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    SubjectRe: How to make Linux scale up WRT bandwidth and size?
       From: alan@lxorguk.ukuu.org.uk (Alan Cox)
    Date: Thu, 29 Apr 1999 19:32:31 +0100 (BST)

    > frames per second. This means a sustained rate of roughly 10MB per
    > second. Lossy compression would falsify the results and is thus not

    Thats about the bandwidth of a disk

    Yes. So on a perfect real-time operating system this would be
    entirely sufficient for monochrome operation.

    > Now how to do the recording? The easiest solution would probably be
    > memory-mapping a recording file and doing a single large read-call
    > from the framegrabber (video4linux device) to the file.

    Or memory map the frame grabber and write to the file - thats how the
    existing video4linux rtjpeg tools work

    Well, if the frame grabber driver does not mind mapping 20GB in one
    piece... I have to test both and see what I will do.

    > File size: the ext2 file system tops out at 2GB file size, which is

    Not on a 64bit machine like the Alpha

    Well, if it does not, it is 2.2.x news. On our stock RedHat 5.2
    Alpha, a write via dd if=/dev/zero is aborted after 2GB, with an
    appropriate error message. Of course I will be glad to hear of any
    change made. I would guess that >2GB files on the Alpha are possible
    on NFS and some foreign file systems.

    > Efficiency of memory map: will all that happens in that scenario be a
    > transfer from framegrabber to memory in one copy done in busmaster
    > mode by the framegrabber, and from memory to disk in another busmaster
    > done by the SCSI controller? Is there even a possibility that a

    Right now you can set it up so that you DMA from the capture card to
    the memory and then write it ought through the buffer cache. Stephen
    Tweedie has O_DIRECT patches for Linux (I dont know how tested they are
    on Alpha but they survive Oracle on x86 so ..) that willl mean you are
    doing DMA direct back out from the same memory


    Sounds good.

    Thank you,

    David Kastrup Phone: +49-234-700-5570
    Email: dak@neuroinformatik.ruhr-uni-bochum.de Fax: +49-234-709-4209
    Institut für Neuroinformatik, Universitätsstr. 150, 44780 Bochum, Germany

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