Hyperthreading Issues (on Athlon64?)

Brooks Davis brooks at one-eyed-alien.net
Tue Oct 10 11:11:29 PDT 2006


On Tue, Oct 10, 2006 at 01:59:39PM -0400, Garance A Drosihn wrote:
> At 10:15 AM +1000 10/10/06, Arthur Hartwig wrote:
> >ext kumaresh pandian wrote:
> >>Hi all
> >>  
> >>  
> >>  How do i Find if the CPU is Hyperthreading enabled or not? I have 
> >>tried using the mtables but i'm not able to figure it out.
> >>
> >Look for the HTT bit in the CPU features displayed when FreeBSD starts
> >up. Here's an example from one of my systems, HTT is third from the
> >right hand end of the CPU features list.
> >
> >Copyright (c) 1992-2005 The FreeBSD Project.
> >Copyright (c) 1979, 1980, 1983, 1986, 1988, 1989, 1991, 1992, 1993, 1994
> >       The Regents of the University of California. All rights reserved.
> >hartwig 1 10.09.2006-151040MPTable: <OEM ID   PRODUCT ID  >
> >Timecounter "i8254" frequency 1193182 Hz quality 0
> >CPU: Intel(R) Xeon(TM) CPU 2.80GHz (2794.59-MHz 686-class CPU)
> > Origin = "GenuineIntel"  Id = 0xf27  Stepping = 7
> >
> >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>
> > Features2=0x4400<CNTX-ID,<b14>>
> > Hyperthreading: 2 logical CPUs
> >real memory  = 2147483648 (2048 MB)
> >avail memory = 2100936704 (2003 MB)
> 
> Huh.
> I just got a dual-core Athlon64 machine, and the features list for
> it says:
> 
>   Features=0x178bfbff<FPU,VME,DE,PSE,TSC,MSR,PAE,MCE,CX8,APIC,SEP,
>            MTRR,PGE,MCA,CMOV,PAT,PSE36,CLFLUSH,MMX,FXSR,SSE,SSE2,
>            HTT>
>            ---
> 
> My dmesg does not have the line about "Hyperthreading: 2 logical CPUs",
> though.  But I had been pretty sure the Athlon64 chips didn't have any
> hyperthreading support.  Why is the HTT there?

To trick license managers and schedulers that know HTT, but not true
multi-core.  They report the physical cores a logical cores and then add
an additional flag saying that they are really physical.

-- Brooks
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