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    SubjectRe: [PATCH 1/11] ksm: allow trees per NUMA node
    On Mon, 28 Jan 2013, Andrew Morton wrote:
    > On Fri, 25 Jan 2013 17:54:53 -0800 (PST)
    > Hugh Dickins <hughd@google.com> wrote:
    >
    > > --- mmotm.orig/Documentation/vm/ksm.txt 2013-01-25 14:36:31.724205455 -0800
    > > +++ mmotm/Documentation/vm/ksm.txt 2013-01-25 14:36:38.608205618 -0800
    > > @@ -58,6 +58,13 @@ sleep_millisecs - how many milliseconds
    > > e.g. "echo 20 > /sys/kernel/mm/ksm/sleep_millisecs"
    > > Default: 20 (chosen for demonstration purposes)
    > >
    > > +merge_across_nodes - specifies if pages from different numa nodes can be merged.
    > > + When set to 0, ksm merges only pages which physically
    > > + reside in the memory area of same NUMA node. It brings
    > > + lower latency to access to shared page. Value can be
    > > + changed only when there is no ksm shared pages in system.
    > > + Default: 1
    > > +
    >
    > The explanation doesn't really tell the operator whether or not to set
    > merge_across_nodes for a particular machine/workload.
    >
    > I guess most people will just shrug, turn the thing on and see if it
    > improved things, but that's rather random.

    Right. I don't think we can tell them which is going to be better,
    but surely we could do a better job of hinting at the tradeoffs.

    I think we expect large NUMA machines with lots of memory to want the
    better NUMA behavior of !merge_across_nodes, but machines with more
    limited memory across short-distance NUMA nodes, to prefer the greater
    deduplication of merge_across nodes.

    Petr, do you have a more informative text for this?

    Hugh


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