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    SubjectRe: [rfc] increase struct page size?!
    On Fri, May 18, 2007 at 11:14:26AM -0700, Christoph Lameter wrote:
    > On Fri, 18 May 2007, Nick Piggin wrote:
    >
    > > However we don't have to let those 8 bytes go to waste: we can use them
    > > to store the virtual address of the page, which kind of makes sense for
    > > 64-bit, because they can likely to use complicated memory models.
    >
    > That is not a valid consideration anymore. There is virtual memmap update
    > pending with the sparsemem folks that will simplify things.
    >
    > > Many batch operations on struct page are completely random, and as such, I
    > > think it is better if each struct page fits completely into a single
    > > cacheline even if it means being slightly larger.
    >
    > Right. That would simplify the calculations.

    It isn't the calculations I'm worried about, although they'll get simpler
    too. It is the cache cost.


    > > Don't let this space go to waste though, we can use page->virtual in order
    > > to optimise page_address operations.
    >
    > page->virtual is a benefit if the page is cache hot. Otherwise it may
    > cause a useless lookup.

    It would be very rare for the page not to be in L1 cache at this point,
    because we've likely taken a reference on it and/or locked it or moved it
    between lists etc.

    > I wonder if there are other uses for the free space?

    Hugh points out that we should make _count and _mapcount atomic_long_t's,
    which would probably be a better use of the space once your vmemmap goes
    in.
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