SMP vs UP on single P4 CPU systems with hyperthreading

From: Zoltan Frombach (tssajo_at_hotmail.com)
Date: 11/07/04

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    To: <freebsd-current@freebsd.org>
Date: Sat, 6 Nov 2004 18:48:33 -0800

    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.

    Zoltan
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