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    SubjectRe: RT cache management
    At 6/22/98 04:18 AM , Rik van Riel wrote:
    > On Sun, 21 Jun 1998, Jeff Millar wrote:
    > > Recalling the questions: What cache control features do RTOS's have that
    > > Linux doesn't? Does Linux have another approach to achieve the
    benefits of
    > > cache control? Do applications really need cache control?
    >
    > Linux has some cache controlling features, but they're not
    > application program tunable. All Linux machines are
    > automatically cache coherent, so programs don't really need
    > that kind of control anyway...
    >
    > If memory is shared, you probably want to pay the price
    > for cache coherency, if memory is not shared, you're not
    > paying the price anyway...

    I believe the original poster was discussing multi-processor "shared
    memory" (also called "reflected" memory) hardware systems, where the shared
    memory (usually on a peripheral card for PCs) is multi-ported via a
    high-speed serial/parallel bus and has a "back-door" port from one system
    to another which is not controlled / controllable by the mobo chipset.
    See, for example, Systran's SCRAMnet series at :

    <http://www.systran.com/scramnet.htm>

    In such a system, the mobo chipset memory controller isn't integrated with
    the shared memory, and a remote system can write to a local shared memory
    location without the local system's chipset / CPU being notified,
    invalidating the local / CPU cache without notifying the local system of
    the fact. In RTOSs which allow for such a setup, you can tell the OS that
    a block of memory is to be locked down (not pageable) and not cached, so
    that the CPU / chipset is always forced to fetch data directly from the
    shared memory rather than from cache.

    ---------------------------------------------
    Alex P. Madarasz, Jr. -- madarasz@erols.com


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