Fusion and RT Rreempt [Was: RTAI/fusion 0.7.2]
rpm at xenomai.org
Thu Jun 2 19:05:02 CEST 2005
Max Krasnyansky wrote:
> Philippe Gerum wrote:
>>> Here is something even more interesting.
>>> I just ran the same test on the IBM Z-Pro machine:
> >> [snip]
>>> latency -p 4000 + high network load
>>> RTS| 1262| 780892| 1724248| 0|
>>> How about that :) ? 1.7 milliseconds. Wow.
>>> This is totally reproduceable btw. I played with the BIOS settings
>>> and stuff, makes no difference. MSI workaround is enabled but makes
>>> no difference.
>> If you look at the average figure, it says that the pathological
>> latency is almost continuously happening (> 780 us), so the problem is
>> likely related to this specific hw there. It would be interesting to
>> know whether running a kernel with IO-APIC and ACPI disabled exhibits
>> the same problem.
> Compiling :)
>>> I was wondering what kind of numbers are you guys seeing on IA64
>>> machines ?
>> 4-way (1.3Ghz) Bull Novascale servers currently give ~50 us worst-case
>> for an earlier version of fusion used by the HYADES project that
>> contributed this port. UP gives ~30 us.
> I see. That still kinda suxx :)
Well, yes and now. SMP adds a fair amount of intrinsic latency, and
those figures have been obtained with a test equivalent to
testsuite/latency, i.e. user-space. This said, you would not want an
ia64-based server in your living room anyway, it's definitely too noisy...
> I need an architecture that gives ulta
> low latencies (say under 10 usec) and loads of CPU horse power. Looks
> like dedicated
> CPU (keep the cache hot, etc) is the only option.
Likely, yes. Even with a very smart latency compensation mechanism, I
don't see how x86 or ia64-based archs could fit such stringent requirement.
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