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    SubjectRe: Virtual address space exhaustion (was Discontigmem virt_to_page() )
    On Friday 03 May 2002 17:17, Martin J. Bligh wrote:
    > Andrea apparently wrote:
    > > Ah, and of course you could also use 2M pagetables by default to make it
    > > more usable but still you would run in some huge ram wastage in certain
    > > usages with small files, huge pageins and reads swapout and swapins,
    > > plus it wouldn't be guaranteed to be transparent to the userspace
    > > binaries (for istance mmap offset fields would break backwards
    > > compatibility on the required alignment, that's probably the last
    > > problem though). Despite its also significant drawbacks and the
    > > complexity of the change, probably the 4M pagetables would be the saner
    > > approch to manage more efficiently 64G with only a 800M kernel window.
    > Though that'd reduce the size of some of the structures, I'd still
    > have other concerns (such as tlb size, which is something stupid
    > like 4 pages, IIRC), and the space wastage you mentioned. Page
    > clustering is probably a more useful technique - letting the existing
    > control structures control groups of pages. For example, one struct
    > page could control aligned groups of 4 4K pages, giving us an
    > effective page size of 16K from the management overhead point of
    > view (swap in and out in 4 page chunks, etc).

    IMHO, this will be a much easier change than storing mem_map in highmem,
    and solves 75% of the problem. It's not just ia32 numa that will benefit
    from it. For example, MIPS supports 16K pages in software, which will
    take a lot of load off the tlb. According to Ralf, there are benefits
    re virtual aliasing as well.

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