To PREEMPT or not to PREEMPT

goku8499 · n00b Joined: 20 Feb 2004 Posts: 22

Is there any real problems with compling in preemption or not? i have been using with out a single problem. what is all the hoopla about not using it?

erik258 · Posted: Thu Jun 29, 2006 4:54 pm Post subject:

what hoopla?
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goku8499 · n00b Joined: 20 Feb 2004 Posts: 22

its just that everything you read tells you not to, but being adventurous, i went ahead and did, no instablity at all just wondering what is broken or just not quite right enough to say don't do it.

erik258 · Posted: Thu Jun 29, 2006 6:04 pm Post subject:

i have never been told not to preempt

furthermore i have noticed that preempting the big kernel lock seems to help with realtime stuff, especially media on a crappy sound card
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fb · l33t Joined: 08 Dec 2003 Posts: 636 Location: New Zealand

preempt on ppc was buggy for a while but it has improved a lot and is safe
to use since 2.6.16 at least (may be even 2.6.15). It was also reported that
it worked all right earlier in some kernel where smp has been enabled.

As someone that has beeen bitten by the buggy "preempt" behavior I can
tell you it was real. Compilation of big package (firefox) would fail in a random
place. Not really pretty.

But it is safe to use now.

Hope that answer your question.

JoseJX · Retired Dev Joined: 28 Apr 2002 Posts: 2774

It's certainly a whole lot more stable since 2.6.15, but there are still a few unverified issues with it (they're all hard to reproduce), and if you do end up having any problems, enabling SMP seems to always take care of it. Feel free to give it a go, and let us know how it works. Some people were never hit by it at all. On my G4 500, I would have issues rarely (say, once every few weeks). But then again, I never noticed much benefit either.

Good luck either way. :)
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ruben · Guru Joined: 04 Jul 2003 Posts: 462

erik258 · Posted: Fri Jul 07, 2006 3:41 pm Post subject: fair enough

fair enough. I have never had problems either. I can't say that I have had much improvement either but to be fair i rarely leave it out of my kernel, only maybe for servers. and i'm not even sure it is disabled on my server maybe just a handheld pc on my shelf somewhere ...

one thing i did notice though was i think kernel-lock preemption helped with crappy sound cards using the AC97 codec, noteably from via. sound was a lot more crappy without preemption. highly distorted. i would say for real-time stuff, especially media, preeption seems to be a good idea.

is SMP resource-maniacal or is it pretty good to go on any system? I thought it introduced a significant amount of overhead.
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JoseJX · Retired Dev Joined: 28 Apr 2002 Posts: 2774

erik258:
You sound like you're using an x86 machine, this is the PPC forum, and things are a little different on PPC. This is the PPC warning about PREEMPT: http://www.gentoo.org/doc/en/gentoo-ppc-faq.xml#gccsegfaults (just fyi). :)

As for SMP, don't bother on single processor systems, it's only suggested here as a workaround for PPC's preempt issues.
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erik258 · Posted: Fri Jul 07, 2006 7:14 pm Post subject:

Gentoo Forums Forum Index Gentoo on PPC

ah yes, that was pertinent ; ) i forgot about you wierdos in MAC World ; )

just kidding. the only mac I have is an old imac (233 mhz) and it definitely is neat, blows any x86 233 mhz computer out of the water as far as i can tell but yes, certainly disregard my stupidity ; )
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qleak · n00b Joined: 17 Jun 2005 Posts: 67

You may want to keep a bootable kernel image around without prempt.

As I recall, I had some issues with prempt in much older kernels. It worked for some time and then without warning made my system quite unworkable.
I didn't have any bootable kernels without preempt, so I had to boot from a live CD to swap out the kernels.
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