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Date: Tue, 17 Dec 2002 14:27:04 -0500 (EST)
From: Bill Davidsen <davidsen@tmr.com>
To: "J.A. Magallon" <jamagallon@able.es>
cc: Linux-Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: HT Benchmarks (was: /proc/cpuinfo and hyperthreading)
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On Mon, 16 Dec 2002, J.A. Magallon wrote:

> Number of threads	Elapsed time   User Time   System Time
> 1                   53:216           53:220    00:000
> 2                   29:272           58:180    00:320
> 3                   27:162         1:21:450    00:540
> 4                   25:094         1:41:080    01:250
> 
> Elapsed is measured by the parent thread, that is not doing anything
> but wait on a pthread_join. User and system times are the sum of
> times for all the children threads, that do real work.
> 
> The jump from 1->2 threads is fine, the one from 2->4 is ridiculous...
> I have my cpus doubled but each one has half the pipelining for floating
> point...see the user cpu time increased due to 'worst' processors and
> cache pollution on each package.
> 
> So, IMHO and for my apps, HyperThreading is just a bad joke.

I must be misreading this, it looks to me as though having threads running
HT is reducing the clock time, and frankly that's what I want. It may not
be as good as having more processors, but it certainly is better for
nothing, even for your application. I read that as about 10% faster, and I
know people who spend more on fans to o/c their CPU than the premium for a
Xeon.

More to the point, since you have no choice if you want to go fast or have
>2 CPUs, you get HT included. Clearly if you want good latency you don't
run SMP at all due to the extra locking, that's a kernel issue, not HT.

-- 
bill davidsen <davidsen@tmr.com>
  CTO, TMR Associates, Inc
Doing interesting things with little computers since 1979.

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