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From: Andrew Theurer <habanero@us.ibm.com>
To: linux-kernel@vger.kernel.org
Subject: Re: Linux 2.4.19-rc3 (hyperthreading)
Date: Mon, 29 Jul 2002 14:54:30 -0500
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Cc: Marcelo Tosatti <marcelo@conectiva.com.br>
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I would caution against having hyperthreading on by default in the 2.4.19 
release.  I am seeing a significant degrade in network workloads on P4 with 
hyperthreading on.  On 2.4.19-pre10, I get 788 Mbps on NetBench, but on 
2.4.19-rc1 (and probably rc3, should know in an hour), I get 690 Mbps.  It is 
clearly a hyperthreading/interrupt routing issue.  On this system (4 x P4), 
with no hyperthreading, there is enough CPU to handle all interrupts on CPU0 
(this is where all ints go by default).  With hyperthreading on, I get "1/2" 
of a CPU for interrupt processing.  What ends up happenning is that CPU0 is 
at 100%, while CPU1-CPU7 are at 75%.  Now I know the "noth" is available, but 
since hyperthreading is not proven to give a performance boost to more than 
1/2 of the common workloads for linux users (or is it? who has done tests?), 
I'd like to see this default behavior reversed, and still use acpismp=force 
to enable hyperthreading.  

Also, If anyone has performance results for their workloads showing a boost 
with hyperthreading, I would really like to know.  

-Andrew Theurer



<<So here goes rc3. Another -rc is going to come only in the case of really
critical problem(s).

I'm attaching the rc2->rc3 changelog only because the full changelog got
too big (I guess thats why my -rc2 announce mail didnt go to lk).>>