arbitrary restriction:

    A memory region that contains a large page cannot be removed.

    What the fragmentation patches _can_ give us is the ability to have 100%
    success in removing certain areas: the "user-reclaimable" areas
    referenced in the patch. This gives a customer at least the ability to
    plan for how dynamically reconfigurable a system should be.

    After these patches, the next logical steps are to increase the
    knowledge that the slabs have about fragmentation, and to teach some of
    the shrinkers about fragmentation.

    After that, we'll need some kind of virtual remapping, breaking the 1:1
    kernel virtual mapping, so that the most problematic pages can be
    remapped. These pages would retain their virtual address, but getting a
    new physical. However, this is quite far down the road and will require
    some serious evaluation because it impacts how normal devices are able
    to to DMA. The ppc64 proprietary hypervisor has features to work around
    these issues, and any new hypervisors wishing to support partition
    memory hotplug would likely have to follow suit.

    -- Dave

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