Re: SMP question

[Artemio]

Hello!

> > I'm building a hard real-time Linux (RTLinux) system on a 2x Xeon
> > machine. If
>
> then LKML is not the place for you.  RTLinux is a fork of "real" linux.

Well, those guys seem to have success on the same machines that I fail. 

I even used *their* .config file to compile a kernel, but I still fail.

> well, talk to the rtlinux people.  considering what rtlinux changes from
> real linux, this hang is not a big surprise.

Yes, rtlinux runs linux kernel as it's idle thread, and it messes up the 
system - I mean it becomes more difficult to make it work if it doesn't work.

> > So, I have to deside between these two:
> >
> >  - Run rtlinux and hard-realtime applications on a kernel without SMP
> > support. How much performance will I loose this way? Is SMP *THAT*
> > critical?
>
> huh?  it's not critical at all.  though if you have two processors,
> you've just wasted several hundred dollars unless you run SMP.

:-)

> >  - Run all tasks in a usual way, no hard realtime, but with SMP support.
>
> why do you think you need rtlinux-type realtime?

Hmm... Nice question.

We are to run many applications that will be mission-critical. That's why we 
decided to make them in hard-realtime.

The other way is probably making low-priority kernel threads by usual means.

But I think hard-realtime will give us more performance. 

> > Also, if I turn hyperthreading off, how will it influence the system with
> > SMP support? Without SMP support?
>
> HT is just hardware-supported multithreading - imagine that you have
> the same old processor, except that the CPU's instruction-dispatcher
> is reading from two instruction streams.  HT doesn't add any new resources,
> just shares a single CPU among two threads.

Got it.

> I suspect the design of rtlinux is inherently incompatible with HT.

I will try turning HT off and running RTLinux.

Thank you very much for your reply!

Artemio.