Re: Hyperthreading and physical/logical CPU identification

J.A. Magallon (jamagallon@able.es)
Tue, 30 Apr 2002 00:20:19 +0200

On 2002.04.29 Andrew Theurer wrote:
>
>> > I would very much like to believe that in this configuration,
>> > I am only
>> > running on 2 physical, 4 logical processors, but I am getting a 31%
>> > improvement (netbench) when hyperthreading is enabled. Thats
>> > why I want to
>> > confirm I am really only using 2 physical, 4 logical
>> > processors. Is there
>> > any way I can do this? (dmesg? /proc/cpuinfo?)
>>
>> Well the two alternatives are, either A) turning on hyperthreading enabled
>> the two virtual processors or B) turning on hyperthreading somehow enabled
>> the other two processors, right?
>>

I think you should post a more specific "this vs. that" comparison.
Guessing...

If you refer to:
- No hypethreading, box sees 2 processors, performance is 100%
- Hyperthreading active, box sees 4 processors, perf is _only_ 130%,
instead of 200%.

that is perfectly true. Each hyperthreaded pair of processors does
not perform like 2 independent ones, the share cache and other internal
things. So the usual performance improvement on a FOster Xeon is not
200%, but in the order of that 130% you get. Hyperthreading is like
'poor man' SMP.

But perhaps you alredy know all this and your question was following
other paths...

I will put my hands on a dual Xeon SuperMicro system, and will make
more tests, in a week or so...

Hope this helps.

-- 
J.A. Magallon                           #  Let the source be with you...        
mailto:jamagallon@able.es
Mandrake Linux release 8.3 (Cooker) for i586
Linux werewolf 2.4.19-pre7-jam7 #1 SMP jue abr 25 00:19:56 CEST 2002 i686
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