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    SubjectRe: Thread implementations...
    On Thu, Jun 25, 1998 at 10:05:17AM -0700, Larry McVoy wrote:
    > : caddr_t buf = mmap(0, len, PROT_READ, MAP_FILE | MAP_SHARED, ifd, 0);
    > : write(ofd, buf, len);
    > :
    > : If we make the write work directly from user space (if we need
    > : to lock down the memory for DMA, we can put any process/thread
    > : that tries to unmap it to sleep) and get rid of copying for
    > : mmap, then we are down to the minimum number of copies.
    > [...]
    > : > mmap() does have significant overhead
    > :
    > : CPU overhead. The question is whether that is the bottleneck.
    > In the for what it is worth department, you are venturing into charted
    > waters. SGI has done all of these tricks and more for years and I think
    > we can learn from their experiences. A few points:
    > 1) Your model above still does a copy.

    I have to admit, I can't see where. (That is, I'm aware
    there are lots of copies there now, which one don't you
    think one can get rid of?).

    > So the cold cache numbers can
    > never be faster than 1/4 of memory speed: DMA in, copy, DMA out.
    > The best numbers are 1/2 memory speed: DMA in, DMA out.

    Realistically, PC hardware can't do TCP checksumming, so
    the best you can do is 1/3 memory speed. DMA in, checksum,
    DMA out. Or can you copy-and-checksum directly to the
    buffers of the Ethernet card?

    > 2) On SGI's, for server type of operations, the mmap() is the bottleneck.
    > You are setting up and tearing down a virtual mapping that you don't
    > need: the ``currency'' you are dealing in at both ends is physical
    > pages, not virtual pages. This starts to become a bottleneck for
    > files smaller than 8K (Linux) or 32K (most other operating systems).
    > Linux is better because it is lighter.

    I must admit I don't understand why this is so. Surely the mmap
    just sets up some kernel structures, it doesn't actually create
    any virtual-physical mapping. Doesn't that happen when we fault
    and the memory is read in. So isn't the overhead per page, and not
    per mapping? (obviously not, but why?).

    I wonder if we can use this to do lazy mmapping. For a server
    process that doesn't look at its data we can just get tricky
    in copy_from_user and copy from the physical address, without
    ever having to map anything into virtual memory. Some locking

    Erik Corry

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