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Why does app use only 25% of dual cpu

Posted on 2006-11-20
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Last Modified: 2010-04-18
Hi

I am new to multiple cpu's and win3k.

I have an application which performs a batch task. On a 64bit 2400 athlon running 32bit Win3k it uses 100% and performs the task quickly.

On a dual xeon 2.4ghz it only uses 25% cpu usage and takes 4 times as long as the single machine to do the job.

Can somebody please explain why an application that uses 100% cpu on a single cpu machine, only uses 25% of a dual xeon machine?

Thanks
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Question by:zattz
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6 Comments
 
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Accepted Solution

by:
Lee W, MVP earned 2000 total points
ID: 17977988
A dual Xeon presents 4 processors to the OS - each physical CPU is hyperthreaded (HT) and each HT CPU is seen as two.  So 2 physical CPUs times 2 for HT gives you 4 processors (according to Windows).

An Athlon64 doesn't offer HyperThreading (HT is an Intel only feature - AMD does offer HyperTransport, but that's not the same thing).  So unless you have two cores in the athlon or a second CPU, the athlon is only seen as 1 CPU.

So you're thinking, "fine, by why still only 25%?"

Because the application only has ONE Thread.  In order to take advantage of multiple processors, you need the application to utilize MORE than one thread.  Think of it like having a kitchen and each person in the house is capable of washing dishes, but there's only one sink.  You can't have more than one person washing the dishes at the same time (drying doesn't count).  So even if you brought friends over, you couldn't wash the dishes any faster unless you had more sinks (threads).
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Author Comment

by:zattz
ID: 17978072
Leew,

That's what I was thinking, hmm

I forgot to mention the application queries a MS Sql server.

If the application does a single query, would the query run as one thread on MS SQL and use one cpu, or would it be spread over multiple?

Thanks
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LVL 96

Expert Comment

by:Lee W, MVP
ID: 17978209
I'm not sure - SQL is multithreaded, but I'm not sure how it's multithreaded.  A single, SIMPLE query almost certainly executes in one thread.  A query that uses subqueries and/or joins may use more than one thread.  But that's just a guess.
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Author Comment

by:zattz
ID: 17978215
Makes sense to me...

Although if I go to task mngr, the cpu graphs show equal activity across all cpu's... and yes it is a very single query, no joins... Maybe windows is lying?

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LVL 96

Expert Comment

by:Lee W, MVP
ID: 17978230
It's possible task manager isn't being entirely accurate... when looking at the processes tabs I've seen that claim lots of idle time then look at the graph and it's pegged...
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Author Comment

by:zattz
ID: 17978243
Okay. Thanks a lot for your help:)
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