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    SubjectRe: [linux-audio-dev] Re: [announce] [patch] Voluntary Kernel Preemption Patch

    * Ingo Molnar <mingo@elte.hu> wrote:

    > ATA hardirq latency can be as high as 700 usecs under load even on
    > modern hw, when big DMA requests are created with long scatter-gather
    > lists. We also moved some of the page IO completion code into irq
    > context which further increased hardirq latencies. Since these all
    > touch cold cachelines it all adds up quite quickly.

    typically all of this happens with irqs enabled (unmask=1), but it still
    increases scheduling latencies.

    with the default DMA setup of today's ATA hw there can be a maximum of
    256 entries in the sg-table all zapped in ide_end_request() ->
    __end_that_request_first().

    Plus the IDE driver also builds the sg-table of the next request in
    hardirq context. (ide_build_dmatable() and ide_build_sglist()).

    Ingo
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