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    SubjectRe: [PATCH] Document huge memory/cache overhead of memory controller in Kconfig
    Nick Piggin wrote:
    > On Wednesday 20 February 2008 23:52, Balbir Singh wrote:
    >> Andi Kleen wrote:
    >>> Document huge memory/cache overhead of memory controller in Kconfig
    >>> I was a little surprised that 2.6.25-rc* increased struct page for the
    >>> memory controller. At least on many x86-64 machines it will not fit into
    >>> a single cache line now anymore and also costs considerable amounts of
    >>> RAM.
    >> The size of struct page earlier was 56 bytes on x86_64 and with 64 bytes it
    >> won't fit into the cacheline anymore? Please also look at
    > BTW. We'll probably want to increase the width of some counters
    > in struct page at some point for 64-bit,

    You mean change count to atomic64_t? Do you have real evidence
    the 32bit counter is a problem?

    > so then it really will
    > go over with the memory controller!

    Not sure how they are related? The count and the memory controller
    data would be always separate.

    BTW if the memory controllers were limited in number it would
    be also possible on 64bit to encode them in the high bits of
    ->flags. I assume 16bit or so could be spared in there. Probably
    would not be enough though.

    > Actually, an external data structure is a pretty good idea. We
    > could probably do it easily with a radix tree (pfn->memory
    > controller). And that might be a better option for distros.

    I would think just a separate vmalloc()ed array for the counters
    would be easy enough. That array could be allocated the first time
    the memory controller is used (so making it zero cost for
    distribution kernels when it is not used at all) and then also on
    memory hotplug etc. If we assume most memory will be in
    memory controllers that is also more efficient (in terms of
    memory and of cache consumption) than any kind
    of tree.

    Balbir mentioned one reason they didn't do that earlier was
    that they worried about the limited vmalloc space on 32bit,
    but I don't think that's a good reason against it. That is because
    vmalloc on 32bit is limited because of the limited direct
    mapped kernel memory, but increasing mem_map size eats that
    the same limited resource. So rather the 32bit vmalloc
    reservation can be just increased by the same amount as the
    mem_map increase would be (ok modulo hotplug, but that
    is difficult anyways on 32bit)

    Another issue is that it will slightly increase TLB/cache
    cost of the memory controller, but I think that would be a fair
    trade off for it being zero cost when disabled but compiled

    Doing it with vmalloc should be easy enough. I can do such
    a patch later unless someone beats me to it...


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