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    SubjectRe: Abysmal RAID 0 performance on 2.4.0-test10 for IDE?
    On Tue, 26 Dec 2000, Felix von Leitner wrote:
    > Thus spake Rik van Riel (riel@conectiva.com.br):
    > > > One more detail: top says the CPU is 50% system when reading from either
    > > > one of the disk or raid devices. That seems awfully high considering
    > > > that the Promise controller claims to do UDMA.
    > > >
    > > > Any comments?
    > > Your program reads in data at 30MB/second, on a memory bus
    > > that most likely supports something like 60 to 100MB/second.
    >
    > 100.

    So that's 30% for the UDMA controller and maybe
    30% for the CPU (if your program reads in all the
    data).

    > > Part of this memory bandwidth is needed for the UDMA controller
    > > to push the data to memory, probably between 30% and 50%.
    >
    > That would be 30%.

    Add to that the overhead of allocating and reclaiming
    the memory, doing the RAID mapping, sending commands
    to the hard disk, ...

    > > Every time the UDMA controller has the memory bus for itself the
    > > CPU will busy-wait on memory, which shows up as CPU busy time.
    >
    > So, you are saying, when I add a gigabit ethernet card, CPU will hit
    > 100% at about 30 MB/second? That sounds like a weak architecture ;-)

    Hey, there's a reason PCs are so cheap ;)

    regards,

    Rik
    --
    Hollywood goes for world dumbination,
    Trailer at 11.

    http://www.surriel.com/
    http://www.conectiva.com/ http://distro.conectiva.com.br/

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