SMP kernel on single CPU machines?

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SMP kernel on single CPU machines?

polarapfel
Hi everybody,

this may be a really stupid question but I'm going to ask it anyway since I
didn't find anything using Google or in the archives.

I was looking at

http://www.openbsd.org/faq/faq8.html#SMP

I'm wondering if there are any disadvantages if I run a SMP kernel on a
machine with only one CPU. Is there any harm running an SMP kernel on a
machine with only one CPU?

thanks,
Tobias W.

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Re: SMP kernel on single CPU machines?

Nick Holland
Tobias Weisserth wrote:

> Hi everybody,
>
> this may be a really stupid question but I'm going to ask it anyway since I
> didn't find anything using Google or in the archives.
>
> I was looking at
>
> http://www.openbsd.org/faq/faq8.html#SMP
>
> I'm wondering if there are any disadvantages if I run a SMP kernel on a
> machine with only one CPU. Is there any harm running an SMP kernel on a
> machine with only one CPU?

There may be a performance hit (probably unimportant to most users)
There may be some machines that may not run GENERIC.MP.

On the other hand...
There are some cases where GENERIC.MP on an SP machine can give better
performance for SOME applications on some hardware....and the same HW
and kernel can provide WORSE performance for other apps.

again...most people will not notice the difference.

HOWEVER, if it were all equal, there wouldn't be two different kernels. :)

Nick.

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Re: SMP kernel on single CPU machines?

Daniel Ouellet
In reply to this post by polarapfel
Tobias Weisserth wrote:
> this may be a really stupid question but I'm going to ask it anyway since I
> didn't find anything using Google or in the archives.

http://marc.theaimsgroup.com/?l=openbsd-misc&w=2&r=1&s=smp&q=b

Gives me 264 instance of smp.

One of the most noticeable difference, not that most would see any
difference anyway, but one aspect is the Interrupts difference between
the mp version and the single core one.

YMMV, but in some specific applications setup and with some hardware,
like some of the Intel Quad nic cards, there is a difference between them.

Test for your setup to know in your case.

Hope this gives you something to work with.

Best.

Daniel