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Subject: Re: Odd hyperthreading behavior

Author: Robert Hyatt

Date: 20:49:01 10/04/03

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On October 04, 2003 at 21:49:48, Gian-Carlo Pascutto wrote:

>On October 04, 2003 at 21:40:59, Tom Kerrigan wrote:
>
>>(Quick aside: very shortly after HT first came out, a patch for Linux was
>>released to "support" HT--the purpose was not to allow you to use the extra
>>logical processors (because they're transparent to the OS) but to solve exactly
>>the problem you mention.
>
>2.6.x is the first kernel to have improvements in this regard,
>meaning that all normal Linux systems do _not_ have this
>optimization yet.

There isn't a 2.6.x kernel out yet.  There are a few early attempts
in the 2.6.0-test1 through 2.6.0-test6, but no official 2.6.0 has yet
been released.

There are patches for recent 2.4 kernels by Ingo Molnar to correct this
and they work fine.

>
>>I assume Microsoft also made a similar fix and I don't
>>know which versions of Windows have it, but I am running a version of Windows
>>that would.)
>
>You don't know, but you do know?

So far as I know, windows.net is the first windows that will do this
correctly...  I don't think 2K/XP do this according to microsoft.


>
>>Given that, HT would have to be providing a 50% speedup for my program if it
>>were only running on one processor, which seems unrealistically high, and that
>>also means my program should run 100% faster with 4 threads (vs. 2) but in
>>reality it only runs 15% faster.
>
>I don't get your math.
>
>--
>GCP



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