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What is actually the difference between deactivating 2 of the cores as in this screenshot and having a dual core machine enter image description here

I guess is not the same as having a dual core machine, right? Do you know where i can find some information related?

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The answer will heavily depend on the CPU. There will be a completely different answer for a quad-core machine as there would be for a dual-core machine with hyper-threading. You tagged this quad-core and are getting the answers for that case. But if your CPU is a dual-core with HT, the answers are not completely accurate. –  David Schwartz Mar 18 '12 at 23:35
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You're only telling SQL Server 2008 itself to only use 2 of the available cores. The rest of your system will use all 4 cores.

If you had a dual core CPU then your whole system would only use just 2.

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so is would be the same to have a duakl core, and to have a quad core but deactivating 2 cores? –  cMinor Mar 18 '12 at 23:24
    
I wouldn't say this, because even though SQL Server is only taking 2 cores, Windows will still use 4, and therefore perform better overall. That means, I think, that SQL Server 2008 will still run faster on a 4 core server with its process affinity set to only use 2, than on a system with only 2 cores available. Since the OS will eat away some of the processing power of the only 2 cores available, it will degrade the performance of SQL Server further. Do you understand what I mean? –  Yanick Girouard Mar 18 '12 at 23:29
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For example, when disk I/O completes, one of the other two cores that SQL Server is not using can handle the completion of the disk I/O, so SQL Server can continue running on two cores while that happens. On a dual-core machine, if SQL Server were using both cores and disk I/O completed, the OS would have to steal one of those two cores from SQL Server to handle the completion of the disk I/O and start the next I/O operation. When SQL Server got the core back, it would find the caches no longer holding its data. –  David Schwartz Mar 18 '12 at 23:36
    
Much better explanation David, thanks for clarifying my answer. –  Yanick Girouard Mar 18 '12 at 23:38
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