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Date: Wed, 07 Jul 2004 13:33:55 -0400
To: cygwin AT cygwin DOT com
From: Ken Thompson <ken DOT thompson AT gtri DOT gatech DOT edu>
Subject: Re: two instances of a.exe on dual processor - still only 50% performance
In-Reply-To: <cchbki$vt9$1@sea.gmane.org>
References: <OF350E233D DOT 3A8C27A7-ONC1256ECA DOT 0027FEFC-C1256ECA DOT 00282CE1 AT philips DOT com> <40EBF4C7 DOT B2E27B6D AT dessent DOT net> <cchbki$vt9$1 AT sea DOT gmane DOT org>
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At 01:24 PM 7/7/2004, Rolf Campbell wrote:
>Brian Dessent wrote:
>>mathiasdotwagneratphilipsdotcom wrote:
>>>Many thanks for this tip. I tried it out and indeed there is a "Set
>>>Affinity" option in the Taskmanager. Apparently, this option lets you
>>>assign one or more of the 4 virtual processors to a particular task. (W2K
>>>seems to have this concept of virtual processors, I am no expert at all
>>>here). But it doesn't change a thing. still 50% are spent on Idle mode...
>>You see 4 CPUs because of HyperThreading.  A HT CPU registers with the
>>OS as two CPUs, but it's not.  Only in certain circumstances can it run
>>two threads concurrently (such as performing an integer and floating
>>operation at the same time.)  Thus 50% CPU usage means that your system
>>is fully loaded.  On a HT system you've got to double all the CPU usage
>>percentages for it to make sense.  Occasionally you might see it surpass
>>50%, which would mean that the hyperthreading is particularly suited to
>>whatever combination of instructions is being executed and it's using
>>the CPU more efficiently.
>>Brian
>Sorry Brian, that is bogus.  I'm running one HT processor right now. The 
>combined CPU utilization is not an actual display of usage, but 
>theoretical usage, based on scheduling.  It's really how much of the CPU 
>was NOT being used by the idle task, and given that there is only one CPU, 
>if some process is taking up 99% of it, and some other process takse up 
>the other 1% on the other 'hyper thread' then the idle task will not be 
>able to run at all on either virtual cpu.  Thus Task Manager will (and 
>does) show 100% dual CPU utilization.
>
>Now, Mathias's problem:  Sounds like your program is bound by something 
>other than CPU.  If you have two programs that try to access the 
>hard-disk, they are not going to be twice as fast with 2 CPUs (virtual or not).
>
>-Rolf

Sorry Rolf, but at least on my HT processor running Windows XP pro, the 
reporting of utilization by task manager is exactly as described by Brian.


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