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    SubjectRe: scheduling latencies, [patch] `cp /dev/zero /tmp' (patch against 2.2.9)
    On Thu, 17 Jun 1999, Alan Cox wrote:
    > > The process does only sit in a loop and write() data from RAM to /dev/dsp
    > > ( RT FIFO scheduled),
    > > and in background I call scripts which write / copy / read from/to large files,
    > > trying to disturb the RT thread.
    > > Obviously my disk I/O processes are scheduled without realtime priority.
    > >
    > > The scheduling latencies on Linux go up to 70-130ms on pretty high end HW.
    > What hard disk controller, if its IDE how is it tuned (DMA/UDMA, irq masked ?)
    > If it is a random socket7 board what fairness settings have you got in the
    > BIOS as by default on some of these boards IDE DMA can hog the entire bus.
    > Alan

    PII 400 + Mainboard Asus P2B BX Chipset, with a IBM Deskstar 16GB UDMA as

    tu tune the disk I enabled all useful features:

    multicount , DMA , unmask IRQs , enable 32 bit support

    /sbin/hdparm -m 8 -d 1 -u 1 -c /dev/hdc

    note that windoze is able to get the 20ms latency with my machine , using a
    software synthesizer, and copying a file in the background,
    Maybe they use some hacks like IRQ programming, but I think with clever
    scheduling , you can reach similar results using RT processes.

    try my benchmark yourself:

    to note that heavy access to the /proc filesystem (using top -d 0.01 ) causes
    audio drop outs too, not so long as disk access, in the range of 15ms , but that
    seems a bit high for me.

    we really need an *UGLY* (but useful) hack which allows the write() to
    soundcards, to skip some locks of the I/O layer, since audio writes are
    completely unrelated to disk I/O , but I have no idea how hard it would be to
    implement such a mechanism.

    let me know if there are new advances in this area !


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