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    SubjectRe: x86, ARM, PARISC, PPC, MIPS and Sparc folks please run this
    David S. Miller wrote:
    > On Fri, 29 Aug 2003 16:05:21 -0700
    > Mike Fedyk <mfedyk@matchmail.com> wrote:
    >
    > > Does this mean that userspace has to take into consideration that the isn't
    > > coherent for adjacent small memory accesses on sparc? What could happen if
    > > it doesn't, or does it need to at all?
    >
    > For shared memory, we enforce the correct mapping alignment
    > so that coherency issues don't crop up.
    >
    > How does this program work? I haven't taken a close look
    > at it. Does it use MAP_SHARED or IPC shm?

    It uses POSIX shared memory and (necessarily) MAP_SHARED, which
    doesn't constrain the mapping alignment.

    I had wondered if some kernels used page faults to maintain coherence
    between multiple shared mappings of the same file. It's one of the
    things the program checks, and I have seen it mentioned on l-k, which
    made me think it might be implemented. None of the results for any
    architecture show it, though.

    If userspace does create multiple shared mappings at non-coherent
    offsets, what is the recommended method for switching between
    accessing one page (or page cluster?) and accessing the other. Is it
    msync(), a special system call to flush parts of the data cache, a
    machine instruction, or something else?

    Thanks,
    -- Jamie



    ps. The program has code to try IPC shm instead. Change "#ifdef
    SHM_DIR_PREFIX" in __page_alias_map to "#if 0", and add
    -DHAVE_SYSV_SHM to the GCC command line. It should fail the same test
    sizes with a different message.
    -
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