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    SubjectRe: tulip driver in 2.1.11* - 2.1.21 is broken - new driver
    Date (Alan Cox) writes:

    > > > Actually, there is one problem now, which makes this discussion
    > > > academic. We stand on the edge now. Seems, it is not widely known
    > > > fact, but current Linux CANNOT handle >~100Mbit interfaces.
    > >
    > > It seems that you're not aware that people are already doing ATM and
    > > getting reasonable throughputs (apparently mainly limited by memory
    > > bandwidth).
    > No the problem we have is that the time go in and out of irq handlers
    > and net_bh is a per packet constant. At 100Mbit small packet routing
    > we cant cope (nor can BSD). At 1Gbit 1500 byte framing Linux routing
    > is out (which in a way is fine since PCI is too slow - but PCI wont be
    > too slow next year). Jes has accurate data on what 1500->8K packet size
    > transitions do to our 1Gbit performance.

    Is that true on SMP boxes too?

    net_bh is basically

    while (queue not empty) {
    get packet
    higher layers( packet );

    If one CPU processes the device interrupts and always fills the queue and the
    other CPU runs net_bh then it should go without too much overhead. The same
    is true on a UP system, because net_bh can be interrupted by devices while the
    packets are processed in the higher layers (so when it returns to the loop
    there is already a new packet in the queues ready to be processed).

    To optimize for this behaviour it might be worth it to add a output flush
    check into the net_bh loop. The XMIT_EVERY define in 2.0 suggests that you
    once tried that, did you try it with routing and SMP again?

    Or do I miss something here?


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