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    SubjectRe: [QUICKLIST 0/4] Arch independent quicklists V2
    > On Tue, 13 Mar 2007 19:03:38 +1100 Nick Piggin <nickpiggin@yahoo.com.au> wrote:
    > Andrew Morton wrote:
    > >>On Tue, 13 Mar 2007 00:13:25 -0700 (PDT) Christoph Lameter <clameter@sgi.com> wrote:
    > >>Page table pages have the characteristics that they are typically zero
    > >>or in a known state when they are freed.
    > >
    > >
    > > Well if they're zero then perhaps they should be released to the page allocator
    > > to satisfy the next __GFP_ZERO request. If that request is for a pagetable
    > > page, we break even (except we get to remove special-case code). If that
    > > __GFP_ZERO allocation was or some application other than for a pagetable, we
    > > win.
    > >
    > > iow, can we just nuke 'em?
    >
    > Page allocator still requires interrupts to be disabled, which this doesn't.

    Bah. How many cli/sti statements fit into a single cachemiss?

    > Considering there isn't much else that frees known zeroed pages, I wonder if
    > it is worthwhile.

    If you want a zeroed page for pagecache and someone has just stuffed a
    known-zero, cache-hot page into the pagetable quicklists, you have good
    reason to be upset.

    In fact, if you want a _non_-zeroed page and someone has just stuffed a
    known-zero, cache-hot page into the pagetable quicklists, you still have
    reason to be upset. You *want* that cache-hot page.

    Generally, all these little private lists of pages (such as the ones which
    slab had/has) are a bad deal. Cache effects preponderate and I do think
    we're generally better off tossing the things into a central pool.

    Plus, we can get in a situation where take a cache-cold, known-zero page
    from the pte quicklist when there is a cache-hot, non-zero page sitting in
    the page allocator. I suspect that zeroing the cache-hot page would take a
    similar amount of time to a single miss agains the cache-cold page.

    I'm not saying that I _know_ that the quicklists are pointless, but I don't
    think it's established that they are pointful.

    ISTR that experiments with removing the i386 quicklists made zero
    difference, but that was an awfully long time ago. Significantly, it
    predated per-cpu-pages..


    > Last time the zeroidle discussion came up was IIRC not actually real performance
    > gain, just cooking the 1024 CPU threaded pagefault numbers ;)

    Maybe, dunno. It was apparently a win on powerpc many years ago. I had a
    fiddle with it 5-6 years ago on x86 using a cache-disabled mapping of the
    page. But it needed too much support in core VM to bother. Since then
    we've grown per-cpu page magazines and __GFP_ZERO. Plus I'm not aware of
    anyone having tried doing it on x86 with non-temporal stores.

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