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    SubjectRe: The performance and behaviour of the anti-fragmentation related patches
    Andrew Morton wrote:
    > So some urgent questions are: how are we going to do mem hotunplug and
    > per-container RSS?
    > Our basic unit of memory management is the zone. Right now, a zone maps
    > onto some hardware-imposed thing. But the zone-based MM works *well*. I
    > suspect that a good way to solve both per-container RSS and mem hotunplug
    > is to split the zone concept away from its hardware limitations: create a
    > "software zone" and a "hardware zone". All the existing page allocator and
    > reclaim code remains basically unchanged, and it operates on "software
    > zones". Each software zones always lies within a single hardware zone.
    > The software zones are resizeable. For per-container RSS we give each
    > container one (or perhaps multiple) resizeable software zones.
    > For memory hotunplug, some of the hardware zone's software zones are marked
    > reclaimable and some are not; DIMMs which are wholly within reclaimable
    > zones can be depopulated and powered off or removed.
    > NUMA and cpusets screwed up: they've gone and used nodes as their basic
    > unit of memory management whereas they should have used zones. This will
    > need to be untangled.
    > Anyway, that's just a shot in the dark. Could be that we implement unplug
    > and RSS control by totally different means. But I do wish that we'd sort
    > out what those means will be before we potentially complicate the story a
    > lot by adding antifragmentation.

    Paul Menage had suggested something very similar in response to the RFC
    for memory controllers I sent out and it was suggested that we create
    small zones (roughly 64 MB) to avoid the issue of a zone/node not being
    a shareable across containers. Even with a small size, there are some
    issues. The following thread has the details discussed.

    RSS accounting is very easy (with minimal changes to the core mm),
    supplemented with an efficient per-container reclaimer, it should be
    easy to implement a good per-container RSS controller.

    Warm Regards,
    Balbir Singh
    Linux Technology Center
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