[gentoo-amd64] Re: amd64 and kernel configuration

[Duncan]

Dulmandakh Sukhbaatar posted <20050727062947.73020.qmail@××××××××.mn>,
excerpted below,  on Wed, 27 Jul 2005 14:29:47 +0800:

> I'm new to amd64 and don't know how to configure kernel for best
> performance, but I've been using gentoo since 2004.1. Should I enable SMP,
> HyperThreading (name differs from hypertransport), and NUMA with single
> processor? I found out what with hypertransport performance will better
> than without it, but in the help of SMP suggests that if you have single
> processor its better to disable SMP. Thus disabling SMP there is no option
> for hyperthreading. Last is hyperthreading same as hypertransport or not?
> Enabling hyperthreading can i enable hypertransport? Sorry for my poor
> english :D.

Hypertransport is the name of the interconnect technology AMD uses.  It's
how the CPU connects to everything else.  Therefore, you want that on, or
it'll use slower modes.

Hyperthreading is an Intel technology, used to help compensate for their
very deep CPU pipelining, to minimize the time the CPU spends idle in case
of a branch mispredict, by switching to the other thread while the first
one goes back to memory to get all the stuff it thought it wouldn't need
because it predicted the branch choice wrongly.  AMD CPUs don't have such
deep pipelining and have other technology to minimize branch mispredict
penalties, so don't benefit much from hyperthreading, and therefore don't
include it.  If your CPU is indeed an AMD64 CPU, you don't want
hyperthreading.  If it's one of the new Intel x86_64 CPUs, you may or may
not want it, depending on which particular one it is and whether
hyperthreading is enabled on it or not.

SMP is short for Symmetrical Multi-Processing.  Traditionally, it meant
you had two CPUs.  However, hyperthreading is treated by the kernel as two
CPUs, which is why SMP must be enabled to get the hyperthreading option. 
Note that the newest thing to come to x86/x86_64 is dual-core CPUs.  These
CPUs actually have two logical CPUs in one package.  This is better than
hyperthreading because it's the real thing.  Both Intel and AMD have
dual-core units available, but they are quite new and still expensive, so
you aren't likely to have one and not know about it.  Again, dual core is
handled as SMP by the processor, so you'll want SMP on if you have a
dual-core CPU.  If you are using only a single-core AMD64, you'll want SMP
off, because altho the kernel will work with it on, it'll be more bloated
than it needs to be.

Does that clear up the confusion?

-- 
Duncan - List replies preferred.   No HTML msgs.
"Every nonfree program has a lord, a master --
and if you use the program, he is your master."  Richard Stallman in
http://www.linuxdevcenter.com/pub/a/linux/2004/12/22/rms_interview.html


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