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List: linux-sound
Subject: Re: low-latency benchmarks: excellent results of RTC clock + SIGIO notification, audio-latency now d
From: Benno Senoner <sbenno () gardena ! net>
Date: 1999-09-11 16:12:36
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On Sat, 11 Sep 1999, mingo@chiara.csoma.elte.hu wrote:
> On Sat, 11 Sep 1999, Benno Senoner wrote:
>
> > Seems that under high disk I/O load,
> > (very seldom, about every 30-100secs) the process
> > gets woken up one IRQ period later.
> > Ideas why this happens.
>
> this could be a lost RTC IRQ. If you are using 2048 Hz RTC interrupts, it
> just needs a single 1msec IRQ delay to lose an RTC IRQ. Especially SCSI
> disk interrupts are known to sometimes cause 1-2 millisecs IRQ delays.
> But before jumping to conclusions, what exactly are the symptoms, what
> does 'one IRQ period later' mean exactly?
>
> -- mingo
The RTC benchmark measures the time between 2 calls to the SIGIO handler.
In my example RTC freq=2048HZ I used 0.48ms periods, and the max jitter
is exactly 2 * 0.48ms = 0.96ms.
The same thing happens on the audio card:
If I use 1.45ms audio fragments, then max delay between two write() calls is
2.9ms ( 2 * 1.45ms)
When I reduce the fragmentsize to 0.7ms , the max registered peak was 1.4ms =
2 * 0.7ms.
Maybe in the audio case, the same phenomen of "lost IRQ" happens,
But it's interesting that the jitter depends on the IRQ frequency.
(maybe only for very low-latencies)
look at the diagrams , you can see very clearly that the peaks are a multiple
of the IRQ period.
Maybe on lower IRQ frequencies ( intervals > 5-10ms ), these peaks will
not show up , because that there are no losses of IRQs ?
But if my HD is blocking the IRQs for max 0.7ms using 0.7ms IRQ period,
why should it block for max 1.45ms by using 1.45ms IRQ periods ?
regards,
Benno.
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