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    SubjectRe: [QUICKLIST 0/4] Arch independent quicklists V2
    Andrew Morton wrote:
    >>On Tue, 13 Mar 2007 22:06:46 +1100 Nick Piggin <nickpiggin@yahoo.com.au> wrote:
    >>Andrew Morton wrote:
    >>
    >>>>On Tue, 13 Mar 2007 19:03:38 +1100 Nick Piggin <nickpiggin@yahoo.com.au> wrote:
    >>
    >>...
    >>
    >>
    >>>>Page allocator still requires interrupts to be disabled, which this doesn't.
    >
    >
    >>>>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.
    >>
    >>The thing is, pagetable pages are the one really good exception to the
    >>rule that we should keep cache hot and initialise-on-demand. They
    >>typically are fairly sparsely populated and sparsely accessed. Even
    >>for last level page tables, I think it is reasonable to assume they will
    >>usually be pretty cold.
    >
    >
    > eh? I'd have thought that a pte page which has just gone through
    > zap_pte_range() will very often have a _lot_ of hot cachelines, and
    > that's a common case.
    >
    > Still. It's pretty easy to test.

    Well I guess that would be the case if you had just unmapped a 4MB
    chunk that was pretty dense with pages.

    My malloc seems to allocate and free in blocks of 128K, so that's
    only going to give us 3% of the last level pte being cache hot when
    it gets freed. Not sure what common mmap(file) access patterns
    look like.

    The majority of programs I run have a smattering of llpt pages
    pretty sparsely populated, covering text, libraries, heap, stack,
    vdso.

    We don't actually have to zap_pte_range the entire page table in
    order to free it (IIRC we used to have to, before the 4lpt patches).

    But yeah let's see some tests. I would definitely want to avoid this
    extra layer of complexity if it is just as good to return the pages
    to the pcp lists.

    >>>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.
    >>
    >>You can win on specifically constructed benchmarks, easily.
    >>
    >>But considering all the other problems you're going to introduce, we'd need
    >>a significant win on a significant something, IMO.
    >>
    >>You waste memory bandwidth. You also use more CPU and memory cycles
    >>speculatively, ergo you waste more power.
    >
    >
    > Yeah, prezeroing in idle is probably pointless. But I'm not aware of
    > anyone having tried it properly...

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