Date: Sat, 6 Nov 2004 19:06:57 -0800 From: "Zoltan Frombach" <tssajo@hotmail.com> To: "Brian Fundakowski Feldman" <green@freebsd.org>, <freebsd-current@freebsd.org> Subject: Re: SMP vs UP on single P4 CPU systems with hyperthreading Message-ID: <BAY2-DAV4kZIhTrBoe200004288@hotmail.com> References: <BAY2-DAV9tgr3SU0hXC0000427f@hotmail.com> <20041107025844.GC918@green.homeunix.org>
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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@FreeBSD.org \ The Power to Serve! > \ > Opinions expressed are my own. > \,,,,,,,,,,,,,,,,,,,,,,\
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