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    SubjectRe: page fault scalability patch V11 [0/7]: overview
    William Lee Irwin III wrote:
    > William Lee Irwin III wrote:
    >
    >>>Furthermore, see Robin Holt's results regarding the performance of the
    >>>atomic operations and their relation to cacheline sharing.
    >
    >
    > On Sat, Nov 20, 2004 at 01:40:40PM +1100, Nick Piggin wrote:
    >
    >>Well yeah, but a. their patch isn't in 2.6 (or 2.4), and b. anon_rss
    >
    >
    > Irrelevant. Unshare cachelines with hot mm-global ones, and the
    > "problem" goes away.
    >

    That's the idea.

    > This stuff is going on and on about some purist "no atomic operations
    > anywhere" weirdness even though killing the last atomic operation
    > creates problems and doesn't improve performance.
    >

    Huh? How is not wanting to impact single threaded performance being
    "purist weirdness"? Practical, I'd call it.

    >
    > On Sat, Nov 20, 2004 at 01:40:40PM +1100, Nick Piggin wrote:
    >
    >>means another atomic op. While this doesn't immediately make it a
    >>showstopper, it is gradually slowing down the single threaded page
    >>fault path too, which is bad.
    >
    >
    > William Lee Irwin III wrote:
    >
    >>>And frankly, the argument that the space overhead of per-cpu counters
    >>>is problematic is not compelling. Even at 1024 cpus it's smaller than
    >>>an ia64 pagetable page, of which there are numerous instances attached
    >>>to each mm.
    >
    >
    > On Sat, Nov 20, 2004 at 01:40:40PM +1100, Nick Piggin wrote:
    >
    >>1024 CPUs * 64 byte cachelines == 64K, no? Well I'm sure they probably
    >>don't even care about 64K on their large machines, but...
    >>On i386 this would be maybe 32 * 128 byte == 4K per task for distro
    >>kernels. Not so good.
    >
    >
    > Why the Hell would you bother giving each cpu a separate cacheline?
    > The odds of bouncing significantly merely amongst the counters are not
    > particularly high.
    >

    Hmm yeah I guess wouldn't put them all on different cachelines.
    As you can see though, Christoph ran into a wall at 8 CPUs, so
    having them densly packed still might not be enough.
    -
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