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

[Ingo Molnar]

* Ingo Molnar <mingo at 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