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    SubjectRe: [PATCH] Document huge memory/cache overhead of memory controller in Kconfig
    Andi Kleen wrote:
    >> 1. We could create something similar to mem_map, we would need to handle 4
    > 4? At least x86 mainline only has two ways now. flatmem and vmemmap.
    >> different ways of creating mem_map.
    > Well it would be only a single way to create the "aux memory controller
    > map" (or however it will be called). Basically just a call to single
    > function from a few different places.
    >> 2. On x86 with 64 GB ram,
    > First i386 with 64GB just doesn't work, at least not with default 3:1
    > split. Just calculate it yourself how much of the lowmem area is left
    > after the 64GB mem_map is allocated. Typical rule of thumb is that 16GB
    > is the realistic limit for 32bit x86 kernels. Worrying about
    > anything more does not make much sense.

    I understand what you say Andi, but nothing in the kernel stops us from
    supporting 64GB. Should a framework like memory controller make an assumption
    that not more than 16GB will be configured on an x86 box?

    >> if we decided to use vmalloc space, we would need 64
    >> MB of vmalloc'ed memory
    > Yes and if you increase mem_map you need exactly the same space
    > in lowmem too. So increasing the vmalloc reservation for this is
    > equivalent. Just make sure you use highmem backed vmalloc.

    I see two problems with using vmalloc. One, the reservation needs to be done
    across architectures. Two, a big vmalloc chunk is not node aware, if all the
    pages come from the same node, we have a penalty to pay in a NUMA system.

    Warm Regards,
    Balbir Singh
    Linux Technology Center

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      Last update: 2008-02-22 05:57    [W:0.022 / U:13.820 seconds]
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