Messages in this thread | | | Date | Fri, 14 Oct 2005 23:32:18 +0200 (METDST) | From | Esben Nielsen <> | Subject | Re: 1.6ms jitter in rtc_wakeup (Re: 2.6.14-rc4-rt1) |
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On Fri, 14 Oct 2005, Ingo Molnar wrote:
> > * Esben Nielsen <simlo@phys.au.dk> wrote: > > > I set up rtc_wakeup and got a jitter up 1.6ms! > > It came when I cd'en into a nfs-mount and typed ls. > > > ls-11239 0Dn.. 4us : profile_hit (__schedule) > > ls-11239 0Dn.1 4us : sched_clock (__schedule) > > ls-11239 0Dn.1 5us : check_tsc_unstable (sched_clock) > > ls-11239 0Dn.1 5us : tsc_read_c3_time (sched_clock) > > IRQ 8-775 0D..2 6us : __switch_to (__schedule) > > IRQ 8-775 0D..2 7us!: __schedule <ls-11239> (75 0) > > IRQ 8-775 0...1 1594us : trace_stop_sched_switched (__schedule) > > IRQ 8-775 0D..2 1594us : trace_stop_sched_switched <IRQ 8-775> (0 0) > > IRQ 8-775 0D..2 1595us : trace_stop_sched_switched (__schedule) > > ouch! This very much looks like a hardware induced latency, because the > codepath from those two __schedule points is extremely short and there > is no loop there. Have you tested this particular box before too? If > not, can you reproduce this latency with older versions of -rt too on > the same box, or is this completely new? I have been testing on this box before but never with such long latencies before. > Occasionally there are boxes > that show clear signs of hardware latencies - there's little the kernel > can do about those. I actually felt like the box went to sleep.
> > Wild shot in the dark: are there any power-saving modes enabled on the > box?
I have had problems with power-saving before on this box. The effect is usually the other way around: The rtdcs used to meassure the timing slows down. I.e. it ought to meassure very short times, not very long times.
I'll try to disable power-management.
> Another shot in the dark: can you trigger these latencies if the > networking card is ifconfig down-ed? I.e. perhaps it's related to DMA > done by the networking device. Playing with BIOS settings / DMA/PCI > priorities might help reduce DMA related latencies ...
Hmm. If it is a DMA transfer for the network device it ought to make bad latencies for any network traffic not only NFS. I will have to test various things during the weekend.
Esben
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