From: Philippe Gerum <rpm@xenomai.org>
To: CHABAL David <david.chabal@domain.hid>
Cc: xenomai@xenomai.org
Subject: Re: [Xenomai-help] awful latencies (~110us)
Date: Wed, 17 Oct 2007 18:05:58 +0200 [thread overview]
Message-ID: <1192637158.5973.51.camel@domain.hid> (raw)
In-Reply-To: <47162E66.1080406@domain.hid>
On Wed, 2007-10-17 at 17:46 +0200, CHABAL David wrote:
> Philippe Gerum a écrit :
> > On Wed, 2007-10-17 at 16:52 +0200, CHABAL David wrote:
> >>> Grmmff... I think the PIC on this box has some deep troubles; please try
> >>> this patch, in replacement of the previous ipipe_trace_special
> >>> instrumentation:
> >>>
> >>
> >> RTD| 8.348| 10.387| 43.647| 0| 6.098|
> >> 80.440
> >> ---|------------|------------|------------|--------|-------------------------
> >> RTS| 6.098| 10.020| 80.440| 0| 00:17:20/00:17:20
> >> [root@domain.hid bin]#
> >>
> >>
> >> IRQ handling takes 17µs in the worst case.
> >>
> >> It takes a long time to write 3 poor bytes...
> >>
> >
> > Indeed. Each outb to the ISA bus should be somewhere in the 1-2.5 us
> > range depending on the hw, maybe a bit higher in case of contention, but
> > not that much.
> >
> >> May be is it a SMI problem ??? (Globally disable SMI is on)
> >>
> >
> > It's less likely with an ICH-2 chipset.
> >
> > I see that CONFIG_IDEDMA_PCI_AUTO is disabled. Any reason not to use PCI
> > DMA when available for IDE drives with your hw?
> >
>
> I read this about DMA:
> http://www.rtai.dk/cgi-bin/gratiswiki.pl?DMA_And_Jitter
>
As usual, usage of DMA in real-time situations is a trade-off, and
should be evaluated within the context of the hw at hand. DMA does bus
mastering, but OTOH, forcing PIO raises native Linux latencies, which
also has some drawbacks for real-time kernels implementing RT/non-RT
mode transitions for tasks like Xenomai and RTAI do. For this reason,
you may want to try switching PCI DMA on for your platform, until it
does prove bad latency-wise.
--
Philippe.
next prev parent reply other threads:[~2007-10-17 16:05 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2007-10-05 7:52 [Xenomai-help] awful latencies (~110us) CHABAL David
2007-10-05 8:30 ` Gilles Chanteperdrix
2007-10-05 9:04 ` CHABAL David
2007-10-05 9:20 ` Philippe Gerum
2007-10-05 10:29 ` CHABAL David
2007-10-05 12:35 ` Jan Kiszka
2007-10-05 14:02 ` Philippe Gerum
2007-10-08 12:46 ` CHABAL David
2007-10-10 11:30 ` Philippe Gerum
2007-10-12 14:05 ` CHABAL David
2007-10-12 14:21 ` Jan Kiszka
2007-10-12 14:26 ` CHABAL David
2007-10-12 15:34 ` Philippe Gerum
2007-10-12 15:49 ` Jan Kiszka
2007-10-12 16:43 ` Philippe Gerum
2007-10-17 8:25 ` CHABAL David
2007-10-17 8:41 ` Philippe Gerum
2007-10-17 8:55 ` CHABAL David
2007-10-17 9:17 ` Philippe Gerum
2007-10-17 13:14 ` CHABAL David
2007-10-17 13:46 ` Philippe Gerum
2007-10-17 14:52 ` CHABAL David
2007-10-17 15:24 ` Philippe Gerum
2007-10-17 15:46 ` CHABAL David
2007-10-17 16:05 ` Philippe Gerum [this message]
2007-10-18 12:38 ` CHABAL David
2007-10-18 13:18 ` Philippe Gerum
2007-10-22 14:28 ` CHABAL David
2007-10-17 8:56 ` Philippe Gerum
2007-10-17 8:43 ` Philippe Gerum
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