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Date: Mon, 29 Jul 2002 18:56:57 -0600 (MDT)
From: James Bourne <jbourne@mtroyal.ab.ca>
To: Andrew Theurer <habanero@us.ibm.com>
cc: Alan Cox <alan@lxorguk.ukuu.org.uk>, <linux-kernel@vger.kernel.org>,
       Marcelo Tosatti <marcelo@conectiva.com.br>
Subject: Re: Linux 2.4.19-rc3 (hyperthreading)
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On Mon, 29 Jul 2002, Andrew Theurer wrote:

> On Monday 29 July 2002 7:37 pm, Alan Cox wrote:
> > On Mon, 2002-07-29 at 21:58, Andrew Theurer wrote:
> > > Agreed, we need some sort of irqbalance, and I intend to test with Ingo's
> > > and Andrea's approaches. With that addition, I may even see an
> > > improvement with hyperthreading. But for an rc release, I think it would
> > > be prudent to revert the "new code" for default hyperthreading behavior,
> > > and attack the whole problem in 2.4.20 or later release.
> >
> > Because your personal workload is slower ?
> 
> Well, I would think some here would be interested in samba performance.   
> However, If 4-way P4 systems are considered rare at this point I guess it's 
> not important enough to revert.  FYI, after testing 2P with and without 
> hyperthreading, it's much faster.  481 Mbps for no hyperthreading and 605 
> Mbps with.  If I can get even close to that improvement with 4 processors, 
> I'll be very happy.   

Hyperthreading only helps if you are running a process when uses a lot
of concurrent process binding up CPU time (IE, more runnable processes
waiting then there are CPUs).  I/O won't be much effected, and I found
that it actually gave a performance hit to turn it on if you weren't using
it (about 5%, maybe) if you are not utilizing the existing processors.

Of course, if you are already talking a single, dual, or quad P4 at 1.8GHz
or something, 5% sounds like a lot, but if you're not using it
that heavily you won't actually notice the 5% loss in performance (well,
unless you are watching some kind of image rendering software do it's
thing? =).

Some tests I performed in April are at http://www.hardrock.org/HT-results/

You can see in the kernel compile output the difference.

Regards
James Bourne
> 
> -Andrew Theurer
> 
> -
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-- 
James Bourne, Supervisor Data Centre Operations
Mount Royal College, Calgary, AB, CA
www.mtroyal.ab.ca

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