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    SubjectRe: [Patch] numa:x86_64: Cacheline aliasing makes for_each_populated_zone extremely expensive -V2.
    On Thu, Aug 19, 2010 at 03:54:36PM -0700, H. Peter Anvin wrote:
    > On 08/18/2010 11:30 AM, Robin Holt wrote:
    > >
    > > - node_data[nodeid] = early_node_mem(nodeid, start, end, pgdat_size,
    > > + /*
    > > + * Allocate an extra cacheline per node to reduce cacheline
    > > + * aliasing when scanning all node's node_data.
    > > + */
    > > + cache_alias_offset = nodeid * SMP_CACHE_BYTES;
    > > + node_data[nodeid] = cache_alias_offset +
    > > + early_node_mem(nodeid, start, end,
    > > + pgdat_size + cache_alias_offset,
    > > SMP_CACHE_BYTES);
    > > - if (node_data[nodeid] == NULL)
    > > + if (node_data[nodeid] == (void *)cache_alias_offset)
    > > return;
    > > nodedata_phys = __pa(node_data[nodeid]);
    > > reserve_early(nodedata_phys, nodedata_phys + pgdat_size, "NODE_DATA");
    >
    > I'm concerned about this, because it really seems to rely on subtleties
    > in the behavior of early_node_mem, as well as the direction of
    > find_e820_area -- which is pretty much intended to change anyway. It's
    > the "action at a distance" effect.
    >
    > What we really want, I think, is to push the offsetting into
    > find_early_area(). Right now we have an alignment parameter, but what
    > we need is an alignment and a color parameter (this is just a classic
    > case of cache coloring, after all) which indicates the desirable offset
    > from the alignment base.

    That sounds reasonable. Are there other examples you can think of which
    I can build upon?

    Robin


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