On Mon, Feb 27, 2012 at 10:23 PM, Scott Long <scottl_at_samsco.org> wrote: > > On Feb 27, 2012, at 3:38 PM, Andriy Gapon wrote: >>> >>> Turning off the APIC turns off SMP in a very efficient, clean manner. I >>> added this not to isolate the APIC code, but to turn off SMP. That's why >>> it's there, and I'd like the ability to turn off SMP to stay there in some >>> form. >> >> I think that this is a good idea. >> >>> If there's a better way to disable SMP that doesn't get into problems >>> with interrupt delivery, then please propose it. >> >> I think that kern.smp.disabled should be it. >> > > I recall this tunable being problematic, but I don't recall the reason. Reading the code > makes me think that it should be fine; maybe it's time to switch this knob from > hint.apic.0.disabled to kern.smp.disabled, as you've done in your proposed patch. APIC is required for SMP, but works on many older, single CPU systems and removes the massive sharing of IRQs common on non-APIC systems. OTOH, some ThinkPads simply won't boot with APIC. My old T43 (Pentium-M) had this issue. I had to turn off APIC to get a GENERIC kernel to boot. I think that I have heard of other systems that have an issue with FreeBSD APIC. (APIC seems to work fine with Windows, so it's something about the FreeBSD implementation, but the problem is pretty rare, seems limited to 3-5 year old uniprocessor systems, so it's probably not worth trying to track down. -- R. Kevin Oberman, Network Engineer E-mail: kob6558_at_gmail.comReceived on Tue Feb 28 2012 - 21:18:17 UTC
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