Re: SMP vs UP on single P4 CPU systems with hyperthreading

Thanks for the quick response. So I just probably stick with the UP kernel 
for now.

Zoltan

> On Sat, Nov 06, 2004 at 06:48:33PM -0800, Zoltan Frombach wrote:
>> I just upgraded my system from a Pentium III to a Pentium 4 with
>> hyperthreading. This is not a multiprocessor system, as I only have one
>> CPU. I disabled hyperthreading in the BIOS (it is an AWARD BIOS). Then I
>> installed FreeBSD 5.3-RELEASE. The first thing I've noticed in dmesg are
>> the following lines:
>>
>> CPU: Intel(R) Pentium(R) 4 CPU 2.80GHz (2806.38-MHz 686-class CPU)
>>  Origin = "GenuineIntel"  Id = 0xf29  Stepping = 9
>> 
>> Features=0xbfebfbff<FPU,VME,DE,PSE,TSC,MSR,PAE,MCE,CX8,APIC,SEP,MTRR,PGE,MCA,CMOV,PAT,PSE36,CLFLUSH,DTS,ACPI,MMX,FXSR,SSE,SSE2,SS,HTT,TM,PBE>
>>  Hyperthreading: 2 logical CPUs
>>
>> So even though I specifically disabled hyperthreading in the BIOS, 
>> FreeBSD
>> 5.3 still detects 2 logical CPUs. Is this by design? I guess, it is. So
>> therefore my question is this: Should I compile an SMP kernel or a UP
>> kernel? To make my decision more difficult, Scott Long posted recently in 
>> a
>> message the following: "We turned off SMP on i386 and amd64 because it is 
>> a
>> serious performance penalty for UP machines."
>>
>> Should I look at my single CPU system as a UP machine, as it physically 
>> is
>> a uni processor machine. Or should I enable SMP in the kernel to take
>> advantage of the "2 logical SPUs" FreeBSD detects - even when I try to
>> disable this feature of the CPU in the BIOS... What is the official word 
>> in
>> this scenario? I'd really appreciate your advice.
>
> It will be detected that you have the capability of using it, whether
> or not you have enabled SMP.  It's probably going to be no performance
> gain to enable SMP, since it also requires slower locking operation to
> remain safe for SMP versus what is sufficient for a UP system.
>
> -- 
> Brian Fundakowski Feldman                           \'[ 
> FreeBSD ]''''''''''\
>  <> green_at_FreeBSD.org                               \  The Power to Serve! 
> \
> Opinions expressed are my own. 
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