Date: Sat, 6 Nov 2004 18:48:33 -0800 From: "Zoltan Frombach" <tssajo@hotmail.com> To: <freebsd-current@freebsd.org> Subject: SMP vs UP on single P4 CPU systems with hyperthreading Message-ID: <BAY2-DAV9tgr3SU0hXC0000427f@hotmail.com>
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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. Zoltanhome | help
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