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From: Scott Long <scottl@freebsd.org>
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To: Mike Tancsa <mike@sentex.net>
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cc: stable@freebsd.org
Subject: Re: to hyperthread or not to hyperthread
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On Fri, 19 Dec 2003, Mike Tancsa wrote:
>
> We are replacing a box that is fairly busy with many different processes
> (sendmail / pop3).  The new MB and CPU is hyperthreading capable. Is it
> worth it to enable this ?  Does anyone have any real world experiences with
> it in RELENG_4 that say one way or the other to use or not use it ?
>
> dmesg shows,
>
> CPU: Intel(R) Pentium(R) 4 CPU 3.00GHz (2992.52-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,S
> SE2,SS,HTT,TM,PBE>
>    Hyperthreading: 2 logical CPUs
>
> 	---Mike

HyperThreading usually requires special smarts in the OS process
scheduler so that the cores don't bottleneck each other with cache
thrashing and pipeline stalls/starvation.  Without this, Hyperthreading
is usually slightly slower than non-Hyperthreading.  4.x does not have
a scheduler that understand HTT, but 5.2-CURRENT does.

Scott