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    SubjectRe: Quick question about hyper-threading (also some NUMA stuff)
    On Mon, Apr 14, 2003 at 09:37:07AM -0700, Martin J. Bligh wrote:
    > >> OK, unless I misunderstand you, I think that happens naturally for that
    > >> kind of thing - when we do the COW split, we'll get a node-local page
    > >> by default (unless the local node is out of memory).
    > >
    > > Yes, it happens naturaly, but it's done when we try to write to it.
    > > What I meant was, at fork time, it we are forking to a different node,
    > > instead of COW-marking, do the COW-mark and the immediately do a sort-of
    > > for_each_page(touch_as_if_written(page)), so that nodes would not have to
    > > reference the memory from others.
    > Ah, you probably don't want to do that ... it's very expensive. Moreover,
    > if you exec 2ns later, all the effort will be wasted ... and it's very hard
    > to deterministically predict whether you'll exec or not (stupid UNIX
    > semantics). Doing it lazily is probably best, and as to "nodes would not
    > have to reference the memory from others" - you're still doing that, you're
    > just batching it on the front end.

    True... What about a vma-level COW-ahead just like we have a file-level
    read-ahead, then? I mean batching the COW at unCOW-because-of-write time.

    btw, COW-ahead sound really silly :)

    > > I don't know if it's really usefull, and anyways I could not try to code it
    > > unless there is a sort of NUMA simulator for "normal" machines.
    > There isn't, but writing one would be very useful (and fairly simple) if
    > you have a 2x machine or something that you could use. I've thought about
    > writing this several times ... just haven't got round to it.
    > M.

    Not possible for me since I've got no SMP. But posting a quick note about
    your proposed "fake-NUMA-on-SMP.patch" would be good only if to have an
    offsite (offbrain also? ;) backup of your ideas :)
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