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    SubjectRe: Cache coherency... and locking
    On Mon, Jul 31, 2000 at 03:04:15PM -0500, Oliver Xymoron wrote:

    > I think you'll find that the really big machines (ASCI, etc) are
    > non-coherent. Coherence is expensive hardware-wise and the programming
    > model it's designed to support just doesn't scale well. People don't
    > ridicule Beowulfs and it's only a matter of time before commodity systems
    > have competitive memory bandwidth with the SGIs and DECs of the world.

    The only ccNUMA ASCI system I know is actually a cluster of 48 128CPU ccNUMAs.
    Why? The Origin 2000 uses directories to avoid broadcasting of invalidates.
    For machines larger than 128 CPUs the directories don't have enough bits to
    allow a per-node granularity of invalidates, so for systems beyond that size
    the hw has to broadcast invalidates into a region of machines thereby
    causing an increase of traffic on the internal network.

    Directories btw. are fairly expensive but yet a proven way to reduce the
    coherency traffic on the internal network. The extra price is why I don't
    believe that we'll see wide offerings of ccNUMAs any time soon. In fact
    even SGI offers two memory configurations (standard / premium) which
    implement different size directories and therefore support different
    system sizes before having to use coarse invalidates.


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