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    SubjectRe: Mmap device performance
    In article <19971210092943.51142@jolt>, Daryll Strauss  <> wrote:
    >On the PPro and PII boxes the performance is roughly half what it is
    >under Windows. Since the application is identical, and the interface to
    >the card is simply writing to a memory mapped region so it seems that
    >the degradation must be happening somewhere in the kernel layers. I'm
    >currently mapping the region shared with read/write access.

    One thing to look out for is the "memory type registers" on the PPro and
    the PII. Somebody had a kernel loadable module to read and set the
    registers, it may be that your windows setup has a different memory type
    setup than the Linux setup.

    Similarly, when you do the mapping, look into the effects of the "cache
    disable" and "write-through" bits in the page tables. You mention that
    you copied the code from the /dev/mem map code, and that code does:

    #if defined(__i386__)
    * hmm.. This disables high-memory caching, as the XFree86 team
    * wondered about that at one time.
    * The surround logic should disable caching for the high device
    * addresses anyway, but right now this seems still needed.
    if (x86 > 3 && offset >= __pa(high_memory))
    pgprot_val(vma->vm_page_prot) |= _PAGE_PCD;

    the code _shouldn't_ make any difference (as per the comment), but
    depending on how the card works it might.


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      Last update: 2005-03-22 13:40    [W:0.019 / U:4.660 seconds]
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