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    SubjectRe: [PATCH 1/4] x86_64: Switch to SPARSE_VIRTUAL
    On Mon, 2 Apr 2007, Andi Kleen wrote:

    > > Correct. 2MB worth of struct page is 128 mb of memory. Are there nodes
    > > with smaller amounts of memory?
    > Yes the discontigmem minimum is 64MB and there are some setups
    > (mostly with numa emulation) where you end up with nodes that small.

    Ok. Then I guess we cannot remove discontigmem.

    > BTW there is no guarantee the node size is a multiple of 128MB so
    > you likely need to handle the overlap case. Otherwise we can
    > get cache corruptions

    How does sparsemem handle that?

    > Sparsemem is still quite experimental; discontigmem is the default
    > on x86-64.

    Ok more fodder for preserving the choice.

    > > > Do you have any benchmarks numbers to prove it? There seem to be a few
    > > > benchmarks where the discontig virt_to_page is a problem
    > > > (although I know ways to make it more efficient), and sparsemem
    > > > is normally slower. Still some numbers would be good.
    > >
    > > You want a benchmark to prove that the removal of memory references and
    > > code improves performance?
    > You're just moving them into MMU, not really removing it. And need more TLB entries.

    No no no. For the gazillions time: All of 1-1 mapped kernel memory on
    x86_64 needs a 2 MB page table entry. The virtual memmap uses the same.
    There are *no* additional TLBs used.

    > It might be faster or it might not. There are some unexpected issues, like most x86-64
    > CPUs have a quite small number of large TLBs so you can get thrashing etc.
    > So numbers with TLB intensive workloads would be good.

    You did not read the descriptions. Sigh.

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